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    Staring out with C++ from control structures through objects

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    STARTING OUT WITH C++ From Control Structures through Objects EIGHTH EDITION This page intentionally left blank STARTING OUT WITH C++ From Control Structures through Objects EIGHTH EDITION Tony Gaddis Haywood Community College Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montreal Toronto Delhi Mexico City São Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo Editorial Director: Marcia Horton Acquisitions Editor: Matt Goldstein Program Manager: Kayla Smith-Tarbox Director of Marketing: Christy Lesko Marketing Coordinator: Kathryn Ferranti Marketing Assistant: Jon Bryant Senior Managing Editor: Scott Disanno Senior Project Manager: Marilyn Lloyd Operations Supervisor: Vincent Scelta Operations Specialist: Linda Sager Art Director, Cover: Jayne Conte Text Designer: Joyce Cosentino Wells Cover Designer: Bruce Kenselaar Manager, Visual Research: Karen Sanatar Permissions Supervisor: Michael Joyce Permission Administrator: Jenell Forschler Cover Image: Sergio37_120/Fotolia Media Project Manager: Renata Butera Full-Service Project Manager: Jogender Taneja Aptara®, Inc. Full-Service Vendor: Aptara®, Inc. Printer/Binder: Courier Kendallville Cover Printer: Lehigh-Phoenix Color/Hagerstown Credits and acknowledgments borrowed from other sources and reproduced, with permission, appear on the Credits page in the endmatter of this textbook. Copyright © 2015, 2012, 2009 Pearson Education, Inc., publishing as Addison-Wesley All rights reserved. Manufactured in the United States of America. This publication is protected by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtain permission(s) to use material from this work, please submit a written request to Pearson Education, Inc., Permissions Department, One Lake Street, Upper Saddle River, New Jersey 07458 or you may fax your request to 201 236-3290. Many of the designations by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed in initial caps or all caps. Library of Congress Cataloging-in-Publication Data Gaddis, Tony. Starting out with C++ : from control structures through objects/Tony Gaddis.—Eighth edition. pages cm Includes bibliographical references and index. Online the following appendices are available at www.pearsonhighered.com/gaddis: Appendix D: Introduction to flowcharting; Appendix E: Using UML in class design; Appendix F: Namespaces; Appendix G: Writing managed C++ code for the .net framework; Appendix H: Passing command line arguments; Appendix I: Header file and library function reference; Appendix J: Binary numbers and bitwise operations; Appendix K: Multi-source file programs; Appendix L: Stream member functions for formatting; Appendix M: Introduction to Microsoft Visual C++ 2010 express edition; Appendix N: Answers to checkpoints; and Appendix O: Solutions to odd-numbered review questions. ISBN-13: 978-0-13-376939-5 ISBN-10: 0-13-376939-9 1. C++ (Computer program language) I. Title. II. Title: From control structures through objects. QA76.73.C153G33 2014b 005.13’3—dc23 2014000213 10 9 8 7 6 5 4 3 2 1 ISBN 13: 978-0-13-376939-5 ISBN 10: 0-13-376939-9 Contents at a Glance Preface xv CHAPTER 1 CHAPTER 2 CHAPTER 3 CHAPTER 4 CHAPTER 5 CHAPTER 6 CHAPTER 7 CHAPTER 8 CHAPTER 9 CHAPTER 10 CHAPTER 11 CHAPTER 12 CHAPTER 13 CHAPTER 14 CHAPTER 15 CHAPTER 16 CHAPTER 17 CHAPTER 18 CHAPTER 19 CHAPTER 20 Introduction to Computers and Programming 1 Introduction to C++ 27 Expressions and Interactivity 83 Making Decisions 149 Loops and Files 227 Functions 299 Arrays 375 Searching and Sorting Arrays 457 Pointers 495 Characters, C-Strings, and More About the string Class 547 Structured Data 599 Advanced File Operations 657 Introduction to Classes 711 More About Classes 811 Inheritance, Polymorphism, and Virtual Functions 891 Exceptions, Templates, and the Standard Template Library (STL) 971 Linked Lists 1025 Stacks and Queues 1063 Recursion 1121 Binary Trees 1155 Appendix A: Getting Started with Alice 1185 Appendix B: The ASCII Character Set 1211 Appendix C: Operator Precedence and Associativity 1213 Quick References 1215 v vi Contents at a Glance Online Index 1217 Credit 1237 The following appendices are available at www.pearsonhighered.com/gaddis. Appendix D: Introduction to Flowcharting Appendix E: Using UML in Class Design Appendix F: Namespaces Appendix G: Passing Command Line Arguments Appendix H: Header File and Library Function Reference Appendix I: Binary Numbers and Bitwise Operations Appendix J: Multi-Source File Programs Appendix K: Stream Member Functions for Formatting Appendix L: Answers to Checkpoints Appendix M: Solutions to Odd-Numbered Review Questions Contents Preface xv CHAPTER 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Introduction to Computers and Programming 1 Why Program? 1 Computer Systems: Hardware and Software 2 Programs and Programming Languages 8 What Is a Program Made of? 14 Input, Processing, and Output 17 The Programming Process 18 Procedural and Object-Oriented Programming 22 CHAPTER 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 Introduction to C++ 27 The Parts of a C++ Program 27 The cout Object 31 The #include Directive 36 Variables and Literals 37 Identifiers 41 Integer Data Types 42 The char Data Type 48 The C++ string Class 52 Floating-Point Data Types 54 The bool Data Type 57 Determining the Size of a Data Type 58 Variable Assignments and Initialization 59 Scope 61 Arithmetic Operators 61 Comments 69 Named Constants 71 Programming Style 73 vii viii Contents CHAPTER 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 Expressions and Interactivity 83 The cin Object 83 Mathematical Expressions 89 When You Mix Apples and Oranges: Type Conversion 98 Overflow and Underflow 100 Type Casting 101 Multiple Assignment and Combined Assignment 104 Formatting Output 108 Working with Characters and string Objects 118 More Mathematical Library Functions 124 Focus on Debugging: Hand Tracing a Program 130 Focus on Problem Solving: A Case Study 132 CHAPTER 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 Making Decisions 149 Relational Operators 149 The if Statement 154 Expanding the if Statement 162 The if/else Statement 166 Nested if Statements 169 The if/else if Statement 176 Flags 181 Logical Operators 182 Checking Numeric Ranges with Logical Operators 189 Menus 190 Focus on Software Engineering: Validating User Input 193 Comparing Characters and Strings 195 The Conditional Operator 199 The switch Statement 202 More About Blocks and Variable Scope 211 CHAPTER 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 Loops and Files 227 The Increment and Decrement Operators 227 Introduction to Loops: The while Loop 232 Using the while Loop for Input Validation 239 Counters 241 The do-while Loop 242 The for Loop 247 Keeping a Running Total 257 Sentinels 260 Focus on Software Engineering: Deciding Which Loop to Use 261 Nested Loops 262 Using Files for Data Storage 265 Optional Topics: Breaking and Continuing a Loop 284 CHAPTER 6 6.1 6.2 6.3 6.4 Functions 299 Focus on Software Engineering: Modular Programming 299 Defining and Calling Functions 300 Function Prototypes 309 Sending Data into a Function 311 Contents ix 6.5 Passing Data by Value 316 6.6 Focus on Software Engineering: Using Functions in a Menu-Driven Program 318 6.7 The return Statement 322 6.8 Returning a Value from a Function 324 6.9 Returning a Boolean Value 332 6.10 Local and Global Variables 334 6.11 Static Local Variables 342 6.12 Default Arguments 345 6.13 Using Reference Variables as Parameters 348 6.14 Overloading Functions 354 6.15 The exit() Function 358 6.16 Stubs and Drivers 361 CHAPTER 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 Arrays 375 Arrays Hold Multiple Values 375 Accessing Array Elements 377 No Bounds Checking in C++ 384 Array Initialization 387 The Range-Based for Loop 392 Processing Array Contents 396 Focus on Software Engineering: Using Parallel Arrays 404 Arrays as Function Arguments 407 Two-Dimensional Arrays 418 Arrays with Three or More Dimensions 425 Focus on Problem Solving and Program Design: A Case Study 427 If You Plan to Continue in Computer Science: Introduction to the STL vector 429 CHAPTER 8 8.1 8.2 8.3 8.4 8.5 Searching and Sorting Arrays 457 Focus on Software Engineering: Introduction to Search Algorithms 457 Focus on Problem Solving and Program Design: A Case Study 463 Focus on Software Engineering: Introduction to Sorting Algorithms 470 Focus on Problem Solving and Program Design: A Case Study 477 If You Plan to Continue in Computer Science: Sorting and Searching vectors 485 CHAPTER 9 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 Pointers 495 Getting the Address of a Variable 495 Pointer Variables 497 The Relationship Between Arrays and Pointers 504 Pointer Arithmetic 508 Initializing Pointers 510 Comparing Pointers 511 Pointers as Function Parameters 513 Focus on Software Engineering: Dynamic Memory Allocation 522 Focus on Software Engineering: Returning Pointers from Functions 526 Using Smart Pointers to Avoid Memory Leaks 533 Focus on Problem Solving and Program Design: A Case Study 536 x Contents CHAPTER 10 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 Characters, C-Strings, and More About the string Class 547 Character Testing 547 Character Case Conversion 551 C-Strings 554 Library Functions for Working with C-Strings 558 C-String/Numeric Conversion Functions 569 Focus on Software Engineering: Writing Your Own C-String-Handling Functions 575 More About the C++ string Class 581 Focus on Problem Solving and Program Design: A Case Study 590 CHAPTER 11 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 Structured Data 599 Abstract Data Types 599 Focus on Software Engineering: Combining Data into Structures 601 Accessing Structure Members 604 Initializing a Structure 608 Arrays of Structures 611 Focus on Software Engineering: Nested Structures 613 Structures as Function Arguments 617 Returning a Structure from a Function 620 Pointers to Structures 623 Focus on Software Engineering: When to Use ., When to Use ->, and When to Use * 626 Unions 628 Enumerated Data Types 632 CHAPTER 12 Advanced File Operations 657 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10 File Operations 657 File Output Formatting 663 Passing File Stream Objects to Functions 665 More Detailed Error Testing 667 Member Functions for Reading and Writing Files 670 Focus on Software Engineering: Working with Multiple Files 678 Binary Files 680 Creating Records with Structures 685 Random-Access Files 689 Opening a File for Both Input and Output 697 CHAPTER 13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 Introduction to Classes 711 Procedural and Object-Oriented Programming 711 Introduction to Classes 718 Defining an Instance of a Class 723 Why Have Private Members? 736 Focus on Software Engineering: Separating Class Specification from Implementation 737 Inline Member Functions 743 Constructors 746 Passing Arguments to Constructors 750 Contents xi 13.9 13.10 13.11 13.12 13.13 13.14 13.15 13.16 13.17 Destructors 758 Overloading Constructors 762 Private Member Functions 765 Arrays of Objects 767 Focus on Problem Solving and Program Design: An OOP Case Study 771 Focus on Object-Oriented Programming: Simulating Dice with Objects 778 Focus on Object-Oriented Programming: Creating an Abstract Array Data Type 782 Focus on Object-Oriented Design: The Unified Modeling Language (UML) 785 Focus on Object-Oriented Design: Finding the Classes and Their Responsibilities 788 CHAPTER 14 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 More About Classes 811 Instance and Static Members 811 Friends of Classes 819 Memberwise Assignment 824 Copy Constructors 825 Operator Overloading 831 Object Conversion 858 Aggregation 860 Focus on Object-Oriented Design: Class Collaborations 865 Focus on Object-Oriented Programming: Simulating the Game of Cho-Han 869 CHAPTER 15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 Inheritance, Polymorphism, and Virtual Functions 891 What Is Inheritance? 891 Protected Members and Class Access 900 Constructors and Destructors in Base and Derived Classes 906 Redefining Base Class Functions 918 Class Hierarchies 923 Polymorphism and Virtual Member Functions 929 Abstract Base Classes and Pure Virtual Functions 945 Multiple Inheritance 952 CHAPTER 16 Exceptions, Templates, and the Standard Template Library (STL) 971 16.1 Exceptions 971 16.2 Function Templates 990 16.3 Focus on Software Engineering: Where to Start When Defining Templates 996 16.4 Class Templates 996 16.5 Introduction to the Standard Template Library (STL) 1005 CHAPTER 17 17.1 17.2 17.3 17.4 17.5 Linked Lists 1025 Introduction to the Linked List ADT Linked List Operations 1027 A Linked List Template 1043 Variations of the Linked List 1055 The STL list Container 1056 1025 xii Contents CHAPTER 18 18.1 18.2 18.3 18.4 18.5 18.6 Stacks and Queues 1063 Introduction to the Stack ADT 1063 Dynamic Stacks 1080 The STL stack Container 1091 Introduction to the Queue ADT 1093 Dynamic Queues 1105 The STL deque and queue Containers 1112 CHAPTER 19 19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9 19.10 Recursion 1121 Introduction to Recursion 1121 Solving Problems with Recursion 1125 Focus on Problem Solving and Program Design: The Recursive gcd Function 1133 Focus on Problem Solving and Program Design: Solving Recursively Defined Problems 1134 Focus on Problem Solving and Program Design: Recursive Linked List Operations 1135 Focus on Problem Solving and Program Design: A Recursive Binary Search Function 1139 The Towers of Hanoi 1141 Focus on Problem Solving and Program Design: The QuickSort Algorithm Exhaustive Algorithms 1148 Focus on Software Engineering: Recursion vs. Iteration 1151 1144 CHAPTER 20 20.1 20.2 20.3 Binary Trees 1155 Definition and Applications of Binary Trees 1155 Binary Search Tree Operations 1158 Template Considerations for Binary Search Trees 1175 Appendix A: Getting Started with Alice 1185 Appendix B: The ASCII Character Set 1211 Appendix C: Operator Precedence and Associativity Quick References 1215 Index 1217 Credit 1237 1213 Online The following appendices are available at www.pearsonhighered.com/gaddis. Appendix D: Introduction to Flowcharting Appendix E: Using UML in Class Design Appendix F: Namespaces Appendix G: Passing Command Line Arguments Appendix H: Header File and Library Function Reference Appendix I: Binary Numbers and Bitwise Operations Appendix J: Multi-Source File Programs Appendix K: Stream Member Functions for Formatting Appendix L: Answers to Checkpoints Appendix M: Solutions to Odd-Numbered Review Questions LOCATION OF VIDEONOTES IN THE TEXT Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Introduction to Flowcharting, p. 20 Designing a Program with Pseudocode, p. 20 Designing the Account Balance Program, p. 25 Predicting the Result of Problem 33, p. 26 Using cout, p. 31 Variabe Definitions, p. 37 Assignment Statements and Simple Math Expressions, p. 62 Solving the Restaurant Bill Problem, p. 80 Reading Input with cin, p. 83 Formatting Numbers with setprecision, p. 111 Solving the Stadium Seating Problem, p. 142 The if Statement, p. 154 The if/else statement, p. 166 The if/else if Statement, p. 176 Solving the Time Calculator Problem, p. 221 The while Loop, p. 232 The for Loop, p. 247 Reading Data from a File, p. 274 Solving the Calories Burned Problem, p. 293 Functions and Arguments, p. 311 Value-Returnlng Functions, p. 324 Solving the Markup Problem, p. 366 Accessing Array Elements With a Loop, p. 380 Passing an Array to a Function, p. 407 Solving the Chips and Salsa Problem, p. 448 The Binary Search, p. 460 The Selection Sort, p. 474 Solving the Charge Account Validation Modification Problem, p. 492 Dynamically Allocating an Array, p. 523 Solving the Pointer Rewrite Problem, p. 545 Writing a C-String-Handling Function, p. 575 More About the string Class, p. 581 Solving the Backward String Problem, p. 594 (continued on the next page) LOCATION OF VIDEONOTES IN THE TEXT (continued) Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Creating a Structure, p. 601 Passing a Structure to a Function, p. 617 Solving the Weather Statistics Problem, p. 652 Passing File Stream Objects to Functions, p. 665 Working with Multiple Files, p. 678 Solving the File Encryption Filter Problem, p. 708 Writing a Class, p. 718 Defining an Instance of a Class, p. 723 Solving the Employee Class Problem, p. 802 Operator Overloading, p. 831 Class Aggregation, p. 860 Solving the NumDays Problem, p. 885 Redefining a Base Class Function in a Derived Class, p. 918 Polymorphism, p. 929 Solving the Employee and Production-Worker Classes Problem, p. 963 Throwing an Exception, p. 972 Handling an Exception, p. 972 Writing a Function Template, p. 990 Storing Objects in a vector, p. 1010 Solving the Exception Project Problem, p. 1024 Appending a Node to a Linked List, p. 1028 Inserting a Node in a Linked List, p. 1035 Deleting a Node from a Linked List, p. 1039 Solving the Member Insertion by Position Problem, p. 1061 Storing Objects in an STL stack, p. 1091 Storing Objects in an STL queue, p. 1114 Solving the File Compare Problem, p. 1119 Reducing a Problem with Recursion, p. 1126 Solving the Recursive Multiplication Problem, p. 1153 Inserting a Node in a Binary Tree, p. 1160 Deleting a Node from a Binary Tree, p. 1166 Solving the Node Counter Problem, p. 1182 Preface Welcome to Starting Out with C++: From Control Structures through Objects, 8th edition. This book is intended for use in a two-semester C++ programming sequence, or an accelerated one-semester course. Students new to programming, as well as those with prior course work in other languages, will find this text beneficial. The fundamentals of programming are covered for the novice, while the details, pitfalls, and nuances of the C++ language are explored in-depth for both the beginner and more experienced student. The book is written with clear, easy-to-understand language, and it covers all the necessary topics for an introductory programming course. This text is rich in example programs that are concise, practical, and real-world oriented, ensuring that the student not only learns how to implement the features and constructs of C++, but why and when to use them. Changes in the Eighth Edition C++11 is the latest standard version of the C++ language. In previous years, while the standard was being developed, it was known as C++0x. In August 2011, it was approved by the International Standards Organization (ISO), and the name of the standard was officially changed to C++11. Most of the popular compilers now support the C++11 standard. The new C++11 standard was the primary motivation behind this edition. Although this edition introduces many of the new language features, a C++11 compiler is not strictly required to use the book. As you progress through the book, you will see C++11 icons in the margins, next to the new features that are introduced. Programs appearing in sections that are not marked with this icon will still compile using an older compiler. Here is a summary of the new C++11 topics that are introduced in this edition: ● The auto key word is introduced as a way to simplify complex variable definitions. The auto key word causes the compiler to infer a variable’s data type from its initialization value. ● The long long int and unsigned long long int data types, and the LL literal suffix are introduced. ● Chapter 5 shows how to pass a string object directly to a file stream object’s open member function, without the need to call the c_str() member function. (A discussion of the c_str()function still exists for anyone using a legacy compiler.) xv xvi Preface ● The range-based for loop is introduced in Chapter 7. This new looping mechanism automatically iterates over each element of an array, vector, or other collection, without the need of a counter variable or a subscript. ● Chapter 7 shows how a vector can be initialized with an initialization list. ● The nullptr key word is introduced as the standard way of representing a null pointer. ● Smart pointers are introduced in Chapter 9, with an example of dynamic memory allocation using unique_ptr. ● Chapter 10 discusses the new, overloaded to_string functions for converting numeric values to string objects. ● The string class’s new back() and front() member functions are included in Chapter 10’s overview of the string class. ● Strongly typed enums are discussed in Chapter 11. ● Chapter 13 shows how to use the smart pointer unique_ptr to dynamically allocate an object. ● Chapter 15 discusses the override key word and demonstrates how it can help prevent subtle overriding errors. The final key word is discussed as a way of preventing a virtual member function from being overridden. In addition to the C++11 topics, the following general improvements were made: ● Several new programming problems have been added to the text, and many of the existing programming problems have been modified to make them unique from previous editions. ● The discussion of early, historic computers in Chapter 1 is expanded. ● The discussion of literal values in Chapter 2 is improved. ● The introduction of the char data type in Chapter 2 is reorganized to use character literals in variable assignments before using ASCII values in variable assignments. ● The discussion of random numbers in Chapter 3 is expanded and improved, with the addition of a new In the Spotlight section. ● A new Focus on Object-Oriented Programming section has been added to Chapter 13, showing how to write a class that simulates dice. ● A new Focus on Object-Oriented Programming section has been added to Chapter 14, showing an object-oriented program that simulates the game of Cho-Han. The program uses objects for the dealer, two players, and a pair of dice. Organization of the Text This text teaches C++ in a step-by-step fashion. Each chapter covers a major set of topics and builds knowledge as the student progresses through the book. Although the chapters can be easily taught in their existing sequence, some flexibility is provided. The diagram shown in Figure P-1 suggests possible sequences of instruction. Figure P-1 Chapter 1 Introduction Chapters 2–7 Basic Language Elements Preface xvii Chapter 8 Searching and Sorting Arrays Chapter 9 Pointers Chapter 10 Characters, Strings, and the string Class Chapter 11 Structures Chapter 12 Advanced File Operations* *A few subtopics in Chapter 12 require Chapters 9 and 11. Chapter 13 Introduction to Classes Chapter 14 More About Classes Chapter 15 Inheritance and Polymorphism Chapter 16 Exceptions, Templates, and STL Chapter 17 Linked Lists Chapter 18 Stacks and Queues Chapter 19 Recursion Chapter 20 Binary Trees xviii Preface Chapter 1 covers fundamental hardware, software, and programming concepts. You may choose to skip this chapter if the class has already mastered those topics. Chapters 2 through 7 cover basic C++ syntax, data types, expressions, selection structures, repetition structures, functions, and arrays. Each of these chapters builds on the previous chapter and should be covered in the order presented. After Chapter 7 has been covered, you may proceed to Chapter 8, or jump to either Chapter 9 or Chapter 12. (If you jump to Chapter 12 at this point, you will need to postpone sections 12.7, 12.8, and 12.10 until Chapters 9 and 11 have been covered.) After Chapter 9 has been covered, either of Chapters 10 or 11 may be covered. After Chapter 11, you may cover Chapters 13 through 17 in sequence. Next you can proceed to either Chapter 18 or Chapter 19. Finally, Chapter 20 may be covered. This text’s approach starts with a firm foundation in structured, procedural programming before delving fully into object-oriented programming and advanced data structures. Brief Overview of Each Chapter Chapter 1: Introduction to Computers and Programming This chapter provides an introduction to the field of computer science and covers the fundamentals of programming, problem solving, and software design. The components of programs, such as key words, variables, operators, and punctuation are covered. The tools of the trade, such as pseudocode, flow charts, and hierarchy charts are also presented. Chapter 2: Introduction to C++ This chapter gets the student started in C++ by introducing data types, identifiers, variable declarations, constants, comments, program output, simple arithmetic operations, and C-strings. Programming style conventions are introduced and good programming style is modeled here, as it is throughout the text. An optional section explains the difference between ANSI standard and pre-standard C++ programs. Chapter 3: Expressions and Interactivity In this chapter the student learns to write programs that input and handle numeric, character, and string data. The use of arithmetic operators and the creation of mathematical expressions are covered in greater detail, with emphasis on operator precedence. Debugging is introduced, with a section on hand tracing a program. Sections are also included on simple output formatting, on data type conversion and type casting, and on using library functions that work with numbers. Chapter 4: Making Decisions Here the student learns about relational operators, relational expressions and how to control the flow of a program with the if, if/else, and if/else if statements. The conditional operator and the switch statement are also covered. Crucial applications of these constructs are covered, such as menu-driven programs and the validation of input. Preface xix Chapter 5: Loops and Files This chapter covers repetition control structures. The while loop, do-while loop, and for loop are taught, along with common uses for these devices. Counters, accumulators, running totals, sentinels, and other application-related topics are discussed. Sequential file I/O is also introduced. The student learns to read and write text files, and use loops to process the data in a file. Chapter 6: Functions In this chapter the student learns how and why to modularize programs, using both void and value returning functions. Argument passing is covered, with emphasis on when arguments should be passed by value versus when they need to be passed by reference. Scope of variables is covered, and sections are provided on local versus global variables and on static local variables. Overloaded functions are also introduced and demonstrated. Chapter 7: Arrays In this chapter the student learns to create and work with single and multidimensional arrays. Many examples of array processing are provided including examples illustrating how to find the sum, average, highest, and lowest values in an array and how to sum the rows, columns, and all elements of a two-dimensional array. Programming techniques using parallel arrays are also demonstrated, and the student is shown how to use a data file as an input source to populate an array. STL vectors are introduced and compared to arrays. Chapter 8: Sorting and Searching Arrays Here the student learns the basics of sorting arrays and searching for data stored in them. The chapter covers the Bubble Sort, Selection Sort, Linear Search, and Binary Search algorithms. There is also a section on sorting and searching STL vector objects. Chapter 9: Pointers This chapter explains how to use pointers. Pointers are compared to and contrasted with reference variables. Other topics include pointer arithmetic, initialization of pointers, relational comparison of pointers, pointers and arrays, pointers and functions, dynamic memory allocation, and more. Chapter 10: Characters, C-strings, and More About the string Class This chapter discusses various ways to process text at a detailed level. Library functions for testing and manipulating characters are introduced. C-strings are discussed, and the technique of storing C-strings in char arrays is covered. An extensive discussion of the string class methods is also given. Chapter 11: Structured Data The student is introduced to abstract data types and taught how to create them using structures, unions, and enumerated data types. Discussions and examples include using pointers to structures, passing structures to functions, and returning structures from functions. xx Preface Chapter 12: Advanced File Operations This chapter covers sequential access, random access, text, and binary files. The various modes for opening files are discussed, as well as the many methods for reading and writing file contents. Advanced output formatting is also covered. Chapter 13: Introduction to Classes The student now shifts focus to the object-oriented paradigm. This chapter covers the fundamental concepts of classes. Member variables and functions are discussed. The student learns about private and public access specifications, and reasons to use each. The topics of constructors, overloaded constructors, and destructors are also presented. The chapter presents a section modeling classes with UML and how to find the classes in a particular problem. Chapter 14: More About Classes This chapter continues the study of classes. Static members, friends, memberwise assignment, and copy constructors are discussed. The chapter also includes in-depth sections on operator overloading, object conversion, and object aggregation. There is also a section on class collaborations and the use of CRC cards. Chapter 15: Inheritance, Polymorphism, and Virtual Functions The study of classes continues in this chapter with the subjects of inheritance, polymorphism, and virtual member functions. The topics covered include base and derived class constructors and destructors, virtual member functions, base class pointers, static and dynamic binding, multiple inheritance, and class hierarchies. Chapter 16: Exceptions, Templates, and the Standard Template Library (STL) The student learns to develop enhanced error trapping techniques using exceptions. Discussion then turns to function and class templates as a method for reusing code. Finally, the student is introduced to the containers, iterators, and algorithms offered by the Standard Template Library (STL). Chapter 17: Linked Lists This chapter introduces concepts and techniques needed to work with lists. A linked list ADT is developed and the student is taught to code operations such as creating a linked list, appending a node, traversing the list, searching for a node, inserting a node, deleting a node, and destroying a list. A linked list class template is also demonstrated. Chapter 18: Stacks and Queues In this chapter the student learns to create and use static and dynamic stacks and queues. The operations of stacks and queues are defined, and templates for each ADT are demonstrated. Chapter 19: Recursion This chapter discusses recursion and its use in problem solving. A visual trace of recursive calls is provided, and recursive applications are discussed. Many recursive algorithms are presented, including recursive functions for finding factorials, finding a greatest common Preface xxi denominator (GCD), performing a binary search, and sorting (QuickSort). The classic Towers of Hanoi example is also presented. For students who need more challenge, there is a section on exhaustive algorithms. Chapter 20: Binary Trees This chapter covers the binary tree ADT and demonstrates many binary tree operations. The student learns to traverse a tree, insert an element, delete an element, replace an element, test for an element, and destroy a tree. Appendix A: Getting Started with Alice This appendix gives a quick introduction to Alice. Alice is free software that can be used to teach fundamental programming concepts using 3D graphics. Appendix B: ASCII Character Set A list of the ASCII and Extended ASCII characters and their codes. Appendix C: Operator Precedence and Associativity A chart showing the C++ operators and their precedence. The following appendices are available online at www.pearsonhighered.com/gaddis. Appendix D: Introduction to Flowcharting A brief introduction to flowcharting. This tutorial discusses sequence, selection, case, repetition, and module structures. Appendix E: Using UML in Class Design This appendix shows the student how to use the Unified Modeling Language to design classes. Notation for showing access specification, data types, parameters, return values, overloaded functions, composition, and inheritance are included. Appendix F: Namespaces This appendix explains namespaces and their purpose. Examples showing how to define a namespace and access its members are given. Appendix G: Passing Command Line Arguments Teaches the student how to write a C++ program that accepts arguments from the command line. This appendix will be useful to students working in a command line environment, such as Unix, Linux, or the Windows command prompt. Appendix H: Header File and Library Function Reference This appendix provides a reference for the C++ library functions and header files discussed in the book. Appendix I: Binary Numbers and Bitwise Operations A guide to the C++ bitwise operators, as well as a tutorial on the internal storage of integers. xxii Preface Appendix J: Multi-Source File Programs Provides a tutorial on creating programs that consist of multiple source files. Function header files, class specification files, and class implementation files are discussed. Appendix K: Stream Member Functions for Formatting Covers stream member functions for formatting such as setf. Appendix L: Answers to Checkpoints Students may test their own progress by comparing their answers to the checkpoint exercises against this appendix. The answers to all Checkpoints are included. Appendix M: Solutions to Odd-Numbered Review Questions Another tool that students can use to gauge their progress. Features of the Text Concept Statements Each major section of the text starts with a concept statement. This statement summarizes the ideas of the section. Example Programs The text has hundreds of complete example programs, each designed to highlight the topic currently being studied. In most cases, these are practical, real-world examples. Source code for these programs is provided so that students can run the programs themselves. Program Output After each example program there is a sample of its screen output. This immediately shows the student how the program should function. In the Spotlight Each of these sections provides a programming problem and a detailed, step-by-step analysis showing the student how to solve it. VideoNotes A series of online videos, developed specifically for this book, is available for viewing at www.pearsonhighered.com/gaddis. Icons appear throughout the text alerting the student to videos about specific topics. Checkpoints Checkpoints are questions placed throughout each chapter as a self-test study aid. Answers for all Checkpoint questions can be downloaded from the book’s Companion Web site at www. pearsonhighered.com/gaddis. This allows students to check how well they have learned a new topic. Notes Notes appear at appropriate places throughout the text. They are short explanations of interesting or often misunderstood points relevant to the topic at hand. Preface xxiii Warnings Case Studies Review Questions and Exercises Programming Challenges Group Projects Software Development Project: Serendipity Booksellers C++ Quick Reference Guide 11 C++11 Warnings are notes that caution the student about certain C++ features, programming techniques, or practices that can lead to malfunctioning programs or lost data. Case studies that simulate real-world applications appear in many chapters throughout the text. These case studies are designed to highlight the major topics of the chapter in which they appear. Each chapter presents a thorough and diverse set of review questions, such as fill-in-the-blank and short answer, that check the student’s mastery of the basic material presented in the chapter. These are followed by exercises requiring problem solving and analysis, such as the Algorithm Workbench, Predict the Output, and Find the Errors sections. Answers to the odd-numbered review questions and review exercises can be downloaded from the book’s Companion Web site at www.pearsonhighered.com/ gaddis. Each chapter offers a pool of programming exercises designed to solidify the student’s knowledge of the topics currently being studied. In most cases the assignments present real-world problems to be solved. When applicable, these exercises include input validation rules. There are several group programming projects throughout the text, intended to be constructed by a team of students. One student might build the program’s user interface, while another student writes the mathematical code, and another designs and implements a class the program uses. This process is similar to the way many professional programs are written and encourages team work within the classroom. Available for download from the book’s Companion Web site at www.pearsonhighered.com/gaddis. This is an ongoing project that instructors can optionally assign to teams of students. It systematically develops a “real-world” software package: a point-of-sale program for the fictitious Serendipity Booksellers organization. The Serendipity assignment for each chapter adds more functionality to the software, using constructs and techniques covered in that chapter. When complete, the program will act as a cash register, manage an inventory database, and produce a variety of reports. For easy access, a quick reference guide to the C++ language is printed on the last two pages of Appendix C in the book. Throughout the text, new C++11 language features are introduced. Look for the C++11 icon to find these new features. xxiv Preface Supplements Student Online Resources Many student resources are available for this book from the publisher. The following items are available on the Gaddis Series Companion Web site at www.pearsonhighered.com/gaddis: ● The source code for each example program in the book ● Access to the book’s companion VideoNotes ● A full set of appendices, including answers to the Checkpoint questions and answers to the odd-numbered review questions ● A collection of valuable Case Studies ● The complete Serendipity Booksellers Project Integrated Development Environment (IDE) Resource Kits Professors who adopt this text can order it for students with a kit containing five popular C++ IDEs (Microsoft® Visual Studio Express Edition, Dev C++, NetBeans, Eclipse, and CodeLite) and access to a Web site containing written and video tutorials for getting started in each IDE. For ordering information, please contact your campus Pearson Education representative or visit www.pearsonhighered.com/cs. Online Practice and Assessment with MyProgrammingLab MyProgrammingLab helps students fully grasp the logic, semantics, and syntax of programming. Through practice exercises and immediate, personalized feedback, MyProgrammingLab improves the programming competence of beginning students who often struggle with the basic concepts and paradigms of popular high-level programming languages. A self-study and homework tool, a MyProgrammingLab course consists of hundreds of small practice exercises organized around the structure of this textbook. For students, the system automatically detects errors in the logic and syntax of their code submissions and offers targeted hints that enable students to figure out what went wrong—and why. For instructors, a comprehensive gradebook tracks correct and incorrect answers and stores the code inputted by students for review. MyProgrammingLab is offered to users of this book in partnership with Turing’s Craft, the makers of the CodeLab interactive programming exercise system. For a full demonstration, to see feedback from instructors and students, or to get started using MyProgrammingLab in your course, visit www.myprogramminglab.com. Instructor Resources The following supplements are available to qualified instructors only: • Answers to all Review Questions in the text • Solutions for all Programming Challenges in the text • PowerPoint presentation slides for every chapter • Computerized test bank Preface xxv • Answers to all Student Lab Manual questions • Solutions for all Student Lab Manual programs Visit the Pearson Instructor Resource Center (www.pearsonhighered.com/irc) for information on how to access instructor resources. Textbook Web site Student and instructor resources, including links to download Microsoft® Visual Studio Express and other popular IDEs, for all the books in the Gaddis Starting Out With series can be accessed at the following URL: http://www.pearsonhighered.com/gaddis Get this book the way you want it! This book is part of Pearson Education’s custom database for Computer Science textbooks. Use our online PubSelect system to select just the chapters you need from this, and other, Pearson Education CS textbooks. You can edit the sequence to exactly match your course organization and teaching approach. Visit www.pearsoncustom.com/cs for details. Which Gaddis C++ book is right for you? The Starting Out with C++ Series includes three books, one of which is sure to fit your course: ● Starting Out with C++: From Control Structures through Objects ● Starting Out with C++: Early Objects ● Starting Out with C++: Brief Version The following chart will help you determine which book is right for your course. ■ FROM CONTROL STRUCTURES THROUGH OBJECTS ■ BRIEF VERSION LATE INTRODUCTION OF OBJECTS Classes are introduced in Chapter 13 of the standard text and Chapter 11 of the brief text, after control structures, functions, arrays, and pointers. Advanced OOP topics, such as inheritance and polymorphism, are covered in the following two chapters. INTRODUCTION OF DATA STRUCTURES AND RECURSION Linked lists, stacks and queues, and binary trees are introduced in the final chapters of the standard text. Recursion is covered after stacks and queues, but before binary trees. These topics are not covered in the brief text, though it does have appendices dealing with linked lists and recursion. ■ EARLY OBJECTS EARLIER INTRODUCTION OF OBJECTS Classes are introduced in Chapter 7, after control structures and functions, but before arrays and pointers. Their use is then integrated into the remainder of the text. Advanced OOP topics, such as inheritance and polymorphism, are covered in Chapters 11 and 15. INTRODUCTION OF DATA STRUCTURES AND RECURSION Linked lists, stacks and queues, and binary trees are introduced in the final chapters of the text, after the chapter on recursion. xxvi Preface Acknowledgments There have been many helping hands in the development and publication of this text. We would like to thank the following faculty reviewers for their helpful suggestions and expertise. 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Quick Penn State University David Topham Ohlone College Alberto Ramon Diablo Valley College Robert Tureman Paul D. Camp Community College Bazlur Rasheed Arisa K. Ude Sault College of Applied Arts and Technology Richland College Farshad Ravanshad Bergen Community College Peter van der Goes Rose State College Dolly Samson Weber State University Stewart Venit California State University, Los Angeles Ruth Sapir SUNY Farmingdale Judy Walters North Central College Jason Schatz City College of San Francisco John H. Whipple Northampton Community College Dr. Sung Shin South Dakota State University Aurelia Williams Norfolk State University Bari Siddique University of Texas at Brownsville Vida Winans Illinois Institute of Technology William Slater Collin County Community College Preface xxix I would like to thank my family for their love and support in all of my many projects. I am extremely fortunate to have Matt Goldstein as my editor. I am also fortunate to have Kathryn Ferranti as marketing coordinator. She does a great job getting my books out to the academic community. I had a great production team led by Marilyn Lloyd and Kayla Smith-Tarbox. Thanks to you all! About the Author Tony Gaddis is the principal author of the Starting Out with series of textbooks. He has nearly two decades of experience teaching computer science courses, primarily at Haywood Community College. Tony is a highly acclaimed instructor who was previously selected as the North Carolina Community College Teacher of the Year and has received the Teaching Excellence award from the National Institute for Staff and Organizational Development. The Starting Out With series includes introductory textbooks covering Programming Logic and Design, Alice, C++, Java™, Microsoft® Visual Basic®, Microsoft® Visual C#, Python, and App Inventor, all published by Pearson. get with the programming Through the power of practice and immediate personalized feedback, MyProgrammingLab improves your performance. MyProgrammingLab™ Learn more at www.myprogramminglab.com CHAPTER 1 Introduction to Computers and Programming TOPICS 1.1 Why Program? 1.2 Computer Systems: Hardware and Software 1.3 Programs and Programming Languages 1.4 What Is a Program Made of? 1.5 Input, Processing, and Output 1.6 The Programming Process 1.7 Procedural and Object-Oriented Programming 1.1 Why Program? CONCEPT: Computers can do many different jobs because they are programmable. Think about some of the different ways that people use computers. In school, students use computers for tasks such as writing papers, searching for articles, sending e-mail, and participating in online classes. At work, people use computers to analyze data, make presentations, conduct business transactions, communicate with customers and coworkers, control machines in manufacturing facilities, and do many other things. At home, people use computers for tasks such as paying bills, shopping online, social networking, and playing computer games. And don’t forget that smart phones, iPods®, car navigation systems, and many other devices are computers as well. The uses of computers are almost limitless in our everyday lives. Computers can do such a wide variety of things because they can be programmed. This means that computers are not designed to do just one job, but any job that their programs tell them to do. A program is a set of instructions that a computer follows to perform a task. For example, Figure 1-1 shows screens using Microsoft Word and PowerPoint, two commonly used programs. Programs are commonly referred to as software. Software is essential to a computer because without software, a computer can do nothing. All of the software that we use to make our computers useful is created by individuals known as programmers or software developers. A programmer, or software developer, is a person with the training and skills necessary to design, create, and test computer programs. Computer programming is an exciting and rewarding career. Today, you will find programmers working in business, medicine, government, law enforcement, agriculture, academics, entertainment, and almost every other field. 1 2 Chapter 1 Introduction to Computers and Programming Figure 1-1 A word processing program and a presentation program Computer programming is both an art and a science. It is an art because every aspect of a program should be carefully designed. Listed below are a few of the things that must be designed for any real-world computer program: • The logical flow of the instructions • The mathematical procedures • The appearance of the screens • The way information is presented to the user • The program’s “user-friendliness” • Manuals and other forms of written documentation There is also a scientific, or engineering, side to programming. Because programs rarely work right the first time they are written, a lot of testing, correction, and redesigning is required. This demands patience and persistence from the programmer. Writing software demands discipline as well. Programmers must learn special languages like C++ because computers do not understand English or other human languages. Languages such as C++ have strict rules that must be carefully followed. Both the artistic and scientific nature of programming make writing computer software like designing a car: Both cars and programs should be functional, efficient, powerful, easy to use, and pleasing to look at. 1.2 Computer Systems: Hardware and Software CONCEPT: All computer systems consist of similar hardware devices and software components. This section provides an overview of standard computer hardware and software organization. 1.2 Computer Systems: Hardware and Software 3 Hardware Hardware refers to the physical components that a computer is made of. A computer, as we generally think of it, is not an individual device, but a system of devices. Like the instruments in a symphony orchestra, each device plays its own part. A typical computer system consists of the following major components: • The central processing unit (CPU) • Main memory • Secondary storage devices • Input devices • Output devices The organization of a computer system is depicted in Figure 1-2. Figure 1-2 Central Processing Unit Output Devices Input Devices Main Memory (RAM) Secondary Storage Devices The CPU When a computer is performing the tasks that a program tells it to do, we say that the computer is running or executing the program. The central processing unit, or CPU, is the part of a computer that actually runs programs. The CPU is the most important component in a computer because without it, the computer could not run software. In the earliest computers, CPUs were huge devices made of electrical and mechanical components such as vacuum tubes and switches. Figure 1-3 shows such a device. The two women in 4 Chapter 1 Introduction to Computers and Programming Figure 1-3 the photo are working with the historic ENIAC computer. The ENIAC, considered by many to be the world’s first programmable electronic computer, was built in 1945 to calculate artillery ballistic tables for the U.S. Army. This machine, which was primarily one big CPU, was 8 feet tall, 100 feet long, and weighed 30 tons. Today, CPUs are small chips known as microprocessors. Figure 1-4 shows a photo of a lab technician holding a modern-day microprocessor. In addition to being much smaller than the old electro-mechanical CPUs in early computers, microprocessors are also much more powerful. Figure 1-4 1.2 Computer Systems: Hardware and Software 5 The CPU’s job is to fetch instructions, follow the instructions, and produce some result. Internally, the central processing unit consists of two parts: the control unit and the arithmetic and logic unit (ALU). The control unit coordinates all of the computer’s operations. It is responsible for determining where to get the next instruction and regulating the other major components of the computer with control signals. The arithmetic and logic unit, as its name suggests, is designed to perform mathematical operations. The organization of the CPU is shown in Figure 1-5. Figure 1-5 Central Processing Unit Instruction (Input) Arithmetic and Logic Unit Result (Output) Control Unit A program is a sequence of instructions stored in the computer’s memory. When a computer is running a program, the CPU is engaged in a process known formally as the fetch/decode/ execute cycle. The steps in the fetch/decode/execute cycle are as follows: Fetch Decode Execute The CPU’s control unit fetches, from main memory, the next instruction in the sequence of program instructions. The instruction is encoded in the form of a number. The control unit decodes the instruction and generates an electronic signal. The signal is routed to the appropriate component of the computer (such as the ALU, a disk drive, or some other device). The signal causes the component to perform an operation. These steps are repeated as long as there are instructions to perform. Main Memory You can think of main memory as the computer’s work area. This is where the computer stores a program while the program is running, as well as the data that the program is working with. For example, suppose you are using a word processing program to write an essay for one of your classes. While you do this, both the word processing program and the essay are stored in main memory. Main memory is commonly known as random-access memory or RAM. It is called this because the CPU is able to quickly access data stored at any random location in RAM. RAM is usually a volatile type of memory that is used only for temporary storage while a program is running. When the computer is turned off, the contents of RAM are erased. Inside your computer, RAM is stored in small chips. A computer’s memory is divided into tiny storage locations known as bytes. One byte is enough memory to store only a letter of the alphabet or a small number. In order to do 6 Chapter 1 Introduction to Computers and Programming anything meaningful, a computer must have lots of bytes. Most computers today have millions, or even billions, of bytes of memory. Each byte is divided into eight smaller storage locations known as bits. The term bit stands for binary digit. Computer scientists usually think of bits as tiny switches that can be either on or off. Bits aren’t actual “switches,” however, at least not in the conventional sense. In most computer systems, bits are tiny electrical components that can hold either a positive or a negative charge. Computer scientists think of a positive charge as a switch in the on position and a negative charge as a switch in the off position. Each byte is assigned a unique number known as an address. The addresses are ordered from lowest to highest. A byte is identified by its address in much the same way a post office box is identified by an address. Figure 1-6 shows a group of memory cells with their addresses. In the illustration, sample data is stored in memory. The number 149 is stored in the cell with the address 16, and the number 72 is stored at address 23. Figure 1-6 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 149 20 21 22 23 72 24 25 26 27 28 29 Secondary Storage Secondary storage is a type of memory that can hold data for long periods of time—even when there is no power to the computer. Frequently used programs are stored in secondary memory and loaded into main memory as needed. Important information, such as word processing documents, payroll data, and inventory figures, is saved to secondary storage as well. The most common type of secondary storage device is the disk drive. A disk drive stores data by magnetically encoding it onto a circular disk. Most computers have a disk drive mounted inside their case. External disk drives, which connect to one of the computer’s communication ports, are also available. External disk drives can be used to create backup copies of important data or to move data to another computer. In addition to external disk drives, many types of devices have been created for copying data and for moving it to other computers. For many years floppy disk drives were popular. A floppy disk drive records data onto a small floppy disk, which can be removed from the drive. The use of floppy disk drives has declined dramatically in recent years, in favor of superior devices such as USB drives. USB drives are small devices that plug into the computer’s USB (universal serial bus) port and appear to the system as a disk drive. USB drives, which use flash memory to store data, are inexpensive, reliable, and small enough to be carried in your pocket. Optical devices such as the CD (compact disc) and the DVD (digital versatile disc) are also popular for data storage. Data is not recorded magnetically on an optical disc, but is encoded as a series of pits on the disc surface. CD and DVD drives use a laser to detect the pits and thus read the encoded data. Optical discs hold large amounts of data, and because recordable CD and DVD drives are now commonplace, they are good mediums for creating backup copies of data. 1.2 Computer Systems: Hardware and Software 7 Input Devices Input is any information the computer collects from the outside world. The device that collects the information and sends it to the computer is called an input device. Common input devices are the keyboard, mouse, scanner, digital camera, and microphone. Disk drives, CD/DVD drives, and USB drives can also be considered input devices because programs and information are retrieved from them and loaded into the computer’s memory. Output Devices Output is any information the computer sends to the outside world. It might be a sales report, a list of names, or a graphic image. The information is sent to an output device, which formats and presents it. Common output devices are monitors, printers, and speakers. Disk drives, USB drives, and CD/DVD recorders can also be considered output devices because the CPU sends them information to be saved. Software If a computer is to function, software is not optional. Everything that a computer does, from the time you turn the power switch on until you shut the system down, is under the control of software. There are two general categories of software: system software and application software. Most computer programs clearly fit into one of these two categories. Let’s take a closer look at each. System Software The programs that control and manage the basic operations of a computer are generally referred to as system software. System software typically includes the following types of programs: • Operating Systems An operating system is the most fundamental set of programs on a computer. The operating system controls the internal operations of the computer’s hardware, manages all the devices connected to the computer, allows data to be saved to and retrieved from storage devices, and allows other programs to run on the computer. • Utility Programs A utility program performs a specialized task that enhances the computer’s operation or safeguards data. Examples of utility programs are virus scanners, file-compression programs, and data-backup programs. • Software Development Tools The software tools that programmers use to create, modify, and test software are referred to as software development tools. Compilers and integrated development environments, which we discuss later in this chapter, are examples of programs that fall into this category. Application Software Programs that make a computer useful for everyday tasks are known as application software. These are the programs that people normally spend most of their time running on their computers. Figure 1-1, at the beginning of this chapter, shows screens from two commonly used applications—Microsoft Word, a word processing program, and Microsoft 8 Chapter 1 Introduction to Computers and Programming PowerPoint, a presentation program. Some other examples of application software are spreadsheet programs, e-mail programs, Web browsers, and game programs. Checkpoint 1.1 Why is the computer used by so many different people, in so many different professions? 1.2 List the five major hardware components of a computer system. 1.3 Internally, the CPU consists of what two units? 1.4 Describe the steps in the fetch/decode/execute cycle. 1.5 What is a memory address? What is its purpose? 1.6 Explain why computers have both main memory and secondary storage. 1.7 What are the two general categories of software? 1.8 What fundamental set of programs control the internal operations of the computer’s hardware? 1.9 What do you call a program that performs a specialized task, such as a virus scanner, a file-compression program, or a data-backup program? 1.10 Word processing programs, spreadsheet programs, e-mail programs, Web browsers, and game programs belong to what category of software? 1.3 Programs and Programming Languages CONCEPT: A program is a set of instructions a computer follows in order to perform a task. A programming language is a special language used to write computer programs. What Is a Program? Computers are designed to follow instructions. A computer program is a set of instructions that tells the computer how to solve a problem or perform a task. For example, suppose we want the computer to calculate someone’s gross pay. Here is a list of things the computer should do: 1. Display a message on the screen asking “How many hours did you work?” 2. Wait for the user to enter the number of hours worked. Once the user enters a number, store it in memory. 3. Display a message on the screen asking “How much do you get paid per hour?” 4. Wait for the user to enter an hourly pay rate. Once the user enters a number, store it in memory. 5. Multiply the number of hours by the amount paid per hour, and store the result in memory. 6. Display a message on the screen that tells the amount of money earned. The message must include the result of the calculation performed in Step 5. Collectively, these instructions are called an algorithm. An algorithm is a set of well-defined steps for performing a task or solving a problem. Notice these steps are sequentially ordered. Step 1 should be performed before Step 2, and so forth. It is important that these instructions be performed in their proper sequence. 1.3 Programs and Programming Languages 9 Although you and I might easily understand the instructions in the pay-calculating algorithm, it is not ready to be executed on a computer. A computer’s CPU can only process instructions that are written in machine language. If you were to look at a machine language program, you would see a stream of binary numbers (numbers consisting of only 1s and 0s). The binary numbers form machine language instructions, which the CPU interprets as commands. Here is an example of what a machine language instruction might look like: 1011010000000101 As you can imagine, the process of encoding an algorithm in machine language is very tedious and difficult. In addition, each different type of CPU has its own machine language. If you wrote a machine language program for computer A and then wanted to run it on computer B, which has a different type of CPU, you would have to rewrite the program in computer B’s machine language. Programming languages, which use words instead of numbers, were invented to ease the task of programming. A program can be written in a programming language, such as C++, which is much easier to understand than machine language. Programmers save their programs in text files, and then use special software to convert their programs to machine language. Program 1-1 shows how the pay-calculating algorithm might be written in C++. The “Program Output with Example Input” shows what the program will display on the screen when it is running. In the example, the user enters 10 for the number of hours worked and 15 for the hourly pay rate. The program displays the earnings, which are $150. N OTE: The line numbers that are shown in Program 1-1 are not part of the program. This book shows line numbers in all program listings to help point out specific parts of the program. Program 1-1 1 // This program calculates the user's pay. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 double hours, rate, pay; 8 9 // Get the number of hours worked. 10 cout << "How many hours did you work? "; 11 cin >> hours; 12 13 // Get the hourly pay rate. 14 cout << "How much do you get paid per hour? "; 15 cin >> rate; 16 17 // Calculate the pay. 18 pay = hours * rate; (program continues) 10 Chapter 1 Introduction to Computers and Programming Program 1-1 (continued) 19 20 21 22 23 } // Display the pay. cout << "You have earned $" << pay << endl; return 0; Program Output with Example Input Shown in Bold How many hours did you work? 10 [Enter] How much do you get paid per hour? 15 [Enter] You have earned $150 Programming Languages In a broad sense, there are two categories of programming languages: low-level and highlevel. A low-level language is close to the level of the computer, which means it resembles the numeric machine language of the computer more than the natural language of humans. The easiest languages for people to learn are high-level languages. They are called “highlevel” because they are closer to the level of human-readability than computer-readability. Figure 1-7 illustrates the concept of language levels. Figure 1-7 High level (Easily understood by humans) cccccicooionouunuuttt>t>><<><<<< using namespace std; int main() { cout<<"Hello World\n"; return 0; } Compiler Object Code Linker Executable Code 1.3 Programs and Programming Languages 13 Appendix G explains how compiling works in .Net. You can download Appendix G from the book’s companion Web site at www.pearsonhighered.com/gaddis. Many development systems, particularly those on personal computers, have integrated development environments (IDEs). These environments consist of a text editor, compiler, debugger, and other utilities integrated into a package with a single set of menus. Preprocessing, compiling, linking, and even executing a program is done with a single click of a button, or by selecting a single item from a menu. Figure 1-9 shows a screen from the Microsoft Visual Studio IDE. Figure 1-9 Checkpoint 1.11 What is an algorithm? 1.12 Why were computer programming languages invented? 1.13 What is the difference between a high-level language and a low-level language? 1.14 What does portability mean? 1.15 Explain the operations carried out by the preprocessor, compiler, and linker. 1.16 Explain what is stored in a source file, an object file, and an executable file. 1.17 What is an integrated development environment? 14 Chapter 1 Introduction to Computers and Programming 1.4 What Is a Program Made of? CONCEPT: There are certain elements that are common to all programming languages. Language Elements All programming languages have a few things in common. Table 1-2 lists the common elements you will find in almost every language. Table 1-2 Language Element Key Words Programmer-Defined Identifiers Operators Punctuation Syntax Description Words that have a special meaning. Key words may only be used for their intended purpose. Key words are also known as reserved words. Words or names defined by the programmer. They are symbolic names that refer to variables or programming routines. Operators perform operations on one or more operands. An operand is usually a piece of data, like a number. Punctuation characters that mark the beginning or ending of a statement, or separate items in a list. Rules that must be followed when constructing a program. Syntax dictates how key words and operators may be used, and where punctuation symbols must appear. Let’s look at some specific parts of Program 1-1 (the pay-calculating program) to see examples of each element listed in the table above. For your convenience, Program 1-1 is listed again. Program 1-1 1 // This program calculates the user's pay. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 double hours, rate, pay; 8 9 // Get the number of hours worked. 10 cout << "How many hours did you work? "; 11 cin >> hours; 12 13 // Get the hourly pay rate. 14 cout << "How much do you get paid per hour? "; 15 cin >> rate; 16 17 // Calculate the pay. 1.4 What Is a Program Made of? 15 18 19 20 21 22 23 } pay = hours * rate; // Display the pay. cout << "You have earned $" << pay << endl; return 0; Key Words (Reserved Words) Three of C++’s key words appear on lines 3 and 5: using, namespace, and int. The word double, which appears on line 7, is also a C++ key word. These words, which are always written in lowercase, each have a special meaning in C++ and can only be used for their intended purposes. As you will see, the programmer is allowed to make up his or her own names for certain things in a program. Key words, however, are reserved and cannot be used for anything other than their designated purposes. Part of learning a programming language is learning what the key words are, what they mean, and how to use them. N O T E : The #include statement in line 2 is a preprocessor directive. N OTE: In C++, key words are written in all lowercase. Programmer-Defined Identifiers The words hours, rate, and pay that appear in the program on lines 7, 11, 15, 18, and 21 are programmer-defined identifiers. They are not part of the C++ language but rather are names made up by the programmer. In this particular program, these are the names of variables. As you will learn later in this chapter, variables are the names of memory locations that may hold data. Operators On line 18 the following code appears: pay = hours * rate; The = and * symbols are both operators. They perform operations on pieces of data known as operands. The * operator multiplies its two operands, which in this example are the variables hours and rate. The = symbol is called the assignment operator. It takes the value of the expression on the right and stores it in the variable whose name appears on the left. In this example, the = operator stores in the pay variable the result of the hours variable multiplied by the rate variable. In other words, the statement says, “Make the pay variable equal to hours times rate, or “pay is assigned the value of hours times rate.” Punctuation Notice that lines 3, 7, 10, 11, 14, 15, 18, 21, and 22 all end with a semicolon. A semicolon in C++ is similar to a period in English: It marks the end of a complete sentence (or statement, as it is called in programming jargon). Semicolons do not appear at the end of every line in a C++ program, however. There are rules that govern where semicolons are required 16 Chapter 1 Introduction to Computers and Programming and where they are not. Part of learning C++ is learning where to place semicolons and other punctuation symbols. Lines and Statements Often, the contents of a program are thought of in terms of lines and statements. A “line” is just that—a single line as it appears in the body of a program. Program 1-1 is shown with each of its lines numbered. Most of the lines contain something meaningful; however, some of the lines are empty. The blank lines are only there to make the program more readable. A statement is a complete instruction that causes the computer to perform some action. Here is the statement that appears in line 10 of Program 1-1: cout << "How many hours did you work? "; This statement causes the computer to display the message “How many hours did you work?” on the screen. Statements can be a combination of key words, operators, and programmerdefined symbols. Statements often occupy only one line in a program, but sometimes they are spread out over more than one line. Variables A variable is a named storage location in the computer’s memory for holding a piece of information. The information stored in variables may change while the program is running (hence the name “variable”). Notice that in Program 1-1 the words hours, rate, and pay appear in several places. All three of these are the names of variables. The hours variable is used to store the number of hours the user has worked. The rate variable stores the user’s hourly pay rate. The pay variable holds the result of hours multiplied by rate, which is the user’s gross pay. N OTE: Notice the variables in Program 1-1 have names that reflect their purpose. In fact, it would be easy to guess what the variables were used for just by reading their names. This is discussed further in Chapter 2. Variables are symbolic names that represent locations in the computer’s random-access memory (RAM). When information is stored in a variable, it is actually stored in RAM. Assume a program has a variable named length. Figure 1-10 illustrates the way the variable name represents a memory location. Figure 1-10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 72 24 25 26 27 28 29 length 1.5 Input, Processing, and Output 17 In Figure 1-10, the variable length is holding the value 72. The number 72 is actually stored in RAM at address 23, but the name length symbolically represents this storage location. If it helps, you can think of a variable as a box that holds information. In Figure 1-10, the number 72 is stored in the box named length. Only one item may be stored in the box at any given time. If the program stores another value in the box, it will take the place of the number 72. Variable Definitions In programming, there are two general types of data: numbers and characters. Numbers are used to perform mathematical operations, and characters are used to print data on the screen or on paper. Numeric data can be categorized even further. For instance, the following are all whole numbers, or integers: 5 7 −129 32154 The following are real, or floating-point numbers: 3.14159 6.7 1.0002 When creating a variable in a C++ program, you must know what type of data the program will be storing in it. Look at line 7 of Program 1-1: double hours, rate, pay; The word double in this statement indicates that the variables hours, rate, and pay will be used to hold double precision floating-point numbers. This statement is called a variable definition. It is used to define one or more variables that will be used in the program and to indicate the type of data they will hold. The variable definition causes the variables to be created in memory, so all variables must be defined before they can be used. If you review the listing of Program 1-1, you will see that the variable definitions come before any other statements using those variables. NOTE: Programmers often use the term “variable declaration” to mean the same thing as “variable definition.” Strictly speaking, there is a difference between the two terms. A definition statement always causes a variable to be created in memory. Some types of declaration statements, however, do not cause a variable to be created in memory. You will learn more about declarations later in this book. 1.5 Input, Processing, and Output CONCEPT: The three primary activities of a program are input, processing, and output. Computer programs typically perform a three-step process of gathering input, performing some process on the information gathered, and then producing output. Input is information 18 Chapter 1 Introduction to Computers and Programming a program collects from the outside world. It can be sent to the program from the user, who is entering data at the keyboard or using the mouse. It can also be read from disk files or hardware devices connected to the computer. Program 1-1 allows the user to enter two pieces of information: the number of hours worked and the hourly pay rate. Lines 11 and 15 use the cin (pronounced “see in”) object to perform these input operations: cin >> hours; cin >> rate; Once information is gathered from the outside world, a program usually processes it in some manner. In Program 1-1, the hours worked and hourly pay rate are multiplied in line 18 and the result is assigned to the pay variable: pay = hours * rate; Output is information that a program sends to the outside world. It can be words or graphics displayed on a screen, a report sent to the printer, data stored in a file, or information sent to any device connected to the computer. Lines 10, 14, and 21 in Program 1-1 all perform output: cout << "How many hours did you work? "; cout << "How much do you get paid per hour? "; cout << "You have earned $" << pay << endl; These lines use the cout (pronounced “see out”) object to display messages on the computer’s screen. You will learn more details about the cin and cout objects in Chapter 2. Checkpoint 1.18 Describe the difference between a key word and a programmer-defined identifier. 1.19 Describe the difference between operators and punctuation symbols. 1.20 Describe the difference between a program line and a statement. 1.21 Why are variables called “variable”? 1.22 What happens to a variable’s current contents when a new value is stored there? 1.23 What must take place in a program before a variable is used? 1.24 What are the three primary activities of a program? 1.6 The Programming Process CONCEPT: The programming process consists of several steps, which include design, creation, testing, and debugging activities. Designing and Creating a Program Now that you have been introduced to what a program is, it’s time to consider the process of creating a program. Quite often, when inexperienced students are given programming assignments, they have trouble getting started because they don’t know what to do first. If you find yourself in this dilemma, the steps listed in Figure 1-11 may help. These are the steps recommended for the process of writing a program. Figure 1-11 1.6 The Programming Process 19 1. Clearly define what the program is to do. 2. Visualize the program running on the computer. 3. Use design tools such as a hierarchy chart, flowcharts, or pseudocode to create a model of the program. 4. Check the model for logical errors. 5. Type the code, save it, and compile it. 6. Correct any errors found during compilation. Repeat Steps 5 and 6 as many times as necessary. 7. Run the program with test data for input. 8. Correct any errors found while running the program. Repeat Steps 5 through 8 as many times as necessary. 9. Validate the results of the program. The steps listed in Figure 1-11 emphasize the importance of planning. Just as there are good ways and bad ways to paint a house, there are good ways and bad ways to create a program. A good program always begins with planning. With the pay-calculating program as our example, let’s look at each of the steps in more detail. 1. Clearly define what the program is to do. This step requires that you identify the purpose of the program, the information that is to be input, the processing that is to take place, and the desired output. Let’s examine each of these requirements for the example program: Purpose Input Process Output To calculate the user’s gross pay. Number of hours worked, hourly pay rate. Multiply number of hours worked by hourly pay rate. The result is the user’s gross pay. Display a message indicating the user’s gross pay. 2. Visualize the program running on the computer. Before you create a program on the computer, you should first create it in your mind. Step 2 is the visualization of the program. Try to imagine what the computer screen looks like while the program is running. If it helps, draw pictures of the screen, with sample input and output, at various points in the program. For instance, here is the screen produced by the pay-calculating program: How many hours did you work? 10 How much do you get paid per hour? 15 You have earned $150 In this step, you must put yourself in the shoes of the user. What messages should the program display? What questions should it ask? By addressing these concerns, you will have already determined most of the program’s output. 20 Chapter 1 Introduction to Computers and Programming 3. Use design tools such as a hierarchy chart, flowcharts, or pseudocode to create a model of the program. While planning a program, the programmer uses one or more design tools to create a model of the program. Three common design tools are hierarchy charts, flowcharts, and pseudocode. A hierarchy chart is a diagram that graphically depicts the structure of a program. It has boxes that represent each step in the program. The boxes are connected in a way that illustrates their relationship to one another. Figure 1-12 shows a hierarchy chart for the pay-calculating program. Figure 1-12 Calculate Gross Pay Get Payroll Data from User Multiply Hours Worked by Pay Rate Display Gross Pay Read Number of Hours Worked Read Hourly Pay Rate VideoNote Introduction to Flowcharting VideoNote Designing a Program with Pseudocode A hierarchy chart begins with the overall task and then refines it into smaller subtasks. Each of the subtasks is then refined into even smaller sets of subtasks, until each is small enough to be easily performed. For instance, in Figure 1-12, the overall task “Calculate Gross Pay” is listed in the top-level box. That task is broken into three subtasks. The first subtask, “Get Payroll Data from User,” is broken further into two subtasks. This process of “divide and conquer” is known as top-down design. A flowchart is a diagram that shows the logical flow of a program. It is a useful tool for planning each operation a program performs and the order in which the operations are to occur. For more information see Appendix D, Introduction to Flowcharting. Pseudocode is a cross between human language and a programming language. Although the computer can’t understand pseudocode, programmers often find it helpful to write an algorithm in a language that’s “almost” a programming language, but still very similar to natural language. For example, here is pseudocode that describes the pay-calculating program: Get payroll data. Calculate gross pay. Display gross pay. Although the pseudocode above gives a broad view of the program, it doesn’t reveal all the program’s details. A more detailed version of the pseudocode follows. 1.6 The Programming Process 21 Display “How many hours did you work?”. Input hours. Display “How much do you get paid per hour?”. Input rate. Store the value of hours times rate in the pay variable. Display the value in the pay variable. Notice the pseudocode contains statements that look more like commands than the English statements that describe the algorithm in Section 1.4 (What Is a Program Made of?). The pseudocode even names variables and describes mathematical operations. 4. Check the model for logical errors. Logical errors are mistakes that cause the program to produce erroneous results. Once a hierarchy chart, flowchart, or pseudocode model of the program is assembled, it should be checked for these errors. The programmer should trace through the charts or pseudocode, checking the logic of each step. If an error is found, the model can be corrected before the next step is attempted. 5. Type the code, save it, and compile it. Once a model of the program (hierarchy chart, flowchart, or pseudocode) has been created, checked, and corrected, the programmer is ready to write source code on the computer. The programmer saves the source code to a file and begins the process of translating it to machine language. During this step the compiler will find any syntax errors that may exist in the program. 6. Correct any errors found during compilation. Repeat Steps 5 and 6 as many times as necessary. If the compiler reports any errors, they must be corrected. Steps 5 and 6 must be repeated until the program is free of compile-time errors. 7. Run the program with test data for input. Once an executable file is generated, the program is ready to be tested for run-time errors. A run-time error is an error that occurs while the program is running. These are usually logical errors, such as mathematical mistakes. Testing for run-time errors requires that the program be executed with sample data or sample input. The sample data should be such that the correct output can be predicted. If the program does not produce the correct output, a logical error is present in the program. 8. Correct any errors found while running the program. Repeat Steps 5 through 8 as many times as necessary. When run-time errors are found in a program, they must be corrected. You must identify the step where the error occurred and determine the cause. Desk-checking is a process that can help locate run-time errors. The term desk-checking means the programmer starts reading the program, or a portion of the program, and steps through each statement. A sheet of paper is often used in this process to jot down the current contents of all variables and sketch what the screen looks like after each output operation. When a variable’s contents change, or information is displayed on the screen, this is noted. By stepping through each statement, many errors can be located and corrected. If an error is a result of incorrect logic (such as an improperly stated math formula), you must correct the statement or statements involved in the logic. If an error is due to an incomplete 22 Chapter 1 Introduction to Computers and Programming understanding of the program requirements, then you must restate the program purpose and modify the hierarchy and/or flowcharts, pseudocode, and source code. The program must then be saved, recompiled and retested. This means Steps 5 though 8 must be repeated until the program reliably produces satisfactory results. 9. Validate the results of the program. When you believe you have corrected all the run-time errors, enter test data and determine whether the program solves the original problem. What Is Software Engineering? The field of software engineering encompasses the whole process of crafting computer software. It includes designing, writing, testing, debugging, documenting, modifying, and maintaining complex software development projects. Like traditional engineers, software engineers use a number of tools in their craft. Here are a few examples: • Program specifications • Charts and diagrams of screen output • Hierarchy charts and flowcharts • Pseudocode • Examples of expected input and desired output • Special software designed for testing programs Most commercial software applications are very large. In many instances one or more teams of programmers, not a single individual, develop them. It is important that the program requirements be thoroughly analyzed and divided into subtasks that are handled by individual teams, or individuals within a team. In Step 3 of the programming process, you were introduced to the hierarchy chart as a tool for top-down design. The subtasks that are identified in a top-down design can easily become modules, or separate components of a program. If the program is very large or complex, a team of software engineers can be assigned to work on the individual modules. As the project develops, the modules are coordinated to finally become a single software application. 1.7 Procedural and Object-Oriented Programming CONCEPT: Procedural programming and object-oriented programming are two ways of thinking about software development and program design. C++ is a language that can be used for two methods of writing computer programs: procedural programming and object-oriented programming. This book is designed to teach you some of both. In procedural programming, the programmer constructs procedures (or functions, as they are called in C++). The procedures are collections of programming statements that perform a specific task. The procedures each contain their own variables and commonly share variables with other procedures. This is illustrated by Figure 1-13. Figure 1-13 1.7 Procedural and Object-Oriented Programming 23 Program PROCEDURE A Variables Programming END OF PROCEDURE A PROCEDURE B Variables Programming END OF PROCEDURE B Procedural programming is centered on the procedure, or function. Object-oriented programming (OOP), on the other hand, is centered on the object. An object is a programming element that contains data and the procedures that operate on the data. It is a self-contained unit. This is illustrated in Figure 1-14. Figure 1-14 Object A Variables PROCEDURE A Variables Programming END OF PROCEDURE A PROCEDURE B Variables Programming END OF PROCEDURE B Program Object B Variables PROCEDURE A Variables Programming END OF PROCEDURE A PROCEDURE B Variables Programming END OF PROCEDURE B Object C Variables PROCEDURE A Variables Programming END OF PROCEDURE A PROCEDURE B Variables Programming END OF PROCEDURE B The objects contain, within themselves, both information and the ability to manipulate the information. Operations are carried out on the information in an object by sending the object a message. When an object receives a message instructing it to perform some operation, it carries out the instruction. As you study this text, you will encounter many other aspects of object-oriented programming. Checkpoint 1.25 What four items should you identify when defining what a program is to do? 1.26 What does it mean to “visualize a program running”? What is the value of such an activity? 1.27 What is a hierarchy chart? 1.28 Describe the process of desk-checking. 24 Chapter 1 Introduction to Computers and Programming 1.29 Describe what a compiler does with a program’s source code. 1.30 What is a run-time error? 1.31 Is a syntax error (such as misspelling a key word) found by the compiler or when the program is running? 1.32 What is the purpose of testing a program with sample data or input? 1.33 Briefly describe the difference between procedural and object-oriented programming. Review Questions and Exercises Short Answer 1. Both main memory and secondary storage are types of memory. Describe the difference between the two. 2. What is the difference between system software and application software? 3. What type of software controls the internal operations of the computer’s hardware? 4. Why must programs written in a high-level language be translated into machine language before they can be run? 5. Why is it easier to write a program in a high-level language than in machine language? 6. Explain the difference between an object file and an executable file. 7. What is the difference between a syntax error and a logical error? Fill-in-the-Blank 8. Computers can do many different jobs because they can be __________. 9. The job of the __________ is to fetch instructions, carry out the operations commanded by the instructions, and produce some outcome or resultant information. 10. Internally, the CPU consists of the __________ and the __________. 11. A(n) __________ is an example of a secondary storage device. 12. The two general categories of software are __________ and __________. 13. A program is a set of __________. 14. Since computers can’t be programmed in natural human language, algorithms must be written in a(n) __________ language. 15. __________ is the only language computers really process. 16. __________ languages are close to the level of humans in terms of readability. 17. __________ languages are close to the level of the computer. 18. A program’s ability to run on several different types of computer systems is called __________. 19. Words that have special meaning in a programming language are called __________. 20. Words or names defined by the programmer are called __________. 21. __________ are characters or symbols that perform operations on one or more operands. VideoNote Designing the Account Balance Program Review Questions and Exercises 25 22. __________ characters or symbols mark the beginning or ending of programming statements, or separate items in a list. 23. The rules that must be followed when constructing a program are called __________. 24. A(n) __________ is a named storage location. 25. A variable must be __________ before it can be used in a program. 26. The three primary activities of a program are __________, __________, and __________. 27. __________ is information a program gathers from the outside world. 28. __________ is information a program sends to the outside world. 29. A(n) __________ is a diagram that graphically illustrates the structure of a program. Algorithm Workbench Draw hierarchy charts or flowcharts that depict the programs described below. (See Appendix D for instructions on creating flowcharts.) 30. Available Credit The following steps should be followed in a program that calculates a customer’s available credit: 1. Display the message “Enter the customer’s maximum credit.” 2. Wait for the user to enter the customer’s maximum credit. 3. Display the message “Enter the amount of credit used by the customer.” 4. Wait for the user to enter the customer’s credit used. 5. Subtract the used credit from the maximum credit to get the customer’s available credit. 6. Display a message that shows the customer’s available credit. 31. Sales Tax Design a hierarchy chart or flowchart for a program that calculates the total of a retail sale. The program should ask the user for: – The retail price of the item being purchased – The sales tax rate Once these items have been entered, the program should calculate and display: – The sales tax for the purchase – The total of the sale 32. Account Balance Design a hierarchy chart or flowchart for a program that calculates the current balance in a savings account. The program must ask the user for: – The starting balance – The total dollar amount of deposits made – The total dollar amount of withdrawals made – The monthly interest rate Once the program calculates the current balance, it should be displayed on the screen. 26 Chapter 1 Introduction to Computers and Programming VideoNote Predicting the Result of Problem 33 Predict the Result Questions 33–35 are programs expressed as English statements. What would each display on the screen if they were actual programs? 33. The variable x starts with the value 0. The variable y starts with the value 5. Add 1 to x. Add 1 to y. Add x and y, and store the result in y. Display the value in y on the screen. 34. The variable j starts with the value 10. The variable k starts with the value 2. The variable l starts with the value 4. Store the value of j times k in j. Store the value of k times l in l. Add j and l, and store the result in k. Display the value in k on the screen. 35. The variable a starts with the value 1. The variable b starts with the value 10. The variable c starts with the value 100. The variable x starts with the value 0. Store the value of c times 3 in x. Add the value of b times 6 to the value already in x. Add the value of a times 5 to the value already in x. Display the value in x on the screen. Find the Error 36. The following pseudocode algorithm has an error. The program is supposed to ask the user for the length and width of a rectangular room, and then display the room’s area. The program must multiply the width by the length in order to determine the area. Find the error. area ϭ width ϫ length. Display “What is the room’s width?”. Input width. Display “What is the room’s length?”. Input length. Display area. CHAPTER 2 Introduction to C++ TOPICS 2.1 The Parts of a C++ Program 2.2 The cout Object 2.3 The #include Directive 2.4 Variables and Literals 2.5 Identifiers 2.6 Integer Data Types 2.7 The char Data Type 2.8 The C++ string Class 2.9 Floating-Point Data Types 2.10 The bool Data Type 2.11 Determining the Size of a Data Type 2.12 Variable Assignments and Initialization 2.13 Scope 2.14 Arithmetic Operators 2.15 Comments 2.16 Named Constants 2.17 Programming Style 2.1 The Parts of a C++ Program CONCEPT: C++ programs have parts and components that serve specific purposes. Every C++ program has an anatomy. Unlike human anatomy, the parts of C++ programs are not always in the same place. Nevertheless, the parts are there, and your first step in learning C++ is to learn what they are. We will begin by looking at Program 2-1. Let’s examine the program line by line. Here’s the first line: // A simple C++ program The // marks the beginning of a comment. The compiler ignores everything from the double slash to the end of the line. That means you can type anything you want on that line and the compiler will never complain! Although comments are not required, they are very important to programmers. Most programs are much more complicated than the example in Program 2-1, and comments help explain what’s going on. 27 28 Chapter 2 Introduction to C++ Program 2-1 1 // A simple C++ program 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << "Programming is great fun!"; 8 return 0; 9} The output of the program is shown below. This is what appears on the screen when the program runs. Program Output Programming is great fun! Line 2 looks like this: #include Because this line starts with a #, it is called a preprocessor directive. The preprocessor reads your program before it is compiled and only executes those lines beginning with a # symbol. Think of the preprocessor as a program that “sets up” your source code for the compiler. The #include directive causes the preprocessor to include the contents of another file in the program. The word inside the brackets, iostream, is the name of the file that is to be included. The iostream file contains code that allows a C++ program to display output on the screen and read input from the keyboard. Because this program uses cout to display screen output, the iostream file must be included. The contents of the iostream file are included in the program at the point the #include statement appears. The iostream file is called a header file, so it should be included at the head, or top, of the program. Line 3 reads: using namespace std; Programs usually contain several items with unique names. In this chapter you will learn to create variables. In Chapter 6 you will learn to create functions. In Chapter 13 you will learn to create objects. Variables, functions, and objects are examples of program entities that must have names. C++ uses namespaces to organize the names of program entities. The statement using namespace std; declares that the program will be accessing entities whose names are part of the namespace called std. (Yes, even namespaces have names.) The reason the program needs access to the std namespace is because every name created by the iostream file is part of that namespace. In order for a program to use the entities in iostream, it must have access to the std namespace. Line 5 reads: int main() This marks the beginning of a function. A function can be thought of as a group of one or more programming statements that collectively has a name. The name of this function is main, and the set of parentheses that follows the name indicate that it is a function. The 2.1 The Parts of a C++ Program 29 word int stands for “integer.” It indicates that the function sends an integer value back to the operating system when it is finished executing. Although most C++ programs have more than one function, every C++ program must have a function called main. It is the starting point of the program. If you are ever reading someone else’s C++ program and want to find where it starts, just look for the function named main. NOTE: C++ is a case-sensitive language. That means it regards uppercase letters as being entirely different characters than their lowercase counterparts. In C++, the name of the function main must be written in all lowercase letters. C++ doesn’t see “Main” the same as “main,” or “INT” the same as “int.” This is true for all the C++ key words. Line 6 contains a single, solitary character: { This is called a left-brace, or an opening brace, and it is associated with the beginning of the function main. All the statements that make up a function are enclosed in a set of braces. If you look at the third line down from the opening brace you’ll see the closing brace. Everything between the two braces is the contents of the function main. WARN IN G ! Make sure you have a closing brace for every opening brace in your program! After the opening brace you see the following statement in line 7: cout << "Programming is great fun!"; To put it simply, this line displays a message on the screen. You will read more about cout and the << operator later in this chapter. The message “Programming is great fun!” is printed without the quotation marks. In programming terms, the group of characters inside the quotation marks is called a string literal or string constant. N OTE: This is the only line in the program that causes anything to be printed on the screen. The other lines, like #include and int main(), are necessary for the framework of your program, but they do not cause any screen output. Remember, a program is a set of instructions for the computer. If something is to be displayed on the screen, you must use a programming statement for that purpose. At the end of the line is a semicolon. Just as a period marks the end of a sentence, a semicolon marks the end of a complete statement in C++. Comments are ignored by the compiler, so the semicolon isn’t required at the end of a comment. Preprocessor directives, like #include statements, simply end at the end of the line and never require semicolons. The beginning of a function, like int main(), is not a complete statement, so you don’t place a semicolon there either. It might seem that the rules for where to put a semicolon are not clear at all. Rather than worry about it now, just concentrate on learning the parts of a program. You’ll soon get a feel for where you should and should not use semicolons. 30 Chapter 2 Introduction to C++ Line 8 reads: return 0; This sends the integer value 0 back to the operating system upon the program’s completion. The value 0 usually indicates that a program executed successfully. Line 9 contains the closing brace: } This brace marks the end of the main function. Since main is the only function in this program, it also marks the end of the program. In the sample program you encountered several sets of special characters. Table 2-1 provides a short summary of how they were used. Table 2-1 Special Characters Character Name Description // Double slash Marks the beginning of a comment. # Pound sign Marks the beginning of a preprocessor directive. <> Opening and closing brackets Encloses a filename when used with the #include directive. () Opening and closing parentheses Used in naming a function, as in int main() {} Opening and closing braces Encloses a group of statements, such as the contents of a function. "" Opening and closing quotation Encloses a string of characters, such as a message marks that is to be printed on the screen. ; Semicolon Marks the end of a complete programming statement. Checkpoint 2.1 The following C++ program will not compile because the lines have been mixed up. int main() } // A crazy mixed up program return 0; #include cout << "In 1492 Columbus sailed the ocean blue."; { using namespace std; When the lines are properly arranged the program should display the following on the screen: In 1492 Columbus sailed the ocean blue. Rearrange the lines in the correct order. Test the program by entering it on the computer, compiling it, and running it. 2.2 The cout Object 31 2.2 The cout Object CONCEPT: Use the cout object to display information on the computer’s screen. In this section you will learn to write programs that produce output on the screen. The simplest type of screen output that a program can display is console output, which is merely plain text. The word console is an old computer term. It comes from the days when a computer operator interacted with the system by typing on a terminal. The terminal, which consisted of a simple screen and keyboard, was known as the console. On modern computers, running graphical operating systems such as Windows or Mac OS X, console output is usually displayed in a window such as the one shown in Figure 2-1. In C++ you use the cout object to produce console output. (You can think of the word cout as meaning console output.) Figure 2-1 A Console Window VideoNote Using cout cout is classified as a stream object, which means it works with streams of data. To print a message on the screen, you send a stream of characters to cout. Let’s look at line 7 from Program 2-1: cout << "Programming is great fun!"; Notice that the << operator is used to send the string “Programming is great fun!” to cout. When the << symbol is used this way, it is called the stream insertion operator. The item immediately to the right of the operator is sent to cout and then displayed on the screen. The stream insertion operator is always written as two less-than signs with no space between them. Because you are using it to send a stream of data to the cout object, you can think of the stream insertion operator as an arrow that must point toward cout. This is illustrated in Figure 2-2. Program 2-2 is another way to write the same program. 32 Chapter 2 Introduction to C++ Figure 2-2 cout << "Programming is great fun!"; Think of the stream insertion operator as an arrow that points toward cout. cout "Programming is great fun!"; Program 2-2 1 // A simple C++ program 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << "Programming is " << "great fun!"; 8 return 0; 9} Program Output Programming is great fun! As you can see, the stream-insertion operator can be used to send more than one item to cout. The output of this program is identical to that of Program 2-1. Program 2-3 shows yet another way to accomplish the same thing. Program 2-3 1 // A simple C++ program 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << "Programming is "; 8 cout << "great fun!"; 9 return 0; 10 } Program Output Programming is great fun! An important concept to understand about Program 2-3 is that, although the output is broken up into two programming statements, this program will still display the message on a single line. Unless you specify otherwise, the information you send to cout is displayed in a continuous stream. Sometimes this can produce less-than-desirable results. Program 2-4 is an example. The layout of the actual output looks nothing like the arrangement of the strings in the source code. First, notice there is no space displayed between the words “sellers” and “during,” or 2.2 The cout Object 33 between “June:” and “Computer.” cout displays messages exactly as they are sent. If spaces are to be displayed, they must appear in the strings. Program 2-4 1 // An unruly printing program 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << "The following items were top sellers"; 8 cout << "during the month of June:"; 9 cout << "Computer games"; 10 cout << "Coffee"; 11 cout << "Aspirin"; 12 return 0; 13 } Program Output The following items were top sellersduring the month of June:Computer gamesCoffeeAspirin Second, even though the output is broken into five lines in the source code, it comes out as one long line of output. Because the output is too long to fit on one line on the screen, it wraps around to a second line when displayed. The reason the output comes out as one long line is because cout does not start a new line unless told to do so. There are two ways to instruct cout to start a new line. The first is to send cout a stream manipulator called endl (which is pronounced “end-line” or “end-L”). Program 2-5 is an example. Program 2-5 1 // A well-adjusted printing program 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << "The following items were top sellers" << endl; 8 cout << "during the month of June:" << endl; 9 cout << "Computer games" << endl; 10 cout << "Coffee" << endl; 11 cout << "Aspirin" << endl; 12 return 0; 13 } Program Output The following items were top sellers during the month of June: Computer games Coffee Aspirin 34 Chapter 2 Introduction to C++ N O T E : The last character in endl is the lowercase letter L, not the number one. Every time cout encounters an endl stream manipulator it advances the output to the beginning of the next line for subsequent printing. The manipulator can be inserted anywhere in the stream of characters sent to cout, outside the double quotes. The following statements show an example. cout << "My pets are" << endl << "dog"; cout << endl << "cat" << endl << "bird" << endl; Another way to cause cout to go to a new line is to insert an escape sequence in the string itself. An escape sequence starts with the backslash character (\) and is followed by one or more control characters. It allows you to control the way output is displayed by embedding commands within the string itself. Program 2-6 is an example. Program 2-6 1 // Yet another well-adjusted printing program 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << "The following items were top sellers\n"; 8 cout << "during the month of June:\n"; 9 cout << "Computer games\nCoffee"; 10 cout << "\nAspirin\n"; 11 return 0; 12 } Program Output The following items were top sellers during the month of June: Computer games Coffee Aspirin The newline escape sequence is \n. When cout encounters \n in a string, it doesn’t print it on the screen, but interprets it as a special command to advance the output cursor to the next line. You have probably noticed inserting the escape sequence requires less typing than inserting endl. That’s why many programmers prefer it. A common mistake made by beginning C++ students is to use a forward slash (/) instead of a backslash (\) when trying to write an escape sequence. This will not work. For example, look at the following code. // Error! cout << "Four Score/nAnd seven/nYears ago./n"; 2.2 The cout Object 35 In this code, the programmer accidentally wrote /n when he or she meant to write \n. The cout object will simply display the /n characters on the screen. This code will display the following output: Four Score/nAnd seven/nYears ago./n Another common mistake is to forget to put the \n inside quotation marks. For example, the following code will not compile. // Error! This code will not compile. cout << "Good" << \n; cout << "Morning" << \n; This code will result in an error because the \n sequences are not inside quotation marks. We can correct the code by placing the \n sequences inside the string literals, as shown here: // This will work. cout << "Good\n"; cout << "Morning\n"; There are many escape sequences in C++. They give you the ability to exercise greater control over the way information is output by your program. Table 2-2 lists a few of them. Table 2-2 Common Escape Sequences Escape Sequence \n \t \a \b \r \\ \' \" Name Newline Horizontal tab Alarm Backspace Return Backslash Single quote Double quote Description Causes the cursor to go to the next line for subsequent printing. Causes the cursor to skip over to the next tab stop. Causes the computer to beep. Causes the cursor to back up, or move left one position. Causes the cursor to go to the beginning of the current line, not the next line. Causes a backslash to be printed. Causes a single quotation mark to be printed. Causes a double quotation mark to be printed. WARNING! When using escape sequences, do not put a space between the backslash and the control character. When you type an escape sequence in a string, you type two characters (a backslash followed by another character). However, an escape sequence is stored in memory as a single character. For example, consider the following string literal: "One\nTwo\nThree\n" The diagram in Figure 2-3 breaks this string into its individual characters. Notice how each of the \n escape sequences are considered one character. Figure 2-3 O n e \n T w o \n T h r e e \n 36 Chapter 2 Introduction to C++ 2.3 The #include Directive CONCEPT: The #include directive causes the contents of another file to be inserted into the program. Now is a good time to expand our discussion of the #include directive. The following line has appeared near the top of every example program. #include The header file iostream must be included in any program that uses the cout object. This is because cout is not part of the “core” of the C++ language. Specifically, it is part of the input–output stream library. The header file, iostream, contains information describing iostream objects. Without it, the compiler will not know how to properly compile a program that uses cout. Preprocessor directives are not C++ statements. They are commands to the preprocessor, which runs prior to the compiler (hence the name “preprocessor”). The preprocessor’s job is to set programs up in a way that makes life easier for the programmer. For example, any program that uses the cout object must contain the extensive setup information found in iostream. The programmer could type all this information into the program, but it would be too time consuming. An alternative would be to use an editor to “cut and paste” the information into the program, but that would quickly become tiring as well. The solution is to let the preprocessor insert the contents of iostream automatically. W ARN IN G ! Do not put semicolons at the end of processor directives. Because preprocessor directives are not C++ statements, they do not require semicolons. In many cases an error will result from a preprocessor directive terminated with a semicolon. An #include directive must always contain the name of a file. The preprocessor inserts the entire contents of the file into the program at the point it encounters the #include directive. The compiler doesn’t actually see the #include directive. Instead it sees the code that was inserted by the preprocessor, just as if the programmer had typed it there. The code contained in header files is C++ code. Typically it describes complex objects like cout. Later you will learn to create your own header files. Checkpoint 2.2 The following C++ program will not compile because the lines have been mixed up. cout << "Success\n"; cout << " Success\n\n"; int main() cout << "Success"; } 2.4 Variables and Literals 37 using namespace std; // It's a mad, mad program #include cout << "Success\n"; { return 0; When the lines are properly arranged the program should display the following on the screen: Program Output Success Success Success Success Rearrange the lines in the correct order. Test the program by entering it on the computer, compiling it, and running it. 2.3 Study the following program and show what it will print on the screen. // The Works of Wolfgang #include using namespace std; int main() { cout << "The works of Wolfgang\ninclude the following"; cout << "\nThe Turkish March" << endl; cout << "and Symphony No. 40 "; cout << "in G minor." << endl; return 0; } 2.4 On paper, write a program that will display your name on the first line, your street address on the second line, your city, state, and ZIP code on the third line, and your telephone number on the fourth line. Place a comment with today’s date at the top of the program. Test your program by entering, compiling, and running it. 2.4 Variables and Literals VideoNote Variabe Definitions CONCEPT: Variables represent storage locations in the computer’s memory. Literals are constant values that are assigned to variables. As you discovered in Chapter 1, variables allow you to store and work with data in the computer’s memory. They provide an “interface” to RAM. Part of the job of programming is to determine how many variables a program will need and what types of information they will hold. Program 2-7 is an example of a C++ program with a variable. Take a look at line 7: int number; This is called a variable definition. It tells the compiler the variable’s name and the type of data it will hold. This line indicates the variable’s name is number. The word int stands for integer, so number will only be used to hold integer numbers. Later in this chapter you will learn all the types of data that C++ allows. 38 Chapter 2 Introduction to C++ Program 2-7 1 // This program has a variable. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int number; 8 9 number = 5; 10 cout << "The value in number is " << number << endl; 11 return 0; 12 } Program Output The value in number is 5 NOTE: You must have a definition for every variable you intend to use in a program. In C++, variable definitions can appear at any point in the program. Later in this chapter, and throughout the book, you will learn the best places to define variables. Notice that variable definitions end with a semicolon. Now look at line 9: number = 5; This is called an assignment. The equal sign is an operator that copies the value on its right (5) into the variable named on its left (number). After this line executes, number will be set to 5. N OTE: This line does not print anything on the computer’s screen. It runs silently behind the scenes, storing a value in RAM. Look at line 10. cout << "The value in number is " << number << endl; The second item sent to cout is the variable name number. When you send a variable name to cout it prints the variable’s contents. Notice there are no quotation marks around number. Look at what happens in Program 2-8. Program 2-8 1 // This program has a variable. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int number; 8 9 number = 5; 2.4 Variables and Literals 39 10 11 12 } cout << "The value in number is " << "number" << endl; return 0; Program Output The value in number is number When double quotation marks are placed around the word number it becomes a string literal and is no longer a variable name. When string literals are sent to cout they are printed exactly as they appear inside the quotation marks. You’ve probably noticed by now that the endl stream manipulator has no quotation marks around it, for the same reason. Sometimes a Number Isn’t a Number As shown in Program 2-8, just placing quotation marks around a variable name changes the program’s results. In fact, placing double quotation marks around anything that is not intended to be a string literal will create an error of some type. For example, in Program 2-8 the number 5 was assigned to the variable number. It would have been incorrect to perform the assignment this way: number = "5"; In this line, 5 is no longer an integer, but a string literal. Because number was defined as an integer variable, you can only store integers in it. The integer 5 and the string literal “5” are not the same thing. The fact that numbers can be represented as strings frequently confuses students who are new to programming. Just remember that strings are intended for humans to read. They are to be printed on computer screens or paper. Numbers, however, are intended primarily for mathematical operations. You cannot perform math on strings. Before numbers can be displayed on the screen, they must first be converted to strings. (Fortunately, cout handles the conversion automatically when you send a number to it.) Literals A literal is a piece of data that is written directly into a program’s code. One of the most common uses of literals is to assign a value to a variable. For example, in the following statement assume that number is an int variable. The statement assigns the literal value 100 to the variable number: number = 100; Another common use of literals is to display something on the screen. For example, the following statement displays the string literal “Welcome to my program.” cout << "Welcome to my program." << endl; Program 2-9 shows an example that uses a variable and several literals. Program 2-9 1 // This program has literals and a variable. 2 #include 3 using namespace std; (program continues) 40 Chapter 2 Introduction to C++ Program 2-9 (continued) 4 5 int main() 6{ 7 int apples; 8 9 apples = 20; 10 cout << "Today we sold " << apples << " bushels of apples.\n"; 11 return 0; 12 } Program Output Today we sold 20 bushels of apples. Of course, the variable is apples. It is defined as an integer. Table 2-3 lists the literals found in the program. Table 2-3 Literal 20 "Today we sold " "bushels of apples.\n" 0 Type of Literal Integer literal String literal String literal Integer literal N OTE: Literals are also called constants. Checkpoint 2.5 Examine the following program. // This program uses variables and literals. #include using namespace std; int main() { int little; int big; little = 2; big = 2000; cout << "The little number is " << little << endl; cout << "The big number is " << big << endl; return 0; } List all the variables and literals that appear in the program. 2.6 What will the following program display on the screen? #include using namespace std; int main() { int number; number = 712; cout << "The value is " << "number" << endl; return 0; } 2.5 Identifiers 41 2.5 Identifiers CONCEPT: Choose variable names that indicate what the variables are used for. An identifier is a programmer-defined name that represents some element of a program. Variable names are examples of identifiers. You may choose your own variable names in C++, as long as you do not use any of the C++ key words. The key words make up the “core” of the language and have specific purposes. Table 2-4 shows a complete list of the C++ key words. Note that they are all lowercase. Table 2-4 The C++ Key Words alignas alignof and and_eq asm auto bitand bitor bool break case catch char char16_t char32_t class compl const constexpr const_cast continue decltype default delete do double dynamic_cast else enum explicit export extern false float for friend goto if inline int long mutable namespace new noexcept not not_eq nullptr operator or or_eq private protected public register reinterpret_cast return short signed sizeof static static_assert static_cast struct switch template this thread_local throw true try typedef typeid typename union unsigned using virtual void volatile wchar_t while xor xor_eq You should always choose names for your variables that give an indication of what the variables are used for. You may be tempted to define variables with names like this: int x; The rather nondescript name, x, gives no clue as to the variable’s purpose. Here is a better example. int itemsOrdered; 42 Chapter 2 Introduction to C++ The name itemsOrdered gives anyone reading the program an idea of the variable’s use. This way of coding helps produce self-documenting programs, which means you get an understanding of what the program is doing just by reading its code. Because real-world programs usually have thousands of lines, it is important that they be as self-documenting as possible. You probably have noticed the mixture of uppercase and lowercase letters in the name itemsOrdered. Although all of C++’s key words must be written in lowercase, you may use uppercase letters in variable names. The reason the O in itemsOrdered is capitalized is to improve readability. Normally “items ordered” is two words. Unfortunately you cannot have spaces in a variable name, so the two words must be combined into one. When “items” and “ordered” are stuck together you get a variable definition like this: int itemsordered; Capitalization of the first letter of the second word and succeeding words makes itemsOrdered easier to read. It should be mentioned that this style of coding is not required. You are free to use all lowercase letters, all uppercase letters, or any combination of both. In fact, some programmers use the underscore character to separate words in a variable name, as in the following. int items_ordered; Legal Identifiers Regardless of which style you adopt, be consistent and make your variable names as sensible as possible. Here are some specific rules that must be followed with all identifiers. • The first character must be one of the letters a through z, A through Z, or an underscore character (_). • After the first character you may use the letters a through z or A through Z, the digits 0 through 9, or underscores. • Uppercase and lowercase characters are distinct. This means ItemsOrdered is not the same as itemsordered. Table 2-5 lists variable names and tells whether each is legal or illegal in C++. Table 2-5 Some Variable Names Variable Name dayOfWeek 3dGraph _employee_num June1997 Mixture#3 Legal or Illegal? Legal. Illegal. Variable names cannot begin with a digit. Legal. Legal. Illegal. Variable names may only use letters, digits, or underscores. 2.6 Integer Data Types CONCEPT: There are many different types of data. Variables are classified according to their data type, which determines the kind of information that may be stored in them. Integer variables can only hold whole numbers. 2.6 Integer Data Types 43 Computer programs collect pieces of data from the real world and manipulate them in various ways. There are many different types of data. In the realm of numeric information, for example, there are whole numbers and fractional numbers. There are negative numbers and positive numbers. And there are numbers so large, and others so small, that they don’t even have a name. Then there is textual information. Names and addresses, for instance, are stored as groups of characters. When you write a program you must determine what types of information it will be likely to encounter. If you are writing a program to calculate the number of miles to a distant star, you’ll need variables that can hold very large numbers. If you are designing software to record microscopic dimensions, you’ll need to store very small and precise numbers. Additionally, if you are writing a program that must perform thousands of intensive calculations, you’ll want variables that can be processed quickly. The data type of a variable determines all of these factors. Although C++ offers many data types, in the very broadest sense there are only two: numeric and character. Numeric data types are broken into two additional categories: integer and floating point. Integers are whole numbers like 12, 157, −34, and 2. Floating point numbers have a decimal point, like 23.7, 189.0231, and 0.987. Additionally, the integer and floating point data types are broken into even more classifications. Before we discuss the character data type, let’s carefully examine the variations of numeric data. Your primary considerations for selecting a numeric data type are • The largest and smallest numbers that may be stored in the variable • How much memory the variable uses • Whether the variable stores signed or unsigned numbers • The number of decimal places of precision the variable has The size of a variable is the number of bytes of memory it uses. Typically, the larger a variable is, the greater the range it can hold. Table 2-6 shows the C++ integer data types with their typical sizes and ranges. NOTE: The data type sizes and ranges shown in Table 2-6 are typical on many systems. Depending on your operating system, the sizes and ranges may be different. Table 2-6 Integer Data Types Data Type short int unsigned short int int unsigned int long int unsigned long int long long int Typical Size 2 bytes 2 bytes 4 bytes 4 bytes 4 bytes 4 bytes 8 bytes unsigned long long int 8 bytes Typical Range Ϫ32,768 to ϩ32,767 0 to +65,535 Ϫ2,147,483,648 to ϩ2,147,483,647 0 to 4,294,967,295 Ϫ2,147,483,648 to ϩ2,147,483,647 0 to 4,294,967,295 Ϫ9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 0 to 18,446,744,073,709,551,615 44 Chapter 2 Introduction to C++ Here are some examples of variable definitions: int days; unsigned int speed; short int month; unsigned short int amount; long int deficit; unsigned long int insects; Each of the data types in Table 2-6, except int, can be abbreviated as follows: • short int can be abbreviated as short • unsigned short int can be abbreviated as unsigned short • unsigned int can be abbreviated as unsigned • long int can be abbreviated as long • unsigned long int can be abbreviated as unsigned long • long long int can be abbreviated as long long • unsigned long long int can be abbreviated as unsigned long long Because they simplify definition statements, programmers commonly use the abbreviated data type names. Here are some examples: unsigned speed; short month; unsigned short amount; long deficit; unsigned long insects; long long grandTotal; unsigned long long lightYearDistance; Unsigned data types can only store nonnegative values. They can be used when you know your program will not encounter negative values. For example, variables that hold ages or weights would rarely hold numbers less than 0. Notice in Table 2-6 that the int and long data types have the same sizes and ranges, and that the unsigned int data type has the same size and range as the unsigned long data type. This is not always true because the size of integers is dependent on the type of system you are using. Here are the only guarantees: • Integers are at least as big as short integers. • Long integers are at least as big as integers. • Unsigned short integers are the same size as short integers. • Unsigned integers are the same size as integers. • Unsigned long integers are the same size as long integers. • The long long int and the unsigned long long int data types are guaranteed to be at least 8 bytes (64 bits) in size. Later in this chapter you will learn to use the sizeof operator to determine how large all the data types are on your computer. 11 NOTE: The long long int and the unsigned long long int data types were intro- duced in C++ 11. 2.6 Integer Data Types 45 As mentioned before, variables are defined by stating the data type key word followed by the name of the variable. In Program 2-10 an integer, an unsigned integer, and a long integer have been defined. Program 2-10 1 // This program has variables of several of the integer types. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int checking; 8 unsigned int miles; 9 long days; 10 11 checking = −20; 12 miles = 4276; 13 days = 189000; 14 cout << "We have made a long journey of " << miles; 15 cout << " miles.\n"; 16 cout << "Our checking account balance is " << checking; 17 cout << "\nAbout " << days << " days ago Columbus "; 18 cout << "stood on this spot.\n"; 19 return 0; 20 } Program Output We have made a long journey of 4276 miles. Our checking account balance is −20 About 189000 days ago Columbus stood on this spot. In most programs you will need more than one variable of any given data type. If a program uses two integers, length and width, they could be defined separately, like this: int length; int width; It is easier, however, to combine both variable definitions on one line: int length, width; You can define several variables of the same type like this, simply separating their names with commas. Program 2-11 illustrates this. Program 2-11 1 // This program shows three variables defined on the same line. 2 #include 3 using namespace std; 4 5 int main() (program continues) 46 Chapter 2 Introduction to C++ Program 2-11 (continued) 6{ 7 8 9 10 11 12 13 14 15 16 } int floors, rooms, suites; floors = 15; rooms = 300; suites = 30; cout << "The Grande Hotel has " << floors << " floors\n"; cout << "with " << rooms << " rooms and " << suites; cout << " suites.\n"; return 0; Program Output The Grande Hotel has 15 floors with 300 rooms and 30 suites. Integer and Long Integer Literals In C++, if a numeric literal is an integer (not written with a decimal point) and it fits within the range of an int (see Table 2-6 for the minimum and maximum values), then the numeric literal is treated as an int. A numeric literal that is treated as an int is called an integer literal. For example, look at lines 9, 10, and 11 in Program 2-11: floors = 15; rooms = 300; suites = 30; Each of these statements assigns an integer literal to a variable. One of the pleasing characteristics of the C++ language is that it allows you to control almost every aspect of your program. If you need to change the way something is stored in memory, the tools are provided to do that. For example, what if you are in a situation where you have an integer literal, but you need it to be stored in memory as a long integer? (Rest assured, this is a situation that does arise.) C++ allows you to force an integer literal to be stored as a long integer by placing the letter L at the end of the number. Here is an example: long amount; amount = 32L; The first statement defines a long variable named amount. The second statement assigns the literal value 32 to the amount variable. In the second statement, the literal is written as 32L, which makes it a long integer literal. This means the literal is treated as a long. 11 If you want an integer literal to be treated as a long long int, you can append LL at the end of the number. Here is an example: long long amount; amount = 32LL; The first statement defines a long long variable named amount. The second statement assigns the literal value 32 to the amount variable. In the second statement, the literal is written as 32LL, which makes it a long long integer literal. This means the literal is treated as a long long int. 2.6 Integer Data Types 47 TIP: When writing long integer literals or long long integer literals, you can use either an uppercase or lowercase L. Because the lowercase l looks like the number 1, you should always use the uppercase L. If You Plan to Continue in Computer Science: Hexadecimal and Octal Literals Programmers commonly express values in numbering systems other than decimal (or base 10). Hexadecimal (base 16) and octal (base 8) are popular because they make certain programming tasks more convenient than decimal numbers do. By default, C++ assumes that all integer literals are expressed in decimal. You express hexadecimal numbers by placing 0x in front of them. (This is zero-x, not oh-x.) Here is how the hexadecimal number F4 would be expressed in C++: 0xF4 Octal numbers must be preceded by a 0 (zero, not oh). For example, the octal 31 would be written 031 NOTE: You will not be writing programs for some time that require this type of manipulation. It is important, however, that you understand this material. Good programmers should develop the skills for reading other people’s source code. You may find yourself reading programs that use items like long integer, hexadecimal, or octal literals. Checkpoint 2.7 Which of the following are illegal variable names, and why? X 99bottles july97 theSalesFigureForFiscalYear98 r&d grade_report 2.8 Is the variable name Sales the same as sales? Why or why not? 2.9 Refer to the data types listed in Table 2-6 for these questions. A) If a variable needs to hold numbers in the range 32 to 6,000, what data type would be best? B) If a variable needs to hold numbers in the range Ϫ40,000 to ϩ40,000, what data type would be best? C) Which of the following literals uses more memory? 20 or 20L 2.10 On any computer, which data type uses more memory, an integer or an unsigned integer? 48 Chapter 2 Introduction to C++ 2.7 The char Data Type The char data type is used to store individual characters. A variable of the char data type can hold only one character at a time. Here is an example of how you might declare a char variable: char letter; This statement declares a char variable named letter, which can store one character. In C++, character literals are enclosed in single quotation marks. Here is an example showing how we would assign a character to the letter variable: letter = 'g'; This statement assigns the character 'g' to the letter variable. Because char variables can hold only one character, they are not compatible with strings. For example, you cannot assign a string to a char variable, even if the string contains only one character. The following statement, for example, will not compile because it attempts to assign a string literal to a char variable. letter = "g"; // ERROR! Cannot assign a string to a char It is important that you do not confuse character literals, which are enclosed in single quotation marks, with string literals, which are enclosed in double quotation marks. Program 2-12 shows an example program that works with characters. Program 2-12 1 // This program works with characters. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 char letter; 8 9 letter = 'A'; 10 cout << letter << endl; 11 letter = 'B'; 12 cout << letter << endl; 13 return 0; 14 } Program Output A B Although the char data type is used for storing characters, it is actually an integer data type that typically uses 1 byte of memory. (The size is system dependent. On some systems, the char data type is larger than 1 byte.) 2.7 The char Data Type 49 The reason an integer data type is used to store characters is because characters are internally represented by numbers. Each printable character, as well as many nonprintable characters, is assigned a unique number. The most commonly used method for encoding characters is ASCII, which stands for the American Standard Code for Information Interchange. (There are other codes, such as EBCDIC, which is used by many IBM mainframes.) When a character is stored in memory, it is actually the numeric code that is stored. When the computer is instructed to print the value on the screen, it displays the character that corresponds with the numeric code. You may want to refer to Appendix B, which shows the ASCII character set. Notice that the number 65 is the code for A, 66 is the code for B, and so on. Program 2-13 demonstrates that when you work with characters, you are actually working with numbers. Program 2-13 1 // This program demonstrates the close relationship between 2 // characters and integers. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char letter; 9 10 letter = 65; 11 cout << letter << endl; 12 letter = 66; 13 cout << letter << endl; 14 return 0; 15 } Program Output A B Figure 2-4 illustrates that when characters, such as A, B, and C, are stored in memory, it is really the numbers 65, 66, and 67 that are stored. Figure 2-4 A B C is stored in memory as 65 66 67 50 Chapter 2 Introduction to C++ The Difference Between String Literals and Character Literals It is important that you do not confuse character literals with string literals. Strings, which are a series of characters stored in consecutive memory locations, can be virtually any length. This means that there must be some way for the program to know how long a string is. In C++ an extra byte is appended to the end of string literals when they are stored in memory. In this last byte, the number 0 is stored. It is called the null terminator or null character, and it marks the end of the string. Don’t confuse the null terminator with the character '0'. If you look at Appendix B, you will see that ASCII code 48 corresponds to the character '0', whereas the null terminator is the same as the ASCII code 0. If you want to print the character 0 on the screen, you use ASCII code 48. If you want to mark the end of a string, however, you use ASCII code 0. Let’s look at an example of how a string literal is stored in memory. Figure 2-5 depicts the way the string literal "Sebastian" would be stored. Figure 2-5 S e b a s t i a n \0 First, notice the quotation marks are not stored with the string. They are simply a way of marking the beginning and end of the string in your source code. Second, notice the very last byte of the string. It contains the null terminator, which is represented by the \0 character. The addition of this last byte means that although the string "Sebastian" is 9 characters long, it occupies 10 bytes of memory. The null terminator is another example of something that sits quietly in the background. It doesn’t print on the screen when you display a string, but nevertheless, it is there silently doing its job. N OTE: C++ automatically places the null terminator at the end of string literals. Now let’s compare the way a string and a char are stored. Suppose you have the literals 'A' and "A" in a program. Figure 2-6 depicts their internal storage. Figure 2-6 ‘A’ is stored as A “A” is stored as A \0 As you can see, 'A' is a 1-byte element and "A" is a 2-byte element. Since characters are really stored as ASCII codes, Figure 2-7 shows what is actually being stored in memory. Figure 2-7 ‘A’ is stored as 65 2.7 The char Data Type 51 “A” is stored as 65 0 Because char variables are only large enough to hold one character, you cannot assign string literals to them. For example, the following code defines a char variable named letter. The character literal 'A' can be assigned to the variable, but the string literal "A" cannot. char letter; letter = 'A'; // This will work. letter = "A"; // This will not work! One final topic about characters should be discussed. You have learned that some strings look like a single character but really aren’t. It is also possible to have a character that looks like a string. A good example is the newline character, \n. Although it is represented by two characters, a slash and an n, it is internally represented as one character. In fact, all escape sequences, internally, are just 1 byte. Program 2-14 shows the use of \n as a character literal, enclosed in single quotation marks. If you refer to the ASCII chart in Appendix B, you will see that ASCII code 10 is the linefeed character. This is the code C++ uses for the newline character. Program 2-14 1 // This program uses character literals. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 char letter; 8 9 letter = 'A'; 10 cout << letter << '\n'; 11 letter = 'B'; 12 cout << letter << '\n'; 13 return 0; 14 } Program Output A B Let’s review some important points regarding characters and strings: • Printable characters are internally represented by numeric codes. Most computers use ASCII codes for this purpose. • Characters normally occupy a single byte of memory. 52 Chapter 2 Introduction to C++ • Strings are consecutive sequences of characters that occupy consecutive bytes of memory. • String literals are always stored in memory with a null terminator at the end. This marks the end of the string. • Character literals are enclosed in single quotation marks. • String literals are enclosed in double quotation marks. • Escape sequences such as '\n' are stored internally as a single character. 2.8 The C++ string Class CONCEPT: Standard C++ provides a special data type for storing and working with strings. Because a char variable can store only one character in its memory location, another data type is needed for a variable able to hold an entire string. Although C++ does not have a built-in data type able to do this, standard C++ provides something called the string class that allows the programmer to create a string type variable. Using the string Class The first step in using the string class is to #include the string header file. This is accomplished with the following preprocessor directive: #include The next step is to define a string type variable, called a string object. Defining a string object is similar to defining a variable of a primitive type. For example, the following statement defines a string object named movieTitle. string movieTitle; You can assign a string literal to movieTitle with the assignment operator: movieTitle = "Wheels of Fury"; You can use cout to display the value of the movieTitle object, as shown in the next statement: cout << "My favorite movie is " << movieTitle << endl; Program 2-15 is a complete program that demonstrates the preceding statements. Program 2-15 1 // This program demonstrates the string class. 2 #include 3 #include // Required for the string class. 4 using namespace std; 2.8 The C++ string Class 53 5 6 int main() 7{ 8 string movieTitle; 9 10 movieTitle = "Wheels of Fury"; 11 cout << "My favorite movie is " << movieTitle << endl; 12 return 0; 13 } Program Output My favorite movie is Wheels of Fury As you can see, working with string objects is similar to working with variables of other types. Throughout this text we will continue to discuss string class features and capabilities. Checkpoint 2.11 What are the ASCII codes for the following characters? (Refer to Appendix B) C F W 2.12 Which of the following is a character literal? 'B' "B" 2.13 Assuming the char data type uses 1 byte of memory, how many bytes do the following literals use? 'Q' "Q" "Sales" '\n' 2.14 2.15 Write a program that has the following character variables: first, middle, and last. Store your initials in these variables and then display them on the screen. What is wrong with the following program statement? char letter = "Z"; 2.16 What header file must you include in order to use string objects? 2.17 Write a program that stores your name, address, and phone number in three separate string objects. Display the contents of the string objects on the screen. 54 Chapter 2 Introduction to C++ 2.9 Floating-Point Data Types CONCEPT: Floating-point data types are used to define variables that can hold real numbers. Whole numbers are not adequate for many jobs. If you are writing a program that works with dollar amounts or precise measurements, you need a data type that allows fractional values. In programming terms, these are called floating-point numbers. Internally, floating-point numbers are stored in a manner similar to scientific notation. Take the number 47,281.97. In scientific notation this number is 4.728197 * 104. (104 is equal to 10,000, and 4.728197 * 10,000 is 47,281.97.) The first part of the number, 4.728197, is called the mantissa. The mantissa is multiplied by a power of ten. Computers typically use E notation to represent floating-point values. In E notation, the number 47,281.97 would be 4.728197E4. The part of the number before the E is the mantissa, and the part after the E is the power of 10. When a floating point number is stored in memory, it is stored as the mantissa and the power of 10. Table 2-7 shows other numbers represented in scientific and E notation. Table 2-7 Floating Point Representations Decimal Notation 247.91 0.00072 2,900,000 Scientific Notation 2.4791 * 102 7.2 * 10−4 2.9 * 106 E Notation 2.4791E2 7.2E–4 2.9E6 In C++ there are three data types that can represent floating-point numbers. They are float double long double The float data type is considered single precision. The double data type is usually twice as big as float, so it is considered double precision. As you’ve probably guessed, the long double is intended to be larger than the double. Of course, the exact sizes of these data types are dependent on the computer you are using. The only guarantees are • A double is at least as big as a float. • A long double is at least as big as a double. Table 2-8 shows the sizes and ranges of floating-point data types usually found on PCs. Table 2-8 Floating Point Data Types on PCs Data Type Key Word Description Single precision Double precision Long double precision float double long double 4 bytes. Numbers between ±3.4E-38 and ±3.4E38 8 bytes. Numbers between ±1.7E-308 and ±1.7E308 8 bytes*. Numbers between ±1.7E-308 and ±1.7E308 *Some compilers use 10 bytes for long doubles. This allows a range of ±3.4E-4932 to ±1.1E4832 2.9 Floating-Point Data Types 55 You will notice there are no unsigned floating point data types. On all machines, variables of the float, double, and long double data types can store positive or negative numbers. Floating Point Literals Floating point literals may be expressed in a variety of ways. As shown in Program 2-16, E notation is one method. When you are writing numbers that are extremely large or extremely small, this will probably be the easiest way. E notation numbers may be expressed with an uppercase E or a lowercase e. Notice that in the source code the literals were written as 1.495979E11 and 1.989E30, but the program printed them as 1.49598e+ 011 and 1.989e+30. The two sets of numbers are equivalent. (The plus sign in front of the exponent is also optional.) In Chapter 3 you will learn to control the way cout displays E notation numbers. Program 2-16 1 // This program uses floating point data types. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 float distance; 8 double mass; 9 10 distance = 1.495979E11; 11 mass = 1.989E30; 12 cout << "The Sun is " << distance << " meters away.\n"; 13 cout << "The Sun\'s mass is " << mass << " kilograms.\n"; 14 return 0; 15 } Program Output The Sun is 1.49598e+011 meters away. The Sun's mass is 1.989e+030 kilograms. You can also express floating-point literals in decimal notation. The literal 1.495979E11 could have been written as 149597900000.00 Obviously the E notation is more convenient for lengthy numbers, but for numbers like 47.39, decimal notation is preferable to 4.739E1. All of the following floating-point literals are equivalent: 1.4959E11 1.4959e11 1.4959E+11 1.4959e+11 149590000000.00 56 Chapter 2 Introduction to C++ Floating-point literals are normally stored in memory as doubles. But remember, C++ provides tools for handling just about any situation. Just in case you need to force a literal to be stored as a float, you can append the letter F or f to the end of it. For example, the following literals would be stored as floats: 1.2F 45.907f N O T E : Because floating-point literals are normally stored in memory as doubles, many compilers issue a warning message when you assign a floating-point literal to a float variable. For example, assuming num is a float, the following statement might cause the compiler to generate a warning message: num = 14.725; You can suppress the warning message by appending the f suffix to the floating-point literal, as shown below: num = 14.725f; If you want to force a value to be stored as a long double, append an L or l to it, as in the following examples: 1034.56L 89.2l The compiler won’t confuse these with long integers because they have decimal points. (Remember, the lowercase L looks so much like the number 1 that you should always use the uppercase L when suffixing literals.) Assigning Floating-Point Values to Integer Variables When a floating-point value is assigned to an integer variable, the fractional part of the value (the part after the decimal point) is discarded. For example, look at the following code. int number; number = 7.5; // Assigns 7 to number This code attempts to assign the floating-point value 7.5 to the integer variable number. As a result, the value 7 will be assigned to number, with the fractional part discarded. When part of a value is discarded, it is said to be truncated. Assigning a floating-point variable to an integer variable has the same effect. For example, look at the following code. int i; float f; f = 7.5; i = f; // Assigns 7 to i. 2.10 The bool Data Type 57 When the float variable f is assigned to the int variable i, the value being assigned (7.5) is truncated. After this code executes i will hold the value 7 and f will hold the value 7.5. N OTE: When a floating-point value is truncated, it is not rounded. Assigning the value 7.9 to an int variable will result in the value 7 being stored in the variable. WARN IN G ! Floating-point variables can hold a much larger range of values than integer variables can. If a floating-point value is being stored in an integer variable, and the whole part of the value (the part before the decimal point) is too large for the integer variable, an invalid value will be stored in the integer variable. 2.10 The bool Data Type CONCEPT: Boolean variables are set to either true or false. Expressions that have a true or false value are called Boolean expressions, named in honor of English mathematician George Boole (1815–1864). The bool data type allows you to create small integer variables that are suitable for holding true or false values. Program 2-17 demonstrates the definition and assignment of a bool variable. Program 2-17 1 // This program demonstrates boolean variables. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 bool boolValue; 8 9 boolValue = true; 10 cout << boolValue << endl; 11 boolValue = false; 12 cout << boolValue << endl; 13 return 0; 14 } Program Output 1 0 As you can see from the program output, the value true is represented in memory by the number 1, and false is represented by 0. You will not be using bool variables until Chapter 4, however, so just remember they are useful for evaluating conditions that are either true or false. 58 Chapter 2 Introduction to C++ 2.11 Determining the Size of a Data Type CONCEPT: The sizeof operator may be used to determine the size of a data type on any system. Chapter 1 discussed the portability of the C++ language. As you have seen in this chapter, one of the problems of portability is the lack of common sizes of data types on all machines. If you are not sure what the sizes of data types are on your computer, C++ provides a way to find out. A special operator called sizeof will report the number of bytes of memory used by any data type or variable. Program 2-18 illustrates its use. The first line that uses the operator is line 10: cout << "The size of an integer is " << sizeof(int); The name of the data type or variable is placed inside the parentheses that follow the operator. The operator “returns” the number of bytes used by that item. This operator can be invoked anywhere you can use an unsigned integer, including in mathematical operations. Program 2-18 1 // This program determines the size of integers, long 2 // integers, and long doubles. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 long double apple; 9 10 cout << "The size of an integer is " << sizeof(int); 11 cout << " bytes.\n"; 12 cout << "The size of a long integer is " << sizeof(long); 13 cout << " bytes.\n"; 14 cout << "An apple can be eaten in " << sizeof(apple); 15 cout << " bytes!\n"; 16 return 0; 17 } Program Output The size of an integer is 4 bytes. The size of a long integer is 4 bytes. An apple can be eaten in 8 bytes! 2.12 Variable Assignments and Initialization 59 Checkpoint 2.18 Yes or No: Is there an unsigned floating point data type? If so, what is it? 2.19 How would the following number in scientific notation be represented in E notation? 6.31 ϫ 1017 2.20 Write a program that defines an integer variable named age and a float variable named weight. Store your age and weight, as literals, in the variables. The program should display these values on the screen in a manner similar to the following: Program Output My age is 26 and my weight is 180 pounds. (Feel free to lie to the computer about your age and your weight— it’ll never know!) 2.12 Variable Assignments and Initialization CONCEPT: An assignment operation assigns, or copies, a value into a variable. When a value is assigned to a variable as part of the variable’s definition, it is called an initialization. As you have already seen in several examples, a value is stored in a variable with an assignment statement. For example, the following statement copies the value 12 into the variable unitsSold. unitsSold = 12; The = symbol is called the assignment operator. Operators perform operations on data. The data that operators work with are called operands. The assignment operator has two operands. In the previous statement, the operands are unitsSold and 12. In an assignment statement, C++ requires the name of the variable receiving the assignment to appear on the left side of the operator. The following statement is incorrect. 12 = unitsSold; // Incorrect! In C++ terminology, the operand on the left side of the = symbol must be an lvalue. It is called an lvalue because it is a value that may appear on the left side of an assignment operator. An lvalue is something that identifies a place in memory whose contents may be changed. Most of the time this will be a variable name. The operand on the right side of the = symbol must be an rvalue. An rvalue is any expression that has a value. The assignment statement takes the value of the rvalue and puts it in the memory location of the object identified by the lvalue. You may also assign values to variables as part of the definition. This is called initialization. Program 2-19 shows how it is done. 60 Chapter 2 Introduction to C++ Program 2-19 1 // This program shows variable initialization. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int month = 2, days = 28; 8 9 cout << "Month " << month << " has " << days << " days.\n"; 10 return 0; 11 } Program Output Month 2 has 28 days. As you can see, this simplifies the program and reduces the number of statements that must be typed by the programmer. Here are examples of other definition statements that perform initialization. double interestRate = 12.9; char stockCode = 'D'; long customerNum = 459L; Of course, there are always variations on a theme. C++ allows you to define several variables and only initialize some of them. Here is an example of such a definition: int flightNum = 89, travelTime, departure = 10, distance; The variable flightNum is initialized to 89 and departure is initialized to 10. The variables travelTime and distance remain uninitialized. 11 Declaring Variables With the auto Key Word C++ 11 introduces an alternative way to define variables, using the auto key word and an initialization value. Here is an example: auto amount = 100; Notice that this statement uses the word auto instead of a data type. The auto key word tells the compiler to determine the variable’s data type from the initialization value. In this example the initialization value, 100, is an int, so amount will be an int variable. Here are other examples: auto interestRate = 12.0; auto stockCode = 'D'; auto customerNum = 459L; In this code, the interestRate variable will be a double because the initialization value, 12.0, is a double. The stockCode variable will be a char because the initialization value, 'D', is a char. The customerNum variable will be a long because the initialization value, 459L, is a long. 2.14 Arithmetic Operators 61 These examples show how to use the auto key word, but they don’t really show its usefulness. The auto key word is intended to simplify the syntax of declarations that are more complex than the ones shown here. Later in the book, you will see examples of how the auto key word can improve the readability of complex definition statements. 2.13 Scope CONCEPT: A variable’s scope is the part of the program that has access to the variable. Every variable has a scope. The scope of a variable is the part of the program where the variable may be used. The rules that define a variable’s scope are complex, and you will only be introduced to the concept here. In other sections of the book we will revisit this topic and expand on it. The first rule of scope you should learn is that a variable cannot be used in any part of the program before the definition. Program 2-20 illustrates this. Program 2-20 1 // This program can't find its variable. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 cout << value; // ERROR! value not defined yet! 8 9 int value = 100; 10 return 0; 11 } The program will not work because line 7 attempts to send the contents of the variable value to cout before the variable is defined. The compiler reads your program from top to bottom. If it encounters a statement that uses a variable before the variable is defined, an error will result. To correct the program, the variable definition must be put before any statement that uses it. 2.14 Arithmetic Operators CONCEPT: There are many operators for manipulating numeric values and performing arithmetic operations. C++ offers a multitude of operators for manipulating data. Generally, there are three types of operators: unary, binary, and ternary. These terms reflect the number of operands an operator requires. 62 Chapter 2 Introduction to C++ VideoNote Assignment Statements and Simple Math Expressions Unary operators only require a single operand. For example, consider the following expression: −5 Of course, we understand this represents the value negative five. The literal 5 is preceded by the minus sign. The minus sign, when used this way, is called the negation operator. Since it only requires one operand, it is a unary operator. Binary operators work with two operands. The assignment operator is in this category. Ternary operators, as you may have guessed, require three operands. C++ only has one ternary operator, which will be discussed in Chapter 4. Arithmetic operations are very common in programming. Table 2-9 shows the common arithmetic operators in C++. Table 2-9 Fundamental Arithmetic Operators Operator Meaning Type + Addition Binary − Subtraction Binary * Multiplication Binary / Division Binary % Modulus Binary Example total = cost + tax; cost = total − tax; tax = cost * rate; salePrice = original / 2; remainder = value % 3; Each of these operators works as you probably expect. The addition operator returns the sum of its two operands. In the following assignment statement, the variable amount will be assigned the value 12: amount = 4 + 8; The subtraction operator returns the value of its right operand subtracted from its left operand. This statement will assign the value 98 to temperature: temperature = 112 − 14; The multiplication operator returns the product of its two operands. In the following statement, markUp is assigned the value 3: markUp = 12 * 0.25; The division operator returns the quotient of its left operand divided by its right operand. In the next statement, points is assigned the value 5: points = 100 / 20; It is important to note that when both of the division operator’s operands are integers, the result of the division will also be an integer. If the result has a fractional part, it will be thrown away. We will discuss this behavior, which is known as integer division, in greater detail later in this section. The modulus operator, which only works with integer operands, returns the remainder of an integer division. The following statement assigns 2 to leftOver: leftOver = 17 % 3; 2.14 Arithmetic Operators 63 In Chapter 3 you will learn how to use these operators in more complex mathematical formulas. For now we will concentrate on their basic usage. For example, suppose we need to write a program that calculates and displays an employee’s total wages for the week. The regular hours for the work week are 40, and any hours worked over 40 are considered overtime. The employee earns $18.25 per hour for regular hours and $27.78 per hour for overtime hours. The employee has worked 50 hours this week. The following pseudocode algorithm shows the program’s logic. Regular wages = base pay rate × regular hours Overtime wages = overtime pay rate × overtime hours Total wages = regular wages + overtime wages Display the total wages Program 2-21 shows the C++ code for the program. Program 2-21 1 // This program calculates hourly wages, including overtime. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 double regularWages, // To hold regular wages 8 basePayRate = 18.25, // Base pay rate 9 regularHours = 40.0, // Hours worked less overtime 10 overtimeWages, // To hold overtime wages 11 overtimePayRate = 27.78, // Overtime pay rate 12 overtimeHours = 10, // Overtime hours worked 13 totalWages; // To hold total wages 14 15 // Calculate the regular wages. 16 regularWages = basePayRate * regularHours; 17 18 // Calculate the overtime wages. 19 overtimeWages = overtimePayRate * overtimeHours; 20 21 // Calculate the total wages. 22 totalWages = regularWages + overtimeWages; 23 24 // Display the total wages. 25 cout << "Wages for this week are $" << totalWages << endl; 26 return 0; 27 } Program Output Wages for this week are $1007.8 Let’s take a closer look at the program. As mentioned in the comments, there are variables for regular wages, base pay rate, regular hours worked, overtime wages, overtime pay rate, overtime hours worked, and total wages. 64 Chapter 2 Introduction to C++ Here is line 16, which multiplies basePayRate times regularHours and stores the result in regularWages: regularWages = basePayRate * regularHours; Here is line 19, which multiplies overtimePayRate times overtimeHours and stores the result in overtimeWages: overtimeWages = overtimePayRate * overtimeHours; Line 22 adds the regular wages and the overtime wages and stores the result in totalWages: totalWages = regularWages + overtimeWages; Line 25 displays the message on the screen reporting the week’s wages. Integer Division When both operands of a division statement are integers, the statement will result in integer division. This means the result of the division will be an integer as well. If there is a remainder, it will be discarded. For example, look at the following code: double number; number = 5 / 2; This code divides 5 by 2 and assigns the result to the number variable. What will be stored in number? You would probably assume that 2.5 would be stored in number because that is the result your calculator shows when you divide 5 by 2. However, that is not what happens when the previous C++ code is executed. Because the numbers 5 and 2 are both integers, the fractional part of the result will be thrown away, or truncated. As a result, the value 2 will be assigned to the number variable. In the previous code, it doesn’t matter that the number variable is declared as a double because the fractional part of the result is discarded before the assignment takes place. In order for a division operation to return a floating-point value, one of the operands must be of a floating-point data type. For example, the previous code could be written as follows: double number; number = 5.0 / 2; In this code, 5.0 is treated as a floating-point number, so the division operation will return a floating-point number. The result of the division is 2.5. In the Spotlight: Calculating Percentages and Discounts Determining percentages is a common calculation in computer programming. Although the % symbol is used in general mathematics to indicate a percentage, most programming languages (including C++) do not use the % symbol for this purpose. In a program, you have to convert a percentage to a floating-point number, just as you would if you were using a calculator. For example, 50 percent would be written as 0.5 and 2 percent would be written as 0.02. 2.14 Arithmetic Operators 65 Let’s look at an example. Suppose you earn $6,000 per month and you are allowed to contribute a portion of your gross monthly pay to a retirement plan. You want to determine the amount of your pay that will go into the plan if you contribute 5 percent, 7 percent, or 10 percent of your gross wages. To make this determination you write the program shown in Program 2-22. Program 2-22 1 // This program calculates the amount of pay that 2 // will be contributed to a retirement plan if 5%, 3 // 7%, or 10% of monthly pay is withheld. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 // Variables to hold the monthly pay and the 10 // amount of contribution. 11 double monthlyPay = 6000.0, contribution; 12 13 // Calculate and display a 5% contribution. 14 contribution = monthlyPay * 0.05; 15 cout << "5 percent is $" << contribution 16 << " per month.\n"; 17 18 // Calculate and display a 7% contribution. 19 contribution = monthlyPay * 0.07; 20 cout << "7 percent is $" << contribution 21 << " per month.\n"; 22 23 // Calculate and display a 10% contribution. 24 contribution = monthlyPay * 0.1; 25 cout << "10 percent is $" << contribution 26 << " per month.\n"; 27 28 return 0; 29 } Program Output 5 percent is $300 per month. 7 percent is $420 per month. 10 percent is $600 per month. Line 11 defines two variables: monthlyPay and contribution. The monthlyPay variable, which is initialized with the value 6000.0, holds the amount of your monthly pay. The contribution variable will hold the amount of a contribution to the retirement plan. The statements in lines 14 through 16 calculate and display 5 percent of the monthly pay. The calculation is done in line 14, where the monthlyPay variable is multiplied by 0.05. The result is assigned to the contribution variable, which is then displayed in line 15. Similar steps are taken in Lines 18 through 21, which calculate and display 7 percent of the monthly pay, and lines 24 through 26, which calculate and display 10 percent of the monthly pay. 66 Chapter 2 Introduction to C++ Calculating a Percentage Discount Another common calculation is determining a percentage discount. For example, suppose a retail business sells an item that is regularly priced at $59.95 and is planning to have a sale where the item’s price will be reduced by 20 percent. You have been asked to write a program to calculate the sale price of the item. To determine the sale price you perform two calculations: • First, you get the amount of the discount, which is 20 percent of the item’s regular price. • Second, you subtract the discount amount from the item’s regular price. This gives you the sale price. Program 2-23 shows how this is done in C++. Program 2-23 1 // This program calculates the sale price of an item 2 // that is regularly priced at $59.95, with a 20 percent 3 // discount subtracted. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 // Variables to hold the regular price, the 10 // amount of a discount, and the sale price. 11 double regularPrice = 59.95, discount, salePrice; 12 13 // Calculate the amount of a 20% discount. 14 discount = regularPrice * 0.2; 15 16 // Calculate the sale price by subtracting the 17 // discount from the regular price. 18 salePrice = regularPrice - discount; 19 20 // Display the results. 21 cout << "Regular price: $" << regularPrice << endl; 22 cout << "Discount amount: $" << discount << endl; 23 cout << "Sale price: $" << salePrice << endl; 24 return 0; 25 } Program Output Regular price: $59.95 Discount amount: $11.99 Sale price: $47.96 Line 11 defines three variables. The regularPrice variable holds the item’s regular price, and is initialized with the value 59.95. The discount variable will hold the amount of the discount once it is calculated. The salePrice variable will hold the item’s sale price. 2.14 Arithmetic Operators 67 Line 14 calculates the amount of the 20 percent discount by multiplying regularPrice by 0.2. The result is stored in the discount variable. Line 18 calculates the sale price by subtracting discount from regularPrice. The result is stored in the salePrice variable. The cout statements in lines 21 through 23 display the item’s regular price, the amount of the discount, and the sale price. In the Spotlight: Using the Modulus Operator and Integer Division The modulus operator (%) is surprisingly useful. For example, suppose you need to extract the rightmost digit of a number. If you divide the number by 10, the remainder will be the rightmost digit. For instance, 123 ÷ 10 = 12 with a remainder of 3. In a computer program you would use the modulus operator to perform this operation. Recall that the modulus operator divides an integer by another integer, and gives the remainder. This is demonstrated in Program 2-24. The program extracts the rightmost digit of the number 12345. Program 2-24 1 // This program extracts the rightmost digit of a number. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int number = 12345; 8 int rightMost = number % 10; 9 10 cout << "The rightmost digit in " 11 << number << " is " 12 << rightMost << endl; 13 14 return 0; 15 } Program Output The rightmost digit in 12345 is 5 Interestingly, the expression number % 100 will give you the rightmost two digits in number, the expression number % 1000 will give you the rightmost three digits in number, etc. The modulus operator (%) is useful in many other situations. For example, Program 2-25 converts 125 seconds to an equivalent number of minutes, and seconds. 68 Chapter 2 Introduction to C++ Program 2-25 1 // This program converts seconds to minutes and seconds. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // The total seconds is 125. 8 int totalSeconds = 125; 9 10 // Variables for the minutes and seconds 11 int minutes, seconds; 12 13 // Get the number of minutes. 14 minutes = totalSeconds / 60; 15 16 // Get the remaining seconds. 17 seconds = totalSeconds % 60; 18 19 // Display the results. 20 cout << totalSeconds << " seconds is equivalent to:\n"; 21 cout << "Minutes: " << minutes << endl; 22 cout << "Seconds: " << seconds << endl; 23 return 0; 24 } Program Output 125 seconds is equivalent to: Minutes: 2 Seconds: 5 Let’s take a closer look at the code: • Line 8 defines an int variable named totalSeconds, initialized with the value 125. • Line 11 declares the int variables minutes and seconds. • Line 14 calculates the number of minutes in the specified number of seconds. There are 60 seconds in a minute, so this statement divides totalSeconds by 60. Notice that we are performing integer division in this statement. Both totalSeconds and the numeric literal 60 are integers, so the division operator will return an integer result. This is intentional because we want the number of minutes with no fractional part. • Line 17 calculates the number of remaining seconds. There are 60 seconds in a minute, so this statement uses the % operator to divide the totalSeconds by 60, and get the remainder of the division. The result is the number of remaining seconds. • Lines 20 through 22 display the number of minutes and seconds. 2.15 Comments 69 Checkpoint 2.21 Is the following assignment statement valid or invalid? If it is invalid, why? 72 = amount; 2.22 How would you consolidate the following definitions into one statement? int x = 7; int y = 16; int z = 28; 2.23 What is wrong with the following program? How would you correct it? #include using namespace std; 2.24 int main() { number = 62.7; double number; cout << number << endl; return 0; } Is the following an example of integer division or floating-point division? What value will be stored in portion? portion = 70 / 3; 2.15 Comments CONCEPT: Comments are notes of explanation that document lines or sections of a program. Comments are part of the program, but the compiler ignores them. They are intended for people who may be reading the source code. It may surprise you that one of the most important parts of a program has absolutely no impact on the way it runs. In fact, the compiler ignores this part of a program. Of course, I’m speaking of the comments. As a beginning programmer, you might be resistant to the idea of liberally writing comments in your programs. After all, it can seem more productive to write code that actually does something! It is crucial, however, that you develop the habit of thoroughly annotating your code with descriptive comments. It might take extra time now, but it will almost certainly save time in the future. Imagine writing a program of medium complexity with about 8,000 to 10,000 lines of C++ code. Once you have written the code and satisfactorily debugged it, you happily put it away and move on to the next project. Ten months later you are asked to make a modification to the program (or worse, track down and fix an elusive bug). You open the file that contains your source code and stare at thousands of statements that now make no sense at all. If only you had left some notes to yourself explaining the program’s code. Of course it’s too late now. All that’s left to do is decide what will take less time: figuring out the old program or completely rewriting it! 70 Chapter 2 Introduction to C++ This scenario might sound extreme, but it’s one you don’t want to happen to you. Realworld programs are big and complex. Thoroughly documented code will make your life easier, not to mention the other programmers who may have to read your code in the future. Single-Line Comments You have already seen one way to place comments in a C++ program. You simply place two forward slashes (//) where you want the comment to begin. The compiler ignores everything from that point to the end of the line. Program 2-26 shows that comments may be placed liberally throughout a program. Program 2-26 1 // PROGRAM: PAYROLL.CPP 2 // Written by Herbert Dorfmann 3 // This program calculates company payroll 4 // Last modification: 8/20/2014 5 #include 6 using namespace std; 7 8 int main() 9{ 10 double payRate; // Holds the hourly pay rate 11 double hours; // Holds the hours worked 12 int employNumber; // Holds the employee number (The remainder of this program is left out.) In addition to telling who wrote the program and describing the purpose of variables, comments can also be used to explain complex procedures in your code. Multi-Line Comments The second type of comment in C++ is the multi-line comment. Multi-line comments start with /* (a forward slash followed by an asterisk) and end with */ (an asterisk followed by a forward slash). Everything between these markers is ignored. Program 2-27 illustrates how multi-line comments may be used. Notice that a multi-line comment starts in line 1 with the /* symbol, and it ends in line 6 with the */ symbol. Program 2-27 1 /* 2 PROGRAM: PAYROLL.CPP 3 Written by Herbert Dorfmann 4 This program calculates company payroll 5 Last modification: 8/20/2014 6 */ 7 8 #include 2.16 Named Constants 71 9 using namespace std; 10 11 int main() 12 { 13 double payRate; // Holds the hourly pay rate 14 double hours; // Holds the hours worked 15 int employNumber; // Holds the employee number (The remainder of this program is left out.) Unlike a comment started with //, a multi-line comment can span several lines. This makes it more convenient to write large blocks of comments because you do not have to mark every line. Consequently, the multi-line comment is inconvenient for writing single-line comments because you must type both a beginning and ending comment symbol. N OTE: Many programmers use a combination of single-line comments and multi-line comments in their programs. Convenience usually dictates which style to use. Remember the following advice when using multi-line comments: • Be careful not to reverse the beginning symbol with the ending symbol. • Be sure not to forget the ending symbol. Both of these mistakes can be difficult to track down and will prevent the program from compiling correctly. 2.16 Named Constants CONCEPT: Literals may be given names that symbolically represent them in a program. Assume the following statement appears in a banking program that calculates data pertaining to loans: amount = balance * 0.069; In such a program, two potential problems arise. First, it is not clear to anyone other than the original programmer what 0.069 is. It appears to be an interest rate, but in some situations there are fees associated with loan payments. How can the purpose of this statement be determined without painstakingly checking the rest of the program? The second problem occurs if this number is used in other calculations throughout the program and must be changed periodically. Assuming the number is an interest rate, what if the rate changes from 6.9 percent to 7.2 percent? The programmer will have to search through the source code for every occurrence of the number. Both of these problems can be addressed by using named constants. A named constant is like a variable, but its content is read-only and cannot be changed while the program is running. Here is a definition of a named constant: const double INTEREST_RATE = 0.069; 72 Chapter 2 Introduction to C++ It looks just like a regular variable definition except that the word const appears before the data type name, and the name of the variable is written in all uppercase characters. The key word const is a qualifier that tells the compiler to make the variable read-only. Its value will remain constant throughout the program’s execution. It is not required that the variable name be written in all uppercase characters, but many programmers prefer to write them this way so they are easily distinguishable from regular variable names. An initialization value must be given when defining a constant with the const qualifier, or an error will result when the program is compiled. A compiler error will also result if there are any statements in the program that attempt to change the value of a named constant. An advantage of using named constants is that they make programs more self-documenting. The following statement amount = balance * 0.069; can be changed to read amount = balance * INTEREST_RATE; A new programmer can read the second statement and know what is happening. It is evident that balance is being multiplied by the interest rate. Another advantage to this approach is that widespread changes can easily be made to the program. Let’s say the interest rate appears in a dozen different statements throughout the program. When the rate changes, the initialization value in the definition of the named constant is the only value that needs to be modified. If the rate increases to 7.2%, the definition is changed to the following: const double INTEREST_RATE = 0.072; The program is then ready to be recompiled. Every statement that uses INTEREST_RATE will then use the new value. Named constants can also help prevent typographical errors in a program’s code. For example, suppose you use the number 3.14159 as the value of pi in a program that performs various geometric calculations. Each time you type the number 3.14159 in the program’s code, there is a chance that you will make a mistake with one or more of the digits. As a result, the program will not produce the correct results. To help prevent a mistake such as this, you can define a named constant for pi, initialized with the correct value, and then use that constant in all of the formulas that require its value. Program 2-28 shows an example. It calculates the circumference of a circle that has a diameter of 10. Program 2-28 1 // This program calculates the circumference of a circle. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Constants 8 const double PI = 3.14159; 9 const double DIAMETER = 10.0; 10 11 // Variable to hold the circumference 2.17 Programming Style 73 12 13 14 15 16 17 18 19 20 } double circumference; // Calculate the circumference. circumference = PI * DIAMETER; // Display the circumference. cout << "The circumference is: " << circumference << endl; return 0; Program Output The circumference is: 31.4159 Let’s take a closer look at the program. Line 8 defines a constant double named PI, initialized with the value 3.14159. This constant will be used for the value of pi in the program’s calculation. Line 9 defines a constant double named DIAMETER, initialized with the value 10. This will be used for the circle’s diameter. Line 12 defines a double variable named circumference, which will be used to hold the circle’s circumference. Line 15 calculates the circle’s circumference by multiplying PI by DIAMETER. The result of the calculation is assigned to the circumference variable. Line 18 displays the circle’s circumference. Checkpoint 2.25 Write statements using the const qualifier to create named constants for the following literal values: Literal Value 2.71828 5.256E5 32.2 9.8 1609 Description Euler’s number (known in mathematics as e) Number of minutes in a year The gravitational acceleration constant (in feet per second2) The gravitational acceleration constant (in meters per second2) Number of meters in a mile 2.17 Programming Style CONCEPT: Programming style refers to the way a programmer uses identifiers, spaces, tabs, blank lines, and punctuation characters to visually arrange a program’s source code. These are some, but not all, of the elements of programming style. In Chapter 1 you learned that syntax rules govern the way a language may be used. The syntax rules of C++ dictate how and where to place key words, semicolons, commas, braces, and other components of the language. The compiler’s job is to check for syntax errors and, if there are none, generate object code. When the compiler reads a program it processes it as one long stream of characters. The compiler doesn’t care that each statement is on a separate line, or that spaces separate operators from operands. Humans, on the other hand, find it difficult to read programs that aren’t written in a visually pleasing manner. Consider Program 2-29 for example. 74 Chapter 2 Introduction to C++ Program 2-29 1 #include 2 using namespace std;int main(){double shares=220.0; 3 double avgPrice=14.67;cout<<"There were "< 3 using namespace std; 4 5 int main() 6{ 7 double shares = 220.0; 8 double avgPrice = 14.67; 9 10 cout << "There were " << shares << " shares sold at $"; 11 cout << avgPrice << " per share.\n"; 12 return 0; 13 } Program Output There were 220 shares sold at $14.67 per share. Programming style refers to the way source code is visually arranged. Ideally, it is a consistent method of putting spaces and indentions in a program so visual cues are created. These cues quickly tell a programmer important information about a program. For example, notice in Program 2-30 that inside the function main’s braces each line is indented. It is a common C++ style to indent all the lines inside a set of braces. You will also notice the blank line between the variable definitions and the cout statements. This is intended to visually separate the definitions from the executable statements. N OTE: Although you are free to develop your own style, you should adhere to common programming practices. By doing so, you will write programs that visually make sense to other programmers. Review Questions and Exercises 75 Another aspect of programming style is how to handle statements that are too long to fit on one line. Because C++ is a free-flowing language, it is usually possible to spread a statement over several lines. For example, here is a cout statement that uses five lines: cout << "The Fahrenheit temperature is " << fahrenheit << " and the Celsius temperature is " << celsius << endl; This statement will work just as if it were typed on one line. Here is an example of variable definitions treated similarly: int fahrenheit, celsius, kelvin; There are many other issues related to programming style. They will be presented throughout the book. Review Questions and Exercises Short Answer 1. How many operands does each of the following types of operators require? _______ Unary _______ Binary _______ Ternary 2. How may the double variables temp, weight, and age be defined in one statement? 3. How may the int variables months, days, and years be defined in one statement, with months initialized to 2 and years initialized to 3? 4. Write assignment statements that perform the following operations with the variables a, b, and c. A) Adds 2 to a and stores the result in b. B) Multiplies b times 4 and stores the result in a. C) Divides a by 3.14 and stores the result in b. D) Subtracts 8 from b and stores the result in a. E) Stores the value 27 in a. F) Stores the character ‘K’ in c. G) Stores the ASCII code for ‘B’ in c. 5. Is the following comment written using single-line or multi-line comment symbols? /* This program was written by M. A. Codewriter*/ 6. Is the following comment written using single-line or multi-line comment symbols? // This program was written by M. A. Codewriter 76 Chapter 2 Introduction to C++ 7. Modify the following program so it prints two blank lines between each line of text. #include using namespace std; int main() { cout << "Two mandolins like creatures in the"; cout << "dark"; cout << "Creating the agony of ecstasy."; cout << " - George Barker"; return 0; } 8. What will the following programs print on the screen? A) #include using namespace std; int main() { int freeze = 32, boil = 212; freeze = 0; boil = 100; cout << freeze << endl << boil << endl; return 0; } B) #include using namespace std; int main() { int x = 0, y = 2; x = y * 4; cout << x << endl << y << endl; return 0; } C) #include using namespace std; int main() { cout << "I am the incredible"; cout << "computing\nmachine"; cout << "\nand I will\namaze\n"; cout << "you."; return 0; } D) #include using namespace std; int main() { cout << "Be careful\n"; cout << "This might/n be a trick "; cout << "question\n"; return 0; } Review Questions and Exercises 77 E) #include using namespace std; int main() { int a, x = 23; a = x % 2; cout << x << endl << a << endl; return 0; } Multiple Choice 9. Every complete statement ends with a A) period B) # symbol C) semicolon D) ending brace 10. Which of the following statements is correct? A) #include (iostream) B) #include {iostream} C) #include D) #include [iostream] E) All of the above 11. Every C++ program must have a A) cout statement B) function main C) #include statement D) All of the above 12. Preprocessor directives begin with a A) # B) ! C) < D) * E) None of the above 13. The following data 72 'A' "Hello World" 2.8712 are all examples of A) Variables B) Literals or constants C) Strings D) None of the above 14. A group of statements, such as the contents of a function, is enclosed in A) Braces {} B) Parentheses () C) Brackets <> D) All of the above will do 78 Chapter 2 Introduction to C++ 15. Which of the following are not valid assignment statements? (Circle all that apply.) A) total = 9; B) 72 = amount; C) profit = 129 D) letter = 'W'; 16. Which of the following are not valid cout statements? (Circle all that apply.) A) cout << "Hello World"; B) cout << "Have a nice day"\n; C) cout < value; D) cout << Programming is great fun; 17. Assume w = 5, x = 4, y = 8, and z = 2. What value will be stored in result in each of the following statements? A) result = x + y; B) result = z * 2; C) result = y / x; D) result = y − z; E) result = w % 2; 18. How would each of the following numbers be represented in E notation? A) 3.287 × 106 B) −978.65 × 1012 C) 7.65491 × 10−3 D) −58710.23 × 10−4 19. The negation operator is A) Unary B) Binary C) Ternary D) None of the above 20. A(n) ___________ is like a variable, but its value is read-only and cannot be changed during the program’s execution. A) secure variable B) uninitialized variable C) named constant D) locked variable 21. When do preprocessor directives execute? A) Before the compiler compiles your program B) After the compiler compiles your program C) At the same time as the compiler compiles your program D) None of the above True or False 22. T F A variable must be defined before it can be used. 23. T F Variable names may begin with a number. 24. T F Variable names may be up to 31 characters long. 25. T F A left brace in a C++ program should always be followed by a right brace later in the program. 26. T F You cannot initialize a named constant that is declared with the const modifier. Programming Challenges 79 Algorithm Workbench 27. Convert the following pseudocode to C++ code. Be sure to define the appropriate variables. Store 20 in the speed variable. Store 10 in the time variable. Multiply speed by time and store the result in the distance variable. Display the contents of the distance variable. 28. Convert the following pseudocode to C++ code. Be sure to define the appropriate variables. Store 172.5 in the force variable. Store 27.5 in the area variable. Divide area by force and store the result in the pressure variable. Display the contents of the pressure variable. Find the Error 29. There are a number of syntax errors in the following program. Locate as many as you can. */ What's wrong with this program? /* #include iostream using namespace std; int main(); } int a, b, c \\ Three integers a=3 b=4 c=a+b Cout < "The value of c is %d" < C; return 0; { Programming Challenges Visit www.myprogramminglab.com to complete many of these Programming Challenges online and get instant feedback. 1. Sum of Two Numbers Write a program that stores the integers 50 and 100 in variables, and stores the sum of these two in a variable named total. 2. Sales Prediction The East Coast sales division of a company generates 58 percent of total sales. Based on that percentage, write a program that will predict how much the East Coast division will generate if the company has $8.6 million in sales this year. 3. Sales Tax Write a program that will compute the total sales tax on a $95 purchase. Assume the state sales tax is 4 percent and the county sales tax is 2 percent. 80 Chapter 2 Introduction to C++ VideoNote Solving the Restaurant Bill Problem 4. Restaurant Bill Write a program that computes the tax and tip on a restaurant bill for a patron with a $88.67 meal charge. The tax should be 6.75 percent of the meal cost. The tip should be 20 percent of the total after adding the tax. Display the meal cost, tax amount, tip amount, and total bill on the screen. 5. Average of Values To get the average of a series of values, you add the values up and then divide the sum by the number of values. Write a program that stores the following values in five different variables: 28, 32, 37, 24, and 33. The program should first calculate the sum of these five variables and store the result in a separate variable named sum. Then, the program should divide the sum variable by 5 to get the average. Display the average on the screen. T I P : Use the double data type for all variables in this program. 6. Annual Pay Suppose an employee gets paid every two weeks and earns $2,200 each pay period. In a year the employee gets paid 26 times. Write a program that defines the following variables: payAmount This variable will hold the amount of pay the employee earns each pay period. Initialize the variable with 2200.0. payPeriods This variable will hold the number of pay periods in a year. Initialize the variable with 26. annualPay This variable will hold the employee’s total annual pay, which will be calculated. The program should calculate the employee’s total annual pay by multiplying the employee’s pay amount by the number of pay periods in a year and store the result in the annualPay variable. Display the total annual pay on the screen. 7. Ocean Levels Assuming the ocean’s level is currently rising at about 1.5 millimeters per year, write a program that displays: • The number of millimeters higher than the current level that the ocean’s level will be in 5 years • The number of millimeters higher than the current level that the ocean’s level will be in 7 years • The number of millimeters higher than the current level that the ocean’s level will be in 10 years 8. Total Purchase A customer in a store is purchasing five items. The prices of the five items are Price of item 1 = $15.95 Price of item 2 = $24.95 Price of item 3 = $6.95 Price of item 4 = $12.95 Price of item 5 = $3.95 Programming Challenges 81 Write a program that holds the prices of the five items in five variables. Display each item’s price, the subtotal of the sale, the amount of sales tax, and the total. Assume the sales tax is 7%. 9. Cyborg Data Type Sizes You have been given a job as a programmer on a Cyborg supercomputer. In order to accomplish some calculations, you need to know how many bytes the following data types use: char, int, float, and double. You do not have any manuals, so you can’t look this information up. Write a C++ program that will determine the amount of memory used by these types and display the information on the screen. 10. Miles per Gallon A car holds 15 gallons of gasoline and can travel 375 miles before refueling. Write a program that calculates the number of miles per gallon the car gets. Display the result on the screen. Hint: Use the following formula to calculate miles per gallon (MPG): MPG ϭ Miles Driven/Gallons of Gas Used 11. Distance per Tank of Gas A car with a 20-gallon gas tank averages 23.5 miles per gallon when driven in town and 28.9 miles per gallon when driven on the highway. Write a program that calculates and displays the distance the car can travel on one tank of gas when driven in town and when driven on the highway. Hint: The following formula can be used to calculate the distance: Distance ϭ Number of Gallons ϫ Average Miles per Gallon 12. Land Calculation One acre of land is equivalent to 43,560 square feet. Write a program that calculates the number of acres in a tract of land with 391,876 square feet. 13. Circuit Board Price An electronics company sells circuit boards at a 35 percent profit. Write a program that will calculate the selling price of a circuit board that costs $14.95. Display the result on the screen. 14. Personal Information Write a program that displays the following pieces of information, each on a separate line: Your name Your address, with city, state, and ZIP code Your telephone number Your college major Use only a single cout statement to display all of this information. 15. Triangle Pattern Write a program that displays the following pattern on the screen: * *** ***** ******* 82 Chapter 2 Introduction to C++ 16. Diamond Pattern Write a program that displays the following pattern: * *** ***** ******* ***** *** * 17. Stock Commission Kathryn bought 750 shares of stock at a price of $35.00 per share. She must pay her stockbroker a 2 percent commission for the transaction. Write a program that calculates and displays the following: • The amount paid for the stock alone (without the commission) • The amount of the commission • The total amount paid (for the stock plus the commission) 18. Energy Drink Consumption A soft drink company recently surveyed 16,500 of its customers and found that approximately 15 percent of those surveyed purchase one or more energy drinks per week. Of those customers who purchase energy drinks, approximately 58 percent of them prefer citrus-flavored energy drinks. Write a program that displays the following: • The approximate number of customers in the survey who purchase one or more energy drinks per week • The approximate number of customers in the survey who prefer citrus-flavored energy drinks CHAPTER 3 Expressions and Interactivity TOPICS 3.1 The cin Object 3.2 Mathematical Expressions 3.3 When You Mix Apples and Oranges: Type Conversion 3.4 Overflow and Underflow 3.5 Type Casting 3.6 Multiple Assignment and Combined Assignment 3.7 Formatting Output 3.8 Working with Characters and string Objects 3.9 More Mathematical Library Functions 3.10 Focus on Debugging: Hand Tracing a Program 3.11 Focus on Problem Solving: A Case Study 3.1 The cin Object VideoNote Reading Input with cin CONCEPT: The cin object can be used to read data typed at the keyboard. So far you have written programs with built-in data. Without giving the user an opportunity to enter his or her own data, you have initialized the variables with the necessary starting values. These types of programs are limited to performing their task with only a single set of starting data. If you decide to change the initial value of any variable, the program must be modified and recompiled. In reality, most programs ask for values that will be assigned to variables. This means the program does not have to be modified if the user wants to run it several times with different sets of data. For example, a program that calculates payroll for a small business might ask the user to enter the name of the employee, the hours worked, and the hourly pay rate. When the paycheck for that employee has been printed, the program could start over again and ask for the name, hours worked, and hourly pay rate of the next employee. Just as cout is C++’s standard output object, cin is the standard input object. It reads input from the console (or keyboard) as shown in Program 3-1. 83 84 Chapter 3 Expressions and Interactivity Program 3-1 1 // This program asks the user to enter the length and width of 2 // a rectangle. It calculates the rectangle's area and displays 3 // the value on the screen. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int length, width, area; 10 11 cout << "This program calculates the area of a "; 12 cout << "rectangle.\n"; 13 cout << "What is the length of the rectangle? "; 14 cin >> length; 15 cout << "What is the width of the rectangle? "; 16 cin >> width; 17 area = length * width; 18 cout << "The area of the rectangle is " << area << ".\n"; 19 return 0; 20 } Program Output with Example Input Shown in Bold This program calculates the area of a rectangle. What is the length of the rectangle? 10 [Enter] What is the width of the rectangle? 20 [Enter] The area of the rectangle is 200. Instead of calculating the area of one rectangle, this program can be used to get the area of any rectangle. The values that are stored in the length and width variables are entered by the user when the program is running. Look at lines 13 and 14: cout << "What is the length of the rectangle? "; cin >> length; In line 13, the cout object is used to display the question “What is the length of the rectangle?” This question is known as a prompt, and it tells the user what data he or she should enter. Your program should always display a prompt before it uses cin to read input. This way, the user will know that he or she must type a value at the keyboard. Line 14 uses the cin object to read a value from the keyboard. The >> symbol is the stream extraction operator. It gets characters from the stream object on its left and stores them in the variable whose name appears on its right. In this line, characters are taken from the cin object (which gets them from the keyboard) and are stored in the length variable. Gathering input from the user is normally a two-step process: 1. Use the cout object to display a prompt on the screen. 2. Use the cin object to read a value from the keyboard. 3.1 The cin Object 85 The prompt should ask the user a question, or tell the user to enter a specific value. For example, the code we just examined from Program 3-1 displays the following prompt: What is the length of the rectangle? When the user sees this prompt, he or she knows to enter the rectangle’s length. After the prompt is displayed, the program uses the cin object to read a value from the keyboard and store the value in the length variable. Notice that the << and >> operators appear to point in the direction that data is flowing. In a statement that uses the cout object, the << operator always points toward cout. This indicates that data is flowing from a variable or a literal to the cout object. In a statement that uses the cin object, the >> operator always points toward the variable that is receiving the value. This indicates that data is flowing from cin to a variable. This is illustrated in Figure 3-1. Figure 3-1 cout << "What is the length of the rectangle? "; cin >> length; Think of the << and >> operators as arrows that point in the direction that data is flowing. cout "What is the length of the rectangle? "; cin length; The cin object causes a program to wait until data is typed at the keyboard and the [Enter] key is pressed. No other lines in the program will be executed until cin gets its input. cin automatically converts the data read from the keyboard to the data type of the variable used to store it. If the user types 10, it is read as the characters ‘1’ and ‘0’. cin is smart enough to know this will have to be converted to an int value before it is stored in the length variable. cin is also smart enough to know a value like 10.7 cannot be stored in an integer variable. If the user enters a floating-point value for an integer variable, cin will not read the part of the number after the decimal point. N O T E : You must include the iostream file in any program that uses cin. Entering Multiple Values The cin object may be used to gather multiple values at once. Look at Program 3-2, which is a modified version of Program 3-1. Line 15 waits for the user to enter two values. The first is assigned to length and the second to width. cin >> length >> width; 86 Chapter 3 Expressions and Interactivity Program 3-2 1 // This program asks the user to enter the length and width of 2 // a rectangle. It calculates the rectangle's area and displays 3 // the value on the screen. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int length, width, area; 10 11 cout << "This program calculates the area of a "; 12 cout << "rectangle.\n"; 13 cout << "Enter the length and width of the rectangle "; 14 cout << "separated by a space.\n"; 15 cin >> length >> width; 16 area = length * width; 17 cout << "The area of the rectangle is " << area << endl; 18 return 0; 19 } Program Output with Example Input Shown in Bold This program calculates the area of a rectangle. Enter the length and width of the rectangle separated by a space. 10 20 [Enter] The area of the rectangle is 200 In the example output, the user entered 10 and 20, so 10 is stored in length and 20 is stored in width. Notice the user separates the numbers by spaces as they are entered. This is how cin knows where each number begins and ends. It doesn’t matter how many spaces are entered between the individual numbers. For example, the user could have entered 10 20 N OTE: The [Enter] key is pressed after the last number is entered. cin will also read multiple values of different data types. This is shown in Program 3-3. Program 3-3 1 // This program demonstrates how cin can read multiple values 2 // of different data types. 3 #include 4 using namespace std; 5 3.1 The cin Object 87 6 int main() 7{ 8 int whole; 9 double fractional; 10 char letter; 11 12 cout << "Enter an integer, a double, and a character: "; 13 cin >> whole >> fractional >> letter; 14 cout << "Whole: " << whole << endl; 15 cout << "Fractional: " << fractional << endl; 16 cout << "Letter: " << letter << endl; 17 return 0; 18 } Program Output with Example Input Shown in Bold Enter an integer, a double, and a character: 4 5.7 b [Enter] Whole: 4 Fractional: 5.7 Letter: b As you can see in the example output, the values are stored in their respective variables. But what if the user had responded in the following way? Enter an integer, a double, and a character: 5.7 4 b [Enter] When the user types values at the keyboard, those values are first stored in an area of memory known as the keyboard buffer. So, when the user enters the values 5.7, 4, and b, they are stored in the keyboard buffer as shown in Figure 3-2. Figure 3-2 Keyboard buffer 5.7 4 b [Enter] cin begins reading here. When the user presses the Enter key, cin reads the value 5 into the variable whole. It does not read the decimal point because whole is an integer variable. Next it reads .7 and stores that value in the double variable fractional. The space is skipped, and 4 is the next value read. It is stored as a character in the variable letter. Because this cin statement reads only three values, the b is left in the keyboard buffer. So, in this situation the program would have stored 5 in whole, 0.7 in fractional, and the character ‘4’ in letter. It is important that the user enters values in the correct order. Checkpoint 3.1 What header file must be included in programs using cin? 3.2 TRUE or FALSE: cin requires the user to press the [Enter] key when finished entering data. 88 Chapter 3 Expressions and Interactivity 3.3 Assume value is an integer variable. If the user enters 3.14 in response to the following programming statement, what will be stored in value? cin >> value; A) 3.14 B) 3 C) 0 D) Nothing. An error message is displayed. 3.4 A program has the following variable definitions. long miles; int feet; float inches; Write one cin statement that reads a value into each of these variables. 3.5 The following program will run, but the user will have difficulty understanding what to do. How would you improve the program? // This program multiplies two numbers and displays the result. #include using namespace std; int main() { double first, second, product; cin >> first >> second; product = first * second; cout << product; return 0; } 3.6 Complete the following program skeleton so it asks for the user’s weight (in pounds) and displays the equivalent weight in kilograms. #include using namespace std; int main() { double pounds, kilograms; // Write code here that prompts the user // to enter his or her weight and reads // the input into the pounds variable. // The following line does the conversion. kilograms = pounds / 2.2; // Write code here that displays the user's weight // in kilograms. return 0; } 3.2 Mathematical Expressions 89 3.2 Mathematical Expressions CONCEPT: C++ allows you to construct complex mathematical expressions using multiple operators and grouping symbols. In Chapter 2 you were introduced to the basic mathematical operators, which are used to build mathematical expressions. An expression is a programming statement that has a value. Usually, an expression consists of an operator and its operands. Look at the following statement: sum = 21 + 3; Since 21 + 3 has a value, it is an expression. Its value, 24, is stored in the variable sum. Expressions do not have to be in the form of mathematical operations. In the following statement, 3 is an expression. number = 3; Here are some programming statements where the variable result is being assigned the value of an expression: result = x; result = 4; result = 15 / 3; result = 22 * number; result = sizeof(int); result = a + b + c; In each of these statements, a number, variable name, or mathematical expression appears on the right side of the = symbol. A value is obtained from each of these and stored in the variable result. These are all examples of a variable being assigned the value of an expression. Program 3-4 shows how mathematical expressions can be used with the cout object. Program 3-4 1 // This program asks the user to enter the numerator 2 // and denominator of a fraction and it displays the 3 // decimal value. 4 5 #include 6 using namespace std; 7 8 int main() 9{ 10 double numerator, denominator; 11 12 cout << "This program shows the decimal value of "; 13 cout << "a fraction.\n"; (program continues) 90 Chapter 3 Expressions and Interactivity Program 3-4 (continued) 14 15 16 17 18 19 20 21 } cout << "Enter the numerator: "; cin >> numerator; cout << "Enter the denominator: "; cin >> denominator; cout << "The decimal value is "; cout << (numerator / denominator) << endl; return 0; Program Output with Example Input Shown in Bold This program shows the decimal value of a fraction. Enter the numerator: 3 [Enter] Enter the denominator: 16 [Enter] The decimal value is 0.1875 The cout object will display the value of any legal expression in C++. In Program 3-4, the value of the expression numerator / denominator is displayed. N OTE: The example input for Program 3-4 shows the user entering 3 and 16. Since these values are assigned to double variables, they are stored as the double values 3.0 and 16.0. N O T E : When sending an expression that consists of an operator to cout, it is always a good idea to put parentheses around the expression. Some advanced operators will yield unexpected results otherwise. Operator Precedence It is possible to build mathematical expressions with several operators. The following statement assigns the sum of 17, x, 21, and y to the variable answer. answer = 17 + x + 21 + y; Some expressions are not that straightforward, however. Consider the following statement: outcome = 12 + 6 / 3; What value will be stored in outcome? 6 is used as an operand for both the addition and division operators. outcome could be assigned either 6 or 14, depending on whether the addition operation or the division operation takes place first. The answer is 14 because the division operator has higher precedence than the addition operator. Mathematical expressions are evaluated from left to right. When two operators share an operand, the operator with the highest precedence works first. Multiplication and division have higher precedence than addition and subtraction, so the statement above works like this: A) 6 is divided by 3, yielding a result of 2 B) 12 is added to 2, yielding a result of 14 3.2 Mathematical Expressions 91 It could be diagrammed in the following way: outcome = 12 + 6 / 3 \/ outcome = 12 + 2 outcome = 14 Table 3-1 shows the precedence of the arithmetic operators. The operators at the top of the table have higher precedence than the ones below them. Table 3-1 Precedence of Arithmetic Operators (Highest to Lowest) (unary negation) */ % +− The multiplication, division, and modulus operators have the same precedence. This is also true of the addition and subtraction operators. Table 3-2 shows some expressions with their values. Table 3-2 Some Simple Expressions and Their Values Expression Value 5+2*4 13 10 / 2 − 3 2 8 + 12 * 2 − 4 28 4 + 17 % 2 − 1 4 6−3*2+7−1 6 Associativity An operator’s associativity is either left to right, or right to left. If two operators sharing an operand have the same precedence, they work according to their associativity. Table 3-3 lists the associativity of the arithmetic operators. As an example, look at the following expression: 5−3+2 Both the − and + operators in this expression have the same precedence, and they have left to right associativity. So, the operators will work from left to right. This expression is the same as: ((5 − 3) + 2) Here is another example: 12 / 6 * 4 Because the / and * operators have the same precedence, and they have left to right associativity, they will work from left to right. This expression is the same as: ((12 / 6) * 4) 92 Chapter 3 Expressions and Interactivity Table 3-3 Associativity of Arithmetic Operators Operator Associativity (unary negation) − Right to left */% Left to right +− Left to right Grouping with Parentheses Parts of a mathematical expression may be grouped with parentheses to force some operations to be performed before others. In the following statement, the sum of a + b is divided by 4. result = (a + b) / 4; Without the parentheses, however, b would be divided by 4 and the result added to a. Table 3-4 shows more expressions and their values. Table 3-4 More Simple Expressions and Their Values Expression Value (5 + 2) * 4 28 10 / (5 − 3) 5 8 + 12 * (6 − 2) 56 (4 + 17) % 2 − 1 0 (6 − 3) * (2 + 7) / 3 9 Converting Algebraic Expressions to Programming Statements In algebra it is not always necessary to use an operator for multiplication. C++, however, requires an operator for any mathematical operation. Table 3-5 shows some algebraic expressions that perform multiplication and the equivalent C++ expressions. Table 3-5 Algebraic and C++ Multiplication Expressions Algebraic Expression Operation C++ Equivalent 6B (3)(12) 4xy 6 times B 3 times 12 4 times x times y 6*B 3 * 12 4*x*y When converting some algebraic expressions to C++, you may have to insert parentheses that do not appear in the algebraic expression. For example, look at the following expression: x ϭ a ϩ c b To convert this to a C++ statement, a + b will have to be enclosed in parentheses: x = (a + b) / c; Table 3-6 shows more algebraic expressions and their C++ equivalents. 3.2 Mathematical Expressions 93 Table 3-6 Algebraic and C++ Expressions Algebraic Expression y ϭ 3x 2 C++ Expression y = x / 2 * 3; z ϭ 3bc ϩ 4 z = 3 * b * c + 4; a ϭ 3x 4a ϩ Ϫ 2 1 a = (3 * x + 2) / (4 * a − 1) No Exponents Please! Unlike many programming languages, C++ does not have an exponent operator. Raising a number to a power requires the use of a library function. The C++ library isn’t a place where you check out books, but a collection of specialized functions. Think of a library function as a “routine” that performs a specific operation. One of the library functions is called pow, and its purpose is to raise a number to a power. Here is an example of how it’s used: area = pow(4.0, 2.0); This statement contains a call to the pow function. The numbers inside the parentheses are arguments. Arguments are data being sent to the function. The pow function always raises the first argument to the power of the second argument. In this example, 4 is raised to the power of 2. The result is returned from the function and used in the statement where the function call appears. In this case, the value 16 is returned from pow and assigned to the variable area. This is illustrated in Figure 3-3. Figure 3-3 arguments area = pow(4.0, 2.0) ; 16.0 return value The statement area = pow(4.0, 2.0) is equivalent to the following algebraic statement: area ϭ 42 Here is another example of a statement using the pow function. It assigns 3 times 63 to x: x = 3 * pow(6.0, 3.0); And the following statement displays the value of 5 raised to the power of 4: cout << pow(5.0, 4.0); It might be helpful to think of pow as a “black box” that you plug two numbers into, and that then sends a third number out. The number that comes out has the value of the first number raised to the power of the second number, as illustrated in Figure 3-4: 94 Chapter 3 Expressions and Interactivity Figure 3-4 Argument 1 x pow function xy Argument 2 y There are some guidelines that should be followed when the pow function is used. First, the program must include the cmath header file. Second, the arguments that you pass to the pow function should be doubles. Third, the variable used to store pow’s return value should be defined as a double. For example, in the following statement the variable area should be a double: area = pow(4.0, 2.0); Program 3-5 solves a simple algebraic problem. It asks the user to enter the radius of a circle and then calculates the area of the circle. The formula is Area ϭ ␲r2 which is expressed in the program as area = PI * pow(radius, 2.0); Program 3-5 1 // This program calculates the area of a circle. 2 // The formula for the area of a circle is Pi times 3 // the radius squared. Pi is 3.14159. 4 #include 5 #include // needed for pow function 6 using namespace std; 7 8 int main() 9{ 10 const double PI = 3.14159; 11 double area, radius; 12 13 cout << "This program calculates the area of a circle.\n"; 14 cout << "What is the radius of the circle? "; 15 cin >> radius; 16 area = PI * pow(radius, 2.0); 17 cout << "The area is " << area << endl; 18 return 0; 19 } Program Output with Example Input Shown in Bold This program calculates the area of a circle. What is the radius of the circle? 10 [Enter] The area is 314.159 3.2 Mathematical Expressions 95 N O T E : Program 3-5 is presented as a demonstration of the pow function. In reality, there is no reason to use the pow function in such a simple operation. The math statement could just as easily be written as area = PI * radius * radius; The pow function is useful, however, in operations that involve larger exponents. In the Spotlight: Calculating an Average Determining the average of a group of values is a simple calculation: You add all of the values and then divide the sum by the number of values. Although this is a straightforward calculation, it is easy to make a mistake when writing a program that calculates an average. For example, let’s assume that a, b, and c are double variables. Each of the variables holds a value, and we want to calculate the average of those values. If we are careless, we might write a statement such as the following to perform the calculation: average = a + b + c / 3.0; Can you see the error in this statement? When it executes, the division will take place first. The value in c will be divided by 3.0, and then the result will be added to the sum of a + b. That is not the correct way to calculate an average. To correct this error we need to put parentheses around a + b + c, as shown here: average = (a + b + c) / 3.0; Let’s step through the process of writing a program that calculates an average. Suppose you have taken three tests in your computer science class, and you want to write a program that will display the average of the test scores. Here is the algorithm in pseudocode: Get the first test score. Get the second test score. Get the third test score. Calculate the average by adding the three test scores and dividing the sum by 3. Display the average. In the first three steps we prompt the user to enter three test scores. Let’s say we store those test scores in the double variables test1, test2, and test3. Then in the fourth step we calculate the average of the three test scores. We will use the following statement to perform the calculation and store the result in the average variable, which is a double: average = (test1 + test2 + test3) / 3.0; The last step is to display the average. Program 3-6 shows the program. 96 Chapter 3 Expressions and Interactivity Program 3-6 1 // This program calculates the average 2 // of three test scores. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 double test1, test2, test3; // To hold the scores 10 double average; // To hold the average 11 12 // Get the three test scores. 13 cout << "Enter the first test score: "; 14 cin >> test1; 15 cout << "Enter the second test score: "; 16 cin >> test2; 17 cout << "Enter the third test score: "; 18 cin >> test3; 19 20 // Calculate the average of the scores. 21 average = (test1 + test2 + test3) / 3.0; 22 23 // Display the average. 24 cout << "The average score is: " << average << endl; 25 return 0; 26 } Program Output with Example Input Shown in Bold Enter the first test score: 90 [Enter] Enter the second test score: 80 [Enter] Enter the third test score: 100 [Enter] The average score is 90 Checkpoint 3.7 Complete the table below by writing the value of each expression in the “Value” column. Expression Value 6+3*5 12 / 2 − 4 9 + 14 * 2 − 6 5 + 19 % 3 − 1 (6 + 2) * 3 14 / (11 − 4) 9 + 12 * (8 − 3) (6 + 17) % 2 − 1 (9 − 3) * (6 + 9) / 3 3.2 Mathematical Expressions 97 3.8 Write C++ expressions for the following algebraic expressions: y ϭ 6x a ϭ 2b ϩ 4c y ϭ x2 g ϭ x ϩ z2 2 y ϭ x2 z2 3.9 Study the following program and complete the table. #include #include using namespace std; int main() { double value1, value2, value3; cout << "Enter a number: "; cin >> value1; value2 = 2 * pow(value1, 2.0); value3 = 3 + value2 / 2 - 1; cout << value3 << endl; return 0; } If the User Enters… 2 5 4.3 6 The Program Will Display What Number (Stored in value3)? 3.10 Complete the following program skeleton so it displays the volume of a cylindrical fuel tank. The formula for the volume of a cylinder is Volume ϭ πr2h where π is 3.14159 r is the radius of the tank h is the height of the tank #include #include using namespace std; 98 Chapter 3 Expressions and Interactivity int main() { double volume, radius, height; cout << "This program will tell you the volume of\n"; cout << "a cylinder-shaped fuel tank.\n"; cout << "How tall is the tank? "; cin >> height; cout << "What is the radius of the tank? "; cin >> radius; // You must complete the program. } 3.3 When You Mix Apples and Oranges: Type Conversion CONCEPT: When an operator’s operands are of different data types, C++ will automatically convert them to the same data type. This can affect the results of mathematical expressions. If an int is multiplied by a float, what data type will the result be? What if a double is divided by an unsigned int? Is there any way of predicting what will happen in these instances? The answer is yes. C++ follows a set of rules when performing mathematical operations on variables of different data types. It’s helpful to understand these rules to prevent subtle errors from creeping into your programs. Just like officers in the military, data types are ranked. One data type outranks another if it can hold a larger number. For example, a float outranks an int. Table 3-7 lists the data types in order of their rank, from highest to lowest. Table 3-7 Data Type Ranking long double double float unsigned long long unsigned int int One exception to the ranking in Table 3-7 is when an int and a long are the same size. In that case, an unsigned int outranks long because it can hold a higher value. When C++ is working with an operator, it strives to convert the operands to the same type. This automatic conversion is known as type coercion. When a value is converted to a higher data type, it is said to be promoted. To demote a value means to convert it to a lower data type. Let’s look at the specific rules that govern the evaluation of mathematical expressions. Rule 1: chars, shorts, and unsigned shorts are automatically promoted to int. 3.3 When You Mix Apples and Oranges: Type Conversion 99 You will notice that char, short, and unsigned short do not appear in Table 3-7. That’s because anytime they are used in a mathematical expression, they are automatically promoted to an int. The only exception to this rule is when an unsigned short holds a value larger than can be held by an int. This can happen on systems where shorts are the same size as ints. In this case, the unsigned short is promoted to unsigned int. Rule 2: When an operator works with two values of different data types, the lower-ranking value is promoted to the type of the higher-ranking value. In the following expression, assume that years is an int and interestRate is a float: years * interestRate Before the multiplication takes place, years will be promoted to a float. Rule 3: When the final value of an expression is assigned to a variable, it will be converted to the data type of that variable. In the following statement, assume that area is a long int, while length and width are both ints: area = length * width; Since length and width are both ints, they will not be converted to any other data type. The result of the multiplication, however, will be converted to long so it can be stored in area. Watch out for situations where an expression results in a fractional value being assigned to an integer variable. Here is an example: int x, y = 4; float z = 2.7; x = y * z; In the expression y * z, y will be promoted to float and 10.8 will result from the multiplication. Since x is an integer, however, 10.8 will be truncated and 10 will be stored in x. Integer Division When you divide an integer by another integer in C++, the result is always an integer. If there is a remainder, it will be discarded. For example, in the following code, parts is assigned the value 2.0: double parts; parts = 15 / 6; Even though 15 divided by 6 is really 2.5, the .5 part of the result is discarded because we are dividing an integer by an integer. It doesn’t matter that parts is declared as a double because the fractional part of the result is discarded before the assignment takes place. In order for a division operation to return a floating-point value, at least one of the operands must be of a floating-point data type. For example, the previous code could be written as: double parts; parts = 15.0 / 6; In this code the literal value 15.0 is interpreted as a floating-point number, so the division operation will return a floating-point number. The value 2.5 will be assigned to parts. 100 Chapter 3 Expressions and Interactivity 3.4 Overflow and Underflow CONCEPT: When a variable is assigned a value that is too large or too small in range for that variable’s data type, the variable overflows or underflows. Trouble can arise when a variable is being assigned a value that is too large for its type. Here is a statement where a, b, and c are all short integers: a = b * c; If b and c are set to values large enough, the multiplication will produce a number too big to be stored in a. To prepare for this, a should have been defined as an int, or a long int. When a variable is assigned a number that is too large for its data type, it overflows. Likewise, assigning a value that is too small for a variable causes it to underflow. Program 3-7 shows what happens when an integer overflows or underflows. (The output shown is from a system with two-byte short integers.) Program 3-7 1 // This program demonstrates integer overflow and underflow. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // testVar is initialized with the maximum value for a short. 8 short testVar = 32767; 9 10 // Display testVar. 11 cout << testVar << endl; 12 13 // Add 1 to testVar to make it overflow. 14 testVar = testVar + 1; 15 cout << testVar << endl; 16 17 // Subtract 1 from testVar to make it underflow. 18 testVar = testVar − 1; 19 cout << testVar << endl; 20 return 0; 21 } Program Output 32767 −32768 32767 Typically, when an integer overflows, its contents wrap around to that data type’s lowest possible value. In Program 3-7, testVar wrapped around from 32,767 to −32,768 when 1 was added to it. When 1 was subtracted from testVar, it underflowed, which caused its 3.5 Type Casting 101 contents to wrap back around to 32,767. No warning or error message is given, so be careful when working with numbers close to the maximum or minimum range of an integer. If an overflow or underflow occurs, the program will use the incorrect number and therefore produce incorrect results. When floating-point variables overflow or underflow, the results depend upon how the compiler is configured. Your system may produce programs that do any of the following: • Produces an incorrect result and continues running. • Prints an error message and immediately stops when either floating point overflow or underflow occurs. • Prints an error message and immediately stops when floating point overflow occurs, but stores a 0 in the variable when it underflows. • Gives you a choice of behaviors when overflow or underflow occurs. You can find out how your system reacts by compiling and running Program 3-8. Program 3-8 1 // This program can be used to see how your system handles 2 // floating point overflow and underflow. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 float test; 9 10 test = 2.0e38 * 1000; // Should overflow test. 11 cout << test << endl; 12 test = 2.0e-38 / 2.0e38; // Should underflow test. 13 cout << test << endl; 14 return 0; 15 } 3.5 Type Casting CONCEPT: Type casting allows you to perform manual data type conversion. A type cast expression lets you manually promote or demote a value. The general format of a type cast expression is static_cast(Value) where Value is a variable or literal value that you wish to convert and DataType is the data type you wish to convert Value to. Here is an example of code that uses a type cast expression: double number = 3.7; int val; val = static_cast(number); 102 Chapter 3 Expressions and Interactivity This code defines two variables: number, a double, and val, an int. The type cast expression in the third statement returns a copy of the value in number, converted to an int. When a double is converted to an int, the fractional part is truncated so this statement stores 3 in val. The original value in number is not changed, however. Type cast expressions are useful in situations where C++ will not perform the desired conversion automatically. Program 3-9 shows an example where a type cast expression is used to prevent integer division from taking place. The statement that uses the type cast expression is perMonth = static_cast(books) / months; Program 3-9 1 // This program uses a type cast to avoid integer division. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int books; // Number of books to read 8 int months; // Number of months spent reading 9 double perMonth; // Average number of books per month 10 11 cout << "How many books do you plan to read? "; 12 cin >> books; 13 cout << "How many months will it take you to read them? "; 14 cin >> months; 15 perMonth = static_cast(books) / months; 16 cout << "That is " << perMonth << " books per month.\n"; 17 return 0; 18 } Program Output with Example Input Shown in Bold How many books do you plan to read? 30 [Enter] How many months will it take you to read them? 7 [Enter] That is 4.28571 books per month. The variable books is an integer, but its value is converted to a double before the division takes place. Without the type cast expression in line 15, integer division would have been performed resulting in an incorrect answer. W ARN IN G ! In Program 3-9, the following statement would still have resulted in integer division: perMonth = static_cast(books / months); The result of the expression books / months is 4. When 4 is converted to a double, it is 4.0. To prevent the integer division from taking place, one of the operands should be converted to a double prior to the division operation. This forces C++ to automatically convert the value of the other operand to a double. Program 3-10 further demonstrates the type cast expression. 3.5 Type Casting 103 Program 3-10 1 // This program uses a type cast expression to print a character 2 // from a number. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int number = 65; 9 10 // Display the value of the number variable. 11 cout << number << endl; 12 13 // Display the value of number converted to 14 // the char data type. 15 cout << static_cast(number) << endl; 16 return 0; 17 } Program Output 65 A Let’s take a closer look at this program. In line 8 the int variable number is initialized with the value 65. In line 11, number is sent to cout, causing 65 to be displayed. In line 15, a type cast expression is used to convert the value in number to the char data type. Recall from Chapter 2 that characters are stored in memory as integer ASCII codes. The number 65 is the ASCII code for the letter ‘A’, so this statement causes the letter ‘A’ to be displayed. N O T E : C++ provides several different type cast expressions. static_cast is the most commonly used type cast expression, so we will primarily use it in this book. Checkpoint 3.11 Assume the following variable definitions: int a = 5, b = 12; double x = 3.4, z = 9.1; What are the values of the following expressions? A) b / a B) x * a C) static_cast(b / a) D) static_cast(b) / a E) b / static_cast(a) F) static_cast(b) / static_cast(a) G) b / static_cast(x) H) static_cast(x) * static_cast(z) I) static_cast(x * z) J) static_cast(static_cast(x) * static_cast(z)) 104 Chapter 3 Expressions and Interactivity 3.12 3.13 Complete the following program skeleton so it asks the user to enter a character. Store the character in the variable letter. Use a type cast expression with the variable in a cout statement to display the character’s ASCII code on the screen. #include using namespace std; int main() { char letter; // Finish this program // as specified above. return 0; } What will the following program display? #include using namespace std; int main() { int integer1, integer2; double result; integer1 = 19; integer2 = 2; result = integer1 / integer2; cout << result << endl; result = static_cast(integer1) / integer2; cout << result << endl; result = static_cast(integer1 / integer2); cout << result << endl; return 0; } 3.6 Multiple Assignment and Combined Assignment CONCEPT: Multiple assignment means to assign the same value to several variables with one statement. C++ allows you to assign a value to multiple variables at once. If a program has several variables, such as a, b, c, and d, and each variable needs to be assigned a value, such as 12, the following statement may be constructed: a = b = c = d = 12; The value 12 will be assigned to each variable listed in the statement.* * The assignment operator works from right to left. 12 is first assigned to d, then to c, then to b, then to a. 3.6 Multiple Assignment and Combined Assignment 105 Combined Assignment Operators Quite often, programs have assignment statements of the following form: number = number + 1; The expression on the right side of the assignment operator gives the value of number plus 1. The result is then assigned to number, replacing the value that was previously stored there. Effectively, this statement adds 1 to number. In a similar fashion, the following statement subtracts 5 from number. number = number − 5; If you have never seen this type of statement before, it might cause some initial confusion because the same variable name appears on both sides of the assignment operator. Table 3-8 shows other examples of statements written this way. Table 3-8 (Assume x = 6) Statement x = x + 4; x = x − 3; x = x * 10; x = x / 2; x=x%4 What It Does Adds 4 to x Subtracts 3 from x Multiplies x by 10 Divides x by 2 Makes x the remainder of x / 4 Value of x After the Statement 10 3 60 3 2 These types of operations are very common in programming. For convenience, C++ offers a special set of operators designed specifically for these jobs. Table 3-9 shows the combined assignment operators, also known as compound operators, and arithmetic assignment operators. Table 3-9 Operator += −= *= /= %= Example Usage x += 5; y −= 2; z *= 10; a /= b; c %= 3; Equivalent to x = x + 5; y = y − 2; z = z * 10; a = a / b; c = c % 3; As you can see, the combined assignment operators do not require the programmer to type the variable name twice. Also, they give a clear indication of what is happening in the statement. Program 3-11 uses combined assignment operators. 106 Chapter 3 Expressions and Interactivity Program 3-11 1 // This program tracks the inventory of three widget stores 2 // that opened at the same time. Each store started with the 3 // same number of widgets in inventory. By subtracting the 4 // number of widgets each store has sold from its inventory, 5 // the current inventory can be calculated. 6 #include 7 using namespace std; 8 9 int main() 10 { 11 int begInv, // Beginning inventory for all stores 12 sold, // Number of widgets sold 13 store1, // Store 1's inventory 14 store2, // Store 2's inventory 15 store3; // Store 3's inventory 16 17 // Get the beginning inventory for all the stores. 18 cout << "One week ago, 3 new widget stores opened\n"; 19 cout << "at the same time with the same beginning\n"; 20 cout << "inventory. What was the beginning inventory? "; 21 cin >> begInv; 22 23 // Set each store's inventory. 24 store1 = store2 = store3 = begInv; 25 26 // Get the number of widgets sold at store 1. 27 cout << "How many widgets has store 1 sold? "; 28 cin >> sold; 29 store1 −= sold; // Adjust store 1's inventory. 30 31 // Get the number of widgets sold at store 2. 32 cout << "How many widgets has store 2 sold? "; 33 cin >> sold; 34 store2 −= sold; // Adjust store 2's inventory. 35 36 // Get the number of widgets sold at store 3. 37 cout << "How many widgets has store 3 sold? "; 38 cin >> sold; 39 store3 −= sold; // Adjust store 3's inventory. 40 41 // Display each store's current inventory. 42 cout << "\nThe current inventory of each store:\n"; 43 cout << "Store 1: " << store1 << endl; 44 cout << "Store 2: " << store2 << endl; 45 cout << "Store 3: " << store3 << endl; 46 return 0; 47 } 3.6 Multiple Assignment and Combined Assignment 107 Program Output with Example Input Shown in Bold One week ago, 3 new widget stores opened at the same time with the same beginning inventory. What was the beginning inventory? 100 [Enter] How many widgets has store 1 sold? 25 [Enter] How many widgets has store 2 sold? 15 [Enter] How many widgets has store 3 sold? 45 [Enter] The current inventory of each store: Store 1: 75 Store 2: 85 Store 3: 55 More elaborate statements may be expressed with the combined assignment operators. Here is an example: result *= a + 5; In this statement, result is multiplied by the sum of a + 5. When constructing such statements, you must realize the precedence of the combined assignment operators is lower than that of the regular math operators. The statement above is equivalent to result = result * (a + 5); which is different from result = result * a + 5; Table 3-10 shows other examples of such statements and their assignment statement equivalencies. Table 3-10 Example Usage x += b + 5; y −= a * 2; z *= 10 − c; a /= b + c; c %= d − 3; Equivalent to x = x + (b + 5); y = y − (a * 2); z = z * (10 − c); a = a / (b + c); c = c % (d − 3); Checkpoint 3.14 Write a multiple assignment statement that assigns 0 to the variables total, subtotal, tax, and shipping. 3.15 Write statements using combined assignment operators to perform the following: A) Add 6 to x. B) Subtract 4 from amount. C) Multiply y by 4. D) Divide total by 27. E) Store in x the remainder of x divided by 7. F) Add y * 5 to x. G) Subtract discount times 4 from total. H) Multiply increase by salesRep times 5. I) Divide profit by shares minus 1000. 108 Chapter 3 Expressions and Interactivity 3.16 What will the following program display? #include using namespace std; int main() { int unus, duo, tres; unus = duo = tres = 5; unus += 4; duo *= 2; tres −= 4; unus /= 3; duo += tres; cout << unus << endl; cout << duo << endl; cout << tres << endl; return 0; } 3.7 Formatting Output CONCEPT: The cout object provides ways to format data as it is being displayed. This affects the way data appears on the screen. The same data can be printed or displayed in several different ways. For example, all of the following numbers have the same value, although they look different: 720 720.0 720.00000000 7.2e+2 +720.0 The way a value is printed is called its formatting. The cout object has a standard way of formatting variables of each data type. Sometimes, however, you need more control over the way data is displayed. Consider Program 3-12, for example, which displays three rows of numbers with spaces between each one. Program 3-12 1 // This program displays three rows of numbers. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int num1 = 2897, num2 = 5, num3 = 837, 3.7 Formatting Output 109 8 9 10 11 12 13 14 15 16 17 18 19 20 } num4 = 34, num5 = 7, num6 = 1623, num7 = 390, num8 = 3456, num9 = 12; // Display the first row of numbers cout << num1 << " " << num2 << " " << num3 << endl; // Display the second row of numbers cout << num4 << " " << num5 << " " << num6 << endl; // Display the third row of numbers cout << num7 << " " << num8 << " " << num9 << endl; return 0; Program Output 2897 5 837 34 7 1623 390 3456 12 Unfortunately, the numbers do not line up in columns. This is because some of the numbers, such as 5 and 7, occupy one position on the screen, while others occupy two or three positions. cout uses just the number of spaces needed to print each number. To remedy this, cout offers a way of specifying the minimum number of spaces to use for each number. A stream manipulator, setw, can be used to establish print fields of a specified width. Here is an example of how it is used: value = 23; cout << setw(5) << value; The number inside the parentheses after the word setw specifies the field width for the value immediately following it. The field width is the minimum number of character positions, or spaces, on the screen to print the value in. In the example above, the number 23 will be displayed in a field of 5 spaces. Since 23 only occupies 2 positions on the screen, 3 blank spaces will be printed before it. To further clarify how this works, look at the following statements: value = 23; cout << "(" << setw(5) << value << ")"; This will cause the following output: ( 23) Notice that the number occupies the last two positions in the field. Since the number did not use the entire field, cout filled the extra 3 positions with blank spaces. Because the number appears on the right side of the field with blank spaces “padding” it in front, it is said to be right-justified. Program 3-13 shows how the numbers in Program 3-12 can be printed in columns that line up perfectly by using setw. 110 Chapter 3 Expressions and Interactivity Program 3-13 1 // This program displays three rows of numbers. 2 #include 3 #include // Required for setw 4 using namespace std; 5 6 int main() 7{ 8 int num1 = 2897, num2 = 5, num3 = 837, 9 num4 = 34, num5 = 7, num6 = 1623, 10 num7 = 390, num8 = 3456, num9 = 12; 11 12 // Display the first row of numbers 13 cout << setw(6) << num1 << setw(6) 14 << num2 << setw(6) << num3 << endl; 15 16 // Display the second row of numbers 17 cout << setw(6) << num4 << setw(6) 18 << num5 << setw(6) << num6 << endl; 19 20 // Display the third row of numbers 21 cout << setw(6) << num7 << setw(6) 22 << num8 << setw(6) << num9 << endl; 23 return 0; 24 } Program Output 2897 5 837 34 7 1623 390 3456 12 By printing each number in a field of 6 positions, they are displayed in perfect columns. N O T E : A new header file, iomanip, is included in Program 3-13. It must be used in any program that uses setw. Notice how a setw manipulator is used with each value because setw only establishes a field width for the value immediately following it. After that value is printed, cout goes back to its default method of printing. You might wonder what will happen if the number is too large to fit in the field, as in the following statement: value = 18397; cout << setw(2) << value; In cases like this, cout will print the entire number. setw only specifies the minimum number of positions in the print field. Any number larger than the minimum will cause cout to override the setw value. 3.7 Formatting Output 111 You may specify the field width of any type of data. Program 3-14 shows setw being used with an integer, a floating-point number, and a string object. Program 3-14 1 // This program demonstrates the setw manipulator being 2 // used with values of various data types. 3 #include 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 int intValue = 3928; 11 double doubleValue = 91.5; 12 string stringValue = "John J. Smith"; 13 14 cout << "(" << setw(5) << intValue << ")" << endl; 15 cout << "(" << setw(8) << doubleValue << ")" << endl; 16 cout << "(" << setw(16) << stringValue << ")" << endl; 17 return 0; 18 } Program Output ( 3928) ( 91.5) ( John J. Smith) Program 3-14 can be used to illustrate the following points: • The field width of a floating-point number includes a position for the decimal point. • The field width of a string object includes all characters in the string, including spaces. • The values printed in the field are right-justified by default. This means they are aligned with the right side of the print field, and any blanks that must be used to pad it are inserted in front of the value. VideoNote Formatting Numbers with setprecision The setprecision Manipulator Floating-point values may be rounded to a number of significant digits, or precision, which is the total number of digits that appear before and after the decimal point. You can control the number of significant digits with which floating-point values are displayed by using the setprecision manipulator. Program 3-15 shows the results of a division operation displayed with different numbers of significant digits. 112 Chapter 3 Expressions and Interactivity Program 3-15 1 // This program demonstrates how setprecision rounds a 2 // floating point value. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 double quotient, number1 = 132.364, number2 = 26.91; 10 11 quotient = number1 / number2; 12 cout << quotient << endl; 13 cout << setprecision(5) << quotient << endl; 14 cout << setprecision(4) << quotient << endl; 15 cout << setprecision(3) << quotient << endl; 16 cout << setprecision(2) << quotient << endl; 17 cout << setprecision(1) << quotient << endl; 18 return 0; 19 } Program Output 4.91877 4.9188 4.919 4.92 4.9 5 The first value is displayed in line 12 without the setprecision manipulator. (By default, the system in the illustration displays floating-point values with 6 significant digits.) The subsequent cout statements print the same value, but rounded to 5, 4, 3, 2, and 1 significant digits. If the value of a number is expressed in fewer digits of precision than specified by setprecision, the manipulator will have no effect. In the following statements, the value of dollars only has four digits of precision, so the number printed by both cout statements is 24.51. double dollars = 24.51; cout << dollars << endl; cout << setprecision(5) << dollars << endl; // Displays 24.51 // Displays 24.51 Table 3-11 shows how setprecision affects the way various values are displayed. Table 3-11 Number Manipulator Value Displayed 28.92786 setprecision(3) 28.9 21 setprecision(5) 21 109.5 34.28596 setprecision(4) setprecision(2) 109.5 34 3.7 Formatting Output 113 Unlike field width, the precision setting remains in effect until it is changed to some other value. As with all formatting manipulators, you must include the header file iomanip to use setprecision. Program 3-16 shows how the setw and setprecision manipulators may be combined to fully control the way floating point numbers are displayed. Program 3-16 1 // This program asks for sales figures for 3 days. The total 2 // sales are calculated and displayed in a table. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 double day1, day2, day3, total; 10 11 // Get the sales for each day. 12 cout << "Enter the sales for day 1: "; 13 cin >> day1; 14 cout << "Enter the sales for day 2: "; 15 cin >> day2; 16 cout << "Enter the sales for day 3: "; 17 cin >> day3; 18 19 // Calculate the total sales. 20 total = day1 + day2 + day3; 21 22 // Display the sales figures. 23 cout << "\nSales Figures\n"; 24 cout << "-------------\n"; 25 cout << setprecision(5); 26 cout << "Day 1: " << setw(8) << day1 << endl; 27 cout << "Day 2: " << setw(8) << day2 << endl; 28 cout << "Day 3: " << setw(8) << day3 << endl; 29 cout << "Total: " << setw(8) << total << endl; 30 return 0; 31 } Program Output with Example Input Shown in Bold Enter the sales for day 1: 321.57 [Enter] Enter the sales for day 2: 269.62 [Enter] Enter the sales for day 3: 307.77 [Enter] Sales Figures ------------Day 1: 321.57 Day 2: 269.62 Day 3: 307.77 Total: 898.96 114 Chapter 3 Expressions and Interactivity The fixed Manipulator The setprecision manipulator can sometimes surprise you in an undesirable way. When the precision of a number is set to a lower value, numbers tend to be printed in scientific notation. For example, here is the output of Program 3-16 with larger numbers being input: Program 3-16 Program Output with Example Input Shown in Bold Enter the sales for day 1: 145678.99 [Enter] Enter the sales for day 2: 205614.85 [Enter] Enter the sales for day 3: 198645.22 [Enter] Sales Figures ------------Day 1: 1.4568e+005 Day 2: 2.0561e+005 Day 3: 1.9865e+005 Total: 5.4994e+005 Another stream manipulator, fixed, forces cout to print the digits in fixed-point notation, or decimal. Program 3-17 shows how the fixed manipulator is used. Program 3-17 1 // This program asks for sales figures for 3 days. The total 2 // sales are calculated and displayed in a table. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 double day1, day2, day3, total; 10 11 // Get the sales for each day. 12 cout << "Enter the sales for day 1: "; 13 cin >> day1; 14 cout << "Enter the sales for day 2: "; 15 cin >> day2; 16 cout << "Enter the sales for day 3: "; 17 cin >> day3; 18 19 // Calculate the total sales. 20 total = day1 + day2 + day3; 21 3.7 Formatting Output 115 22 23 24 25 26 27 28 29 30 31 } // Display the sales figures. cout << "\nSales Figures\n"; cout << "-------------\n"; cout << setprecision(2) << fixed; cout << "Day 1: " << setw(8) << day1 << endl; cout << "Day 2: " << setw(8) << day2 << endl; cout << "Day 3: " << setw(8) << day3 << endl; cout << "Total: " << setw(8) << total << endl; return 0; Program Output with Example Input Shown in Bold Enter the sales for day 1: 1321.87 [Enter] Enter the sales for day 2: 1869.26 [Enter] Enter the sales for day 3: 1403.77 [Enter] Sales Figures ------------Day 1: 1321.87 Day 2: 1869.26 Day 3: 1403.77 Total: 4594.90 The statement in line 25 uses the fixed manipulator: cout << setprecision(2) << fixed; When the fixed manipulator is used, all floating point numbers that are subsequently printed will be displayed in fixed point notation, with the number of digits to the right of the decimal point specified by the setprecision manipulator. When the fixed and setprecision manipulators are used together, the value specified by the setprecision manipulator will be the number of digits to appear after the decimal point, not the number of significant digits. For example, look at the following code. double x = 123.4567; cout << setprecision(2) << fixed << x << endl; Because the fixed manipulator is used, the setprecision manipulator will cause the number to be displayed with two digits after the decimal point. The value will be displayed as 123.46. The showpoint Manipulator By default, floating-point numbers are not displayed with trailing zeroes, and floating-point numbers that do not have a fractional part are not displayed with a decimal point. For example, look at the following code. double x = 123.4, y = 456.0; cout << setprecision(6) << x << endl; cout << y << endl; 116 Chapter 3 Expressions and Interactivity The cout statements will produce the following output. 123.4 456 Although six significant digits are specified for both numbers, neither number is displayed with trailing zeroes. If we want the numbers padded with trailing zeroes, we must use the showpoint manipulator as shown in the following code. double x = 123.4, y = 456.0; cout << setprecision(6) << showpoint << x << endl; cout << y << endl; These cout statements will produce the following output. 123.400 456.000 N O T E : With most compilers, trailing zeroes are displayed when the setprecision and fixed manipulators are used together. The left and right Manipulators Normally output is right justified. For example, look at the following code. double x = 146.789, y = 24.2, z = 1.783; cout << setw(10) << x << endl; cout << setw(10) << y << endl; cout << setw(10) << z << endl; Each of the variables, x, y, and z, is displayed in a print field of 10 spaces. The output of the cout statements is 146.789 24.2 1.783 Notice that each value is right-justified, or aligned to the right of its print field. You can cause the values to be left-justified by using the left manipulator, as shown in the following code. double x = 146.789, y = 24.2, z = 1.783; cout << left << setw(10) << x << endl; cout << setw(10) << y << endl; cout << setw(10) << z << endl; The output of these cout statements is 146.789 24.2 1.783 In this case, the numbers are aligned to the left of their print fields. The left manipulator remains in effect until you use the right manipulator, which causes all subsequent output to be right-justified. Table 3-12 summarizes the manipulators we have discussed. Table 3-12 Stream Manipulator setw(n) fixed showpoint setprecision(n) left right 3.7 Formatting Output 117 Description Establishes a print field of n spaces. Displays floating-point numbers in fixed point notation. Causes a decimal point and trailing zeroes to be displayed, even if there is no fractional part. Sets the precision of floating-point numbers. Causes subsequent output to be left justified. Causes subsequent output to be right justified. Checkpoint 3.17 Write cout statements with stream manipulators that perform the following: A) Display the number 34.789 in a field of nine spaces with two decimal places of precision. B) Display the number 7.0 in a field of five spaces with three decimal places of precision. The decimal point and any trailing zeroes should be displayed. C) Display the number 5.789e+12 in fixed point notation. D) Display the number 67 left justified in a field of seven spaces. 3.18 The following program will not compile because the lines have been mixed up. #include } cout << person << endl; string person = "Wolfgang Smith"; int main() cout << person << endl; { #include return 0; cout << left; using namespace std; cout << setw(20); cout << right; When the lines are properly arranged the program should display the following: Wolfgang Smith Wolfgang Smith Rearrange the lines in the correct order. Test the program by entering it on the computer, compiling it, and running it. 118 Chapter 3 Expressions and Interactivity 3.19 The following program skeleton asks for an angle in degrees and converts it to radians. The formatting of the final output is left to you. #include #include using namespace std; int main() { const double PI = 3.14159; double degrees, radians; cout << "Enter an angle in degrees and I will convert it\n"; cout << "to radians for you: "; cin >> degrees; radians = degrees * PI / 180; // Display the value in radians left justified, in fixed // point notation, with 4 places of precision, in a field // 5 spaces wide, making sure the decimal point is always // displayed. return 0; } 3.8 Working with Characters and string Objects CONCEPT: Special functions exist for working with characters and string objects. Although it is possible to use cin with the >> operator to input strings, it can cause problems that you need to be aware of. When cin reads input, it passes over and ignores any leading whitespace characters (spaces, tabs, or line breaks). Once it comes to the first nonblank character and starts reading, it stops reading when it gets to the next whitespace character. Program 3-18 illustrates this problem. Program 3-18 1 // This program illustrates a problem that can occur if 2 // cin is used to read character data into a string object. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 string name; 10 string city; 11 12 cout << "Please enter your name: "; 13 cin >> name; 14 cout << "Enter the city you live in: "; 15 cin >> city; 16 3.8 Working with Characters and string Objects 119 17 18 19 20 } cout << "Hello, " << name << endl; cout << "You live in " << city << endl; return 0; Program Output with Example Input Shown in Bold Please enter your name: Kate Smith [Enter] Enter the city you live in: Hello, Kate You live in Smith Notice that the user was never given the opportunity to enter the city. In the first input statement, when cin came to the space between Kate and Smith, it stopped reading, storing just Kate as the value of name. In the second input statement, cin used the leftover characters it found in the keyboard buffer and stored Smith as the value of city. To work around this problem, you can use a C++ function named getline. The getline function reads an entire line, including leading and embedded spaces, and stores it in a string object. The getline function looks like the following, where cin is the input stream we are reading from and inputLine is the name of the string object receiving the input. getline(cin, inputLine); Program 3-19 illustrates using the getline function. Program 3-19 1 // This program demonstrates using the getline function 2 // to read character data into a string object. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 string name; 10 string city; 11 12 cout << "Please enter your name: "; 13 getline(cin, name); 14 cout << "Enter the city you live in: "; 15 getline(cin, city); 16 17 cout << "Hello, " << name << endl; 18 cout << "You live in " << city << endl; 19 return 0; 20 } Program Output with Example Input Shown in Bold Please enter your name: Kate Smith [Enter] Enter the city you live in: Raleigh [Enter] Hello, Kate Smith You live in Raleigh 120 Chapter 3 Expressions and Interactivity Inputting a Character Sometimes you want to read only a single character of input. For example, some programs display a menu of items for the user to choose from. Often the selections are denoted by the letters A, B, C, and so forth. The user chooses an item from the menu by typing a character. The simplest way to read a single character is with cin and the >> operator, as illustrated in Program 3-20. Program 3-20 1 // This program reads a single character into a char variable. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 char ch; 8 9 cout << "Type a character and press Enter: "; 10 cin >> ch; 11 cout << "You entered " << ch << endl; 12 return 0; 13 } Program Output with Example Input Shown in Bold Type a character and press Enter: A [Enter] You entered A Using cin.get As with string input, however, there are times when using cin >> to read a character does not do what you want. For example, because it passes over all leading whitespace, it is impossible to input just a blank or [Enter] with cin >>. The program will not continue past the cin statement until some character other than the spacebar, tab key, or [Enter] key has been pressed. (Once such a character is entered, the [Enter] key must still be pressed before the program can continue to the next statement.) Thus, programs that ask the user to "Press the Enter key to continue." cannot use the >> operator to read only the pressing of the [Enter] key. In those situations, the cin object has a built-in function named get that is helpful. Because the get function is built into the cin object, we say that it is a member function of cin. The get member function reads a single character, including any whitespace character. If the program needs to store the character being read, the get member function can be called in either of the following ways. In both examples, assume that ch is the name of a char variable that the character is being read into. cin.get(ch); ch = cin.get(); 3.8 Working with Characters and string Objects 121 If the program is using the cin.get function simply to pause the screen until the [Enter] key is pressed and does not need to store the character, the function can also be called like this: cin.get(); Program 3-21 illustrates all three ways to use the cin.get function. Program 3-21 1 // This program demonstrates three ways 2 // to use cin.get() to pause a program. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char ch; 9 10 cout << "This program has paused. Press Enter to continue."; 11 cin.get(ch); 12 cout << "It has paused a second time. Please press Enter again."; 13 ch = cin.get(); 14 cout << "It has paused a third time. Please press Enter again."; 15 cin.get(); 16 cout << "Thank you!"; 17 return 0; 18 } Program Output with Example Input Shown in Bold This program has paused. Press Enter to continue. [Enter] It has paused a second time. Please press Enter again. [Enter] It has paused a third time. Please press Enter again. [Enter] Thank you! Mixing cin >> and cin.get Mixing cin >> with cin.get can cause an annoying and hard-to-find problem. For example, look at Program 3-22. Program 3-22 1 // This program demonstrates a problem that occurs 2 // when you mix cin >> with cin.get(). 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char ch; // Define a character variable 9 int number; // Define an integer variable 10 (program continues) 122 Chapter 3 Expressions and Interactivity Program 3-22 (continued) 11 12 13 14 15 16 17 } cout << "Enter a number: "; cin >> number; // Read an integer cout << "Enter a character: "; ch = cin.get(); // Read a character cout << "Thank You!\n"; return 0; Program Output with Example Input Shown in Bold Enter a number: 100 [Enter] Enter a character: Thank You! When this program runs, line 12 lets the user enter a number, but it appears as though the statement in line 14 is skipped. This happens because cin >> and cin.get use slightly different techniques for reading data. In the example run of the program, when line 12 executed, the user entered 100 and pressed the [Enter] key. Pressing the [Enter] key causes a newline character ('\n') to be stored in the keyboard buffer, as shown in Figure 3-5. The cin >> statement in line 12 begins reading the data that the user entered, and stops reading when it comes to the newline character. This is shown in Figure 3-6. The newline character is not read, but remains in the keyboard buffer. Figure 3-5 Keyboard buffer 1 0 0 \n cin begins reading here. Figure 3-6 Keyboard buffer 1 0 0 \n cin stops reading here, but does not read the \n character. When the cin.get function in line 14 executes, it begins reading the keyboard buffer where the previous input operation stopped. That means that cin.get reads the newline character, without giving the user a chance to enter any more input. You can remedy this situation by using the cin.ignore function, described in the following section. 3.8 Working with Characters and string Objects 123 Using cin.ignore To solve the problem previously described, you can use another of the cin object’s member functions named ignore. The cin.ignore function tells the cin object to skip one or more characters in the keyboard buffer. Here is its general form: cin.ignore(n, c); The arguments shown in the parentheses are optional. If used, n is an integer and c is a character. They tell cin to skip n number of characters, or until the character c is encountered. For example, the following statement causes cin to skip the next 20 characters or until a newline is encountered, whichever comes first: cin.ignore(20,'\n'); If no arguments are used, cin will skip only the very next character. Here’s an example: cin.ignore(); Program 3-23, which is a modified version of Program 3-22, demonstrates the function. Notice that a call to cin.ignore has been inserted in line 13, right after the cin >> statement. Program 3-23 1 // This program successfully uses both 2 // cin >> and cin.get() for keyboard input. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char ch; 9 int number; 10 11 cout << "Enter a number: "; 12 cin >> number; 13 cin.ignore(); // Skip the newline character 14 cout << "Enter a character: "; 15 ch = cin.get(); 16 cout << "Thank You!\n"; 17 return 0; 18 } Program Output with Example Input Shown in Bold Enter a number: 100 [Enter] Enter a character: Z [Enter] Thank You! 124 Chapter 3 Expressions and Interactivity string Member Functions and Operators C++ string objects also have a number of member functions. For example, if you want to know the length of the string that is stored in a string object, you can call the object’s length member function. Here is an example of how to use it. string state = "Texas"; int size = state.length(); The first statement creates a string object named state and initializes it with the string "Texas". The second statement defines an int variable named size and initializes it with the length of the string in the state object. After this code executes, the size variable will hold the value 5. Certain operators also work with string objects. One of them is the + operator. You have already encountered the + operator to add two numeric quantities. Because strings cannot be added, when this operator is used with string operands it concatenates them, or joins them together. Assume we have the following definitions and initializations in a program. string greeting1 = "Hello "; string greeting2; string name1 = "World"; string name2 = "People"; The following statements illustrate how string concatenation works. greeting2 = greeting1 + name1; // greeting2 now holds "Hello World" greeting1 = greeting1 + name2; // greeting1 now holds "Hello People" Notice that the string stored in greeting1 has a blank as its last character. If the blank were not there, greeting2 would have been assigned the string "HelloWorld". The last statement in the previous example could also have been written using the += combined assignment operator, to achieve the same result: greeting1 += name2; You will learn about other useful string member functions and operators in Chapter 10. 3.9 More Mathematical Library Functions CONCEPT: The C++ runtime library provides several functions for performing complex mathematical operations. Earlier in this chapter you learned to use the pow function to raise a number to a power. The C++ library has numerous other functions that perform specialized mathematical operations. These functions are useful in scientific and special-purpose programs. Table 3-13 shows several of these, each of which requires the cmath header file. 3.9 More Mathematical Library Functions 125 Table 3-13 Function Example Description abs y = abs(x); Returns the absolute value of the argument. The argument and the return value are integers. cos y = cos(x); Returns the cosine of the argument. The argument should be an angle expressed in radians. The return type and the argument are doubles. exp y = exp(x); Computes the exponential function of the argument, which is x. The return type and the argument are doubles. fmod y = fmod(x, z); Returns, as a double, the remainder of the first argument divided by the second argument. Works like the modulus operator, but the arguments are doubles. (The modulus operator only works with integers.) Take care not to pass zero as the second argument. Doing so would cause division by zero. log y = log(x); Returns the natural logarithm of the argument. The return type and the argument are doubles. log10 y = log10(x); Returns the base-10 logarithm of the argument. The return type and the argument are doubles. sin sqrt y = sin(x); y = sqrt(x); Returns the sine of the argument. The argument should be an angle expressed in radians. The return type and the argument are doubles. Returns the square root of the argument. The return type and argument are doubles. tan y = tan(x); Returns the tangent of the argument. The argument should be an angle expressed in radians. The return type and the argu- ment are doubles. Each of these functions is as simple to use as the pow function. The following program segment demonstrates the sqrt function, which returns the square root of a number: cout << "Enter a number: "; cin >> num; s = sqrt(num); cout << "The square root of " << num << " is " << s << endl; Here is the output of the program segment, with 25 as the number entered by the user: Enter a number: 25 The square root of 25 is 5 Program 3-24 shows the sqrt function being used to find the hypotenuse of a right triangle. The program uses the following formula, taken from the Pythagorean theorem: c ϭ "a2 ϩ b2 In the formula, c is the length of the hypotenuse, and a and b are the lengths of the other sides of the triangle. 126 Chapter 3 Expressions and Interactivity Program 3-24 1 // This program asks for the lengths of the two sides of a 2 // right triangle. The length of the hypotenuse is then 3 // calculated and displayed. 4 #include 5 #include // For setprecision 6 #include // For the sqrt and pow functions 7 using namespace std; 8 9 int main() 10 { 11 double a, b, c; 12 13 cout << "Enter the length of side a: "; 14 cin >> a; 15 cout << "Enter the length of side b: "; 16 cin >> b; 17 c = sqrt(pow(a, 2.0) + pow(b, 2.0)); 18 cout << "The length of the hypotenuse is "; 19 cout << setprecision(2) << c << endl; 20 return 0; 21 } Program Output with Example Input Shown in Bold Enter the length of side a: 5.0 [Enter] Enter the length of side b: 12.0 [Enter] The length of the hypotenuse is 13 The following statement, taken from Program 3-24, calculates the square root of the sum of the squares of the triangle’s two sides: c = sqrt(pow(a, 2.0) + pow(b, 2.0)); Notice that the following mathematical expression is used as the sqrt function’s argument: pow(a, 2.0) + pow(b, 2.0) This expression calls the pow function twice: once to calculate the square of a and again to calculate the square of b. These two squares are then added together, and the sum is sent to the sqrt function. Random Numbers Random numbers are useful for lots of different programming tasks. The following are just a few examples: • Random numbers are commonly used in games. For example, computer games that let the player roll dice use random numbers to represent the values of the dice. Programs that show cards being drawn from a shuffled deck use random numbers to represent the face values of the cards. 3.9 More Mathematical Library Functions 127 • Random numbers are useful in simulation programs. In some simulations, the computer must randomly decide how a person, animal, insect, or other living being will behave. Formulas can be constructed in which a random number is used to determine various actions and events that take place in the program. • Random numbers are useful in statistical programs that must randomly select data for analysis. • Random numbers are commonly used in computer security to encrypt sensitive data. The C++ library has a function, rand(), that you can use to generate random numbers. (The rand() function requires the cstdlib header file.) The number returned from the function is an int. Here is an example of its usage: y = rand(); After this statement executes, the variable y will contain a random number. In actuality, the numbers produced by rand()are pseudorandom. The function uses an algorithm that produces the same sequence of numbers each time the program is repeated on the same system. For example, suppose the following statements are executed. cout << rand() << endl; cout << rand() << endl; cout << rand() << endl; The three numbers displayed will appear to be random, but each time the program runs, the same three values will be generated. In order to randomize the results of rand(), the srand() function must be used. srand() accepts an unsigned int argument, which acts as a seed value for the algorithm. By specifying different seed values, rand() will generate different sequences of random numbers. A common practice for getting unique seed values is to call the time function, which is part of the standard library. The time function returns the number of seconds that have elapsed since midnight, January 1, 1970. The time function requires the ctime header file, and you pass 0 as an argument to the function. Program 3-25 demonstrates. The program should generate three different random numbers each time it is executed. Program 3-25 1 // This program demonstrates random numbers. 2 #include 3 #include // For rand and srand 4 #include // For the time function 5 using namespace std; 6 7 int main() 8{ 9 // Get the system time. 10 unsigned seed = time(0); 11 12 // Seed the random number generator. 13 srand(seed); 14 (program continues) 128 Chapter 3 Expressions and Interactivity Program 3-25 (continued) 15 16 17 18 19 20 } // Display three random numbers. cout << rand() << endl; cout << rand() << endl; cout << rand() << endl; return 0; Program Output 23861 20884 21941 If you wish to limit the range of the random number, use the following formula: y = (rand() % (maxValue − minValue + 1)) + minValue; In the formula, minValue is the lowest number in the range, and maxValue is the highest number in the range. For example, the following code assigns a random number in the range of 1 through 100 to the variable y: const int MIN_VALUE = 1; const int MAX_VALUE = 100; y = (rand() % (MAX_VALUE - MIN_VALUE + 1)) + MIN_VALUE; As another example, the following code assigns a random number in the range of 100 through 200 to the variable y: const int MIN_VALUE = 100; const int MAX_VALUE = 200; y = (rand() % (MAX_VALUE - MIN_VALUE + 1)) + MIN_VALUE; The following “In the Spotlight” section demonstrates how to use random numbers to simulate rolling dice. In the Spotlight: Using Random Numbers Dr. Kimura teaches an introductory statistics class and has asked you to write a program that he can use in class to simulate the rolling of dice. The program should randomly generate two numbers in the range of 1 through 6 and display them. Program 3-26 shows the program, with three examples of program output. Program 3-26 1 // This program simulates rolling dice. 2 #include 3 #include // For rand and srand 4 #include // For the time function 5 using namespace std; 6 3.9 More Mathematical Library Functions 129 7 int main() 8{ 9 // Constants 10 const int MIN_VALUE = 1; // Minimum die value 11 const int MAX_VALUE = 6; // Maximum die value 12 13 // Variables 14 int die1; // To hold the value of die #1 15 int die2; // To hold the value of die #2 16 17 // Get the system time. 18 unsigned seed = time(0); 19 20 // Seed the random number generator. 21 srand(seed); 22 23 cout << "Rolling the dice…\n"; 24 die1 = (rand() % (MAX_VALUE - MIN_VALUE + 1)) + MIN_VALUE; 25 die2 = (rand() % (MAX_VALUE - MIN_VALUE + 1)) + MIN_VALUE; 26 cout << die1 << endl; 27 cout << die2 << endl; 28 return 0; 29 } Program Output Rolling the dice... 5 2 Program Output Rolling the dice... 4 6 Program Output Rolling the dice... 3 1 Checkpoint 3.20 Write a short description of each of the following functions: cos exp fmod log log10 pow sin sqrt tan 3.21 Assume the variables angle1 and angle2 hold angles stored in radians. Write a statement that adds the sine of angle1 to the cosine of angle2 and stores the result in the variable x. 3.22 To find the cube root (the third root) of a number, raise it to the power of 1/3. To find the fourth root of a number, raise it to the power of ¼. Write a statement that will find the fifth root of the variable x and store the result in the variable y. 130 Chapter 3 Expressions and Interactivity 3.23 The cosecant of the angle a is 1 sin a Write a statement that calculates the cosecant of the angle stored in the variable a, and stores it in the variable y. 3.10 Focus on Debugging: Hand Tracing a Program Hand tracing is a debugging process where you pretend that you are the computer executing a program. You step through each of the program’s statements one by one. As you look at a statement, you record the contents that each variable will have after the statement executes. This process is often helpful in finding mathematical mistakes and other logic errors. To hand trace a program you construct a chart with a column for each variable. The rows in the chart correspond to the lines in the program. For example, Program 3-27 is shown with a hand trace chart. The program uses the following four variables: num1, num2, num3, and avg. Notice that the hand trace chart has a column for each variable and a row for each line of code in function main. Program 3-27 1 // This program asks for three numbers, then 2 // displays the average of the numbers. 3 #include 4 using namespace std; 5 int main() 6{ num1 num2 num3 avg 7 double num1, num2, num3, avg; 8 cout << "Enter the first number: "; 9 cin >> num1; 10 cout << "Enter the second number: "; 11 cin >> num2; 12 cout << "Enter the third number: "; 13 cin >> num3; 14 avg = num1 + num2 + num3 / 3; 15 cout << "The average is " << avg << endl; 16 return 0; 17 } 3.10 Focus on Debugging: Hand Tracing a Program 131 This program, which asks the user to enter three numbers and then displays the average of the numbers, has a bug. It does not display the correct average. The output of a sample session with the program follows. Program Output with Example Input Shown in Bold Enter the first number: 10 [Enter] Enter the second number: 20 [Enter] Enter the third number: 30 [Enter] The average is 40 The correct average of 10, 20, and 30 is 20, not 40. To find the error we will hand trace the program. To hand trace this program, you step through each statement, observing the operation that is taking place, and then record the contents of the variables after the statement executes. After the hand trace is complete, the chart will appear as follows. We have written question marks in the chart where we do not know the contents of a variable. Program 3-27 (with hand trace chart filled) 1 // This program asks for three numbers, then 2 // displays the average of the numbers. 3 #include 4 using namespace std; 5 int main() 6{ num1 7 double num1, num2, num3, avg; ? 8 cout << "Enter the first number: "; ? 9 cin >> num1; 10 10 cout << "Enter the second number: "; 10 11 cin >> num2; 10 12 cout << "Enter the third number: "; 10 13 cin >> num3; 10 14 avg = num1 + num2 + num3 / 3; 10 15 cout << "The average is " << avg << endl; 10 16 return 0; 17 } num2 ? ? ? ? 20 20 20 20 20 num3 avg ? ? ? ? ? ? ? ? ? ? ? ? 30 ? 30 40 30 40 Do you see the error? By examining the statement that performs the math operation in line 14, we find a mistake. The division operation takes place before the addition operations, so we must rewrite that statement as avg = (num1 + num2 + num3) / 3; Hand tracing is a simple process that focuses your attention on each statement in a program. Often this helps you locate errors that are not obvious. 132 Chapter 3 Expressions and Interactivity 3.11 Focus on Problem Solving: A Case Study General Crates, Inc. builds custom-designed wooden crates. With materials and labor, it costs GCI $0.23 per cubic foot to build a crate. In turn, they charge their customers $0.50 per cubic foot for the crate. You have been asked to write a program that calculates the volume (in cubic feet), cost, customer price, and profit of any crate GCI builds. Variables Table 3-14 shows the named constants and variables needed. Table 3-14 Constant or Variable COST_PER_CUBIC_FOOT CHARGE_PER_CUBIC_FOOT length width height volume cost charge profit Description A named constant, declared as a double and initialized with the value 0.23. This represents the cost to build a crate, per cubic foot. A named constant, declared as a double and initialized with the value 0.5. This represents the amount charged for a crate, per cubic foot. A double variable to hold the length of the crate, which is input by the user. A double variable to hold the width of the crate, which is input by the user. A double variable to hold the height of the crate, which is input by the user. A double variable to hold the volume of the crate. The value stored in this variable is calculated. A double variable to hold the cost of building the crate. The value stored in this variable is calculated. A double variable to hold the amount charged to the customer for the crate. The value stored in this variable is calculated. A double variable to hold the profit GCI makes from the crate. The value stored in this variable is calculated. Program Design The program must perform the following general steps: 1. Ask the user to enter the dimensions of the crate (the crate’s length, width, and height). 2. Calculate the crate’s volume, the cost of building the crate, the customer’s charge, and the profit made. 3. Display the data calculated in Step 2. A general hierarchy chart for this program is shown in Figure 3-7. Figure 3-7 3.11 Focus on Problem Solving: A Case Study 133 Calculate Crate Volume, Cost, Price, and Profit. Get Crate Dimensions. Calculate Volume, Cost, Customer Charge, and Profit. Display Calculated Data. The “Get Crate Dimensions” step is shown in greater detail in Figure 3-8. Figure 3-8 Get Crate Dimensions. Get Length. Get Width. Get Height. The “Calculate Volume, Cost, Customer Charge, and Profit” step is shown in greater detail in Figure 3-9. Figure 3-9 Calculate Volume, Cost, Customer Charge, and Profit. Calculate the Crate’s Volume. Calculate the Crate’s Cost. Calculate the Customer Charge. Calculate the Profit Made. The “Display Calculated Data” step is shown in greater detail in Figure 3-10. Figure 3-10 Display Calculated Data. Display the Crate’s Volume. Display the Crate’s Cost. Display the Customer Charge. Display the Profit Made. 134 Chapter 3 Expressions and Interactivity Pseudocode for the program is as follows: Ask the user to input the crate's length. Ask the user to input the crate's width. Ask the user to input the crate's height. Calculate the crate's volume. Calculate the cost of building the crate. Calculate the customer's charge for the crate. Calculate the profit made from the crate. Display the crate's volume. Display the cost of building the crate. Display the customer's charge for the crate. Display the profit made from the crate. Calculations The following formulas will be used to calculate the crate’s volume, cost, charge, and profit: volume ϭ length ϫ width ϫ height cost ϭ volume ϫ 0.23 charge ϭ volume ϫ 0.5 profit ϭ charge Ϫ cost The Program The last step is to expand the pseudocode into the final program, which is shown in Program 3-28. Program 3-28 1 // This program is used by General Crates, Inc. to calculate 2 // the volume, cost, customer charge, and profit of a crate 3 // of any size. It calculates this data from user input, which 4 // consists of the dimensions of the crate. 5 #include 6 #include 7 using namespace std; 8 9 int main() 10 { 11 // Constants for cost and amount charged 12 const double COST_PER_CUBIC_FOOT = 0.23; 13 const double CHARGE_PER_CUBIC_FOOT = 0.5; 14 15 // Variables 16 double length, // The crate's length 17 width, // The crate's width 18 height, // The crate's height 19 volume, // The volume of the crate 20 cost, // The cost to build the crate 3.11 Focus on Problem Solving: A Case Study 135 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 } charge, // The customer charge for the crate profit; // The profit made on the crate // Set the desired output formatting for numbers. cout << setprecision(2) << fixed << showpoint; // Prompt the user for the crate's length, width, and height cout << "Enter the dimensions of the crate (in feet):\n"; cout << "Length: "; cin >> length; cout << "Width: "; cin >> width; cout << "Height: "; cin >> height; // Calculate the crate's volume, the cost to produce it, // the charge to the customer, and the profit. volume = length * width * height; cost = volume * COST_PER_CUBIC_FOOT; charge = volume * CHARGE_PER_CUBIC_FOOT; profit = charge − cost; // Display the calculated data. cout << "The volume of the crate is "; cout << volume << " cubic feet.\n"; cout << "Cost to build: $" << cost << endl; cout << "Charge to customer: $" << charge << endl; cout << "Profit: $" << profit << endl; return 0; Program Output with Example Input Shown in Bold Enter the dimensions of the crate (in feet): Length: 10 [Enter] Width: 8 [Enter] Height: 4 [Enter] The volume of the crate is 320.00 cubic feet. Cost to build: $73.60 Charge to customer: $160.00 Profit: $86.40 Program Output with Different Example Input Shown in Bold Enter the dimensions of the crate (in feet): Length: 12.5 [Enter] Width: 10.5 [Enter] Height: 8 [Enter] The volume of the crate is 1050.00 cubic feet. Cost to build: $241.50 Charge to customer: $525.00 Profit: $283.50 136 Chapter 3 Expressions and Interactivity Review Questions and Exercises Short Answer 1. Assume that the following variables are defined: int age; double pay; char section; Write a single cin statement that will read input into each of these variables. 2. Assume a string object has been defined as follows: string description; A) Write a cin statement that reads in a one-word string. B) Write a statement that reads in a string that can contain multiple words separated by blanks. 3. What header files must be included in the following program? int main() { double amount = 89.7; cout << showpoint << fixed; cout << setw(8) << amount << endl; return 0; } 4. Complete the following table by writing the value of each expression in the Value column. Expression 28 / 4 − 2 6 + 12 * 2 − 8 4+8*2 6 + 17 % 3 − 2 2 + 22 * (9 − 7) (8 + 7) * 2 (16 + 7) % 2 − 1 12 / (10 − 6) (19 − 3) * (2 + 2) / 4 Value 5. Write C++ expressions for the following algebraic expressions: a ϭ 12x z ϭ 5x ϩ 14y ϩ 6k y ϭ x4 gϭ h ϩ 12 4k a3 c ϭ b2k4 Review Questions and Exercises 137 6. Assume a program has the following variable definitions: int units; float mass; double weight; and the following statement: weight = mass * units; Which automatic data type conversion will take place? A) mass is demoted to an int, units remains an int, and the result of mass * units is an int. B) units is promoted to a float, mass remains a float, and the result of mass * units is a float. C) units is promoted to a float, mass remains a float, and the result of mass * units is a double. 7. Assume a program has the following variable definitions: int a, b = 2; float c = 4.2; and the following statement: a = b * c; What value will be stored in a? A) 8.4 B) 8 C) 0 D) None of the above 8. Assume that qty and salesReps are both integers. Use a type cast expression to rewrite the following statement so it will no longer perform integer division. unitsEach = qty / salesReps; 9. Rewrite the following variable definition so the variable is a named constant. int rate; 10. Complete the following table by writing statements with combined assignment operators in the right-hand column. The statements should be equivalent to the statements in the left-hand column. Statements with Assignment Operator x = x + 5; total = total + subtotal; dist = dist / rep; ppl = ppl * period; inv = inv − shrinkage; num = num % 2; Statements with Combined Assignment Operator 138 Chapter 3 Expressions and Interactivity 11. Write a multiple assignment statement that can be used instead of the following group of assignment statements: east = 1; west = 1; north = 1; south = 1; 12. Write a cout statement so the variable divSales is displayed in a field of 8 spaces, in fixed point notation, with a precision of 2 decimal places. The decimal point should always be displayed. 13. Write a cout statement so the variable totalAge is displayed in a field of 12 spaces, in fixed point notation, with a precision of 4 decimal places. 14. Write a cout statement so the variable population is displayed in a field of 12 spaces, left-justified, with a precision of 8 decimal places. The decimal point should always be displayed. Fill-in-the-Blank 15. The __________ library function returns the cosine of an angle. 16. The __________ library function returns the sine of an angle. 17. The __________ library function returns the tangent of an angle. 18. The __________ library function returns the exponential function of a number. 19. The __________ library function returns the remainder of a floating point division. 20. The __________ library function returns the natural logarithm of a number. 21. The __________ library function returns the base-10 logarithm of a number. 22. The __________ library function returns the value of a number raised to a power. 23. The __________ library function returns the square root of a number. 24. The __________ file must be included in a program that uses the mathematical functions. Algorithm Workbench 25. A retail store grants its customers a maximum amount of credit. Each customer’s available credit is his or her maximum amount of credit minus the amount of credit used. Write a pseudocode algorithm for a program that asks for a customer’s maximum amount of credit and amount of credit used. The program should then display the customer’s available credit. After you write the pseudocode algorithm, convert it to a complete C++ program. 26. Write a pseudocode algorithm for a program that calculates the total of a retail sale. The program should ask for the amount of the sale and the sales tax rate. The sales tax rate should be entered as a floating-point number. For example, if the sales tax rate is 6 percent, the user should enter 0.06. The program should display the amount of sales tax and the total of the sale. After you write the pseudocode algorithm, convert it to a complete C++ program. Review Questions and Exercises 139 27. Write a pseudocode algorithm for a program that asks the user to enter a golfer’s score for three games of golf, and then displays the average of the three scores. After you write the pseudocode algorithm, convert it to a complete C++ program. Find the Errors Each of the following programs has some errors. Locate as many as you can. 28. using namespace std; int main () { double number1, number2, sum; Cout << "Enter a number: "; Cin << number1; Cout << "Enter another number: "; Cin << number2; number1 + number2 = sum; Cout "The sum of the two numbers is " << sum return 0; } 29. #include using namespace std; int main() { int number1, number2; float quotient; cout << "Enter two numbers and I will divide\n"; cout << "the first by the second for you.\n"; cin >> number1, number2; quotient = float(number1) / number2; cout << quotient return 0; } 30. #include ; using namespace std; int main() { const int number1, number2, product; cout << "Enter two numbers and I will multiply\n"; cout << "them for you.\n"; cin >> number1 >> number2; product = number1 * number2; cout << product return 0; } 140 Chapter 3 Expressions and Interactivity 31. #include ; using namespace std; main { int number1, number2; cout << "Enter two numbers and I will multiply\n" cout << "them by 50 for you.\n" cin >> number1 >> number2; number1 =* 50; number2 =* 50; cout << number1 << " " << number2; return 0; } 32. #include ; using namespace std; main { double number, half; cout << "Enter a number and I will divide it\n" cout << "in half for you.\n" cin >> number1; half =/ 2; cout << fixedpoint << showpoint << half << endl; return 0; } 33. #include ; using namespace std; int main() { char name, go; cout << "Enter your name: "; getline >> name; cout << "Hi " << name << endl; return 0; } Predict the Output What will each of the following programs display? (Some should be hand traced and require a calculator.) 34. (Assume the user enters 38700. Use a calculator.) #include using namespace std; Review Questions and Exercises 141 int main() { double salary, monthly; cout << "What is your annual salary? "; cin >> salary; monthly = static_cast(salary) / 12; cout << "Your monthly wages are " << monthly << endl; return 0; } 35. #include using namespace std; int main() { long x, y, z; x = y = z = 4; x += 2; y −= 1; z *= 3; cout << x << " " << y << " " << z << endl; return 0; } 36. (Assume the user enters George Washington.) #include #include #include using namespace std; int main() { string userInput; cout << "What is your name? "; getline(cin, userInput); cout << "Hello " << userInput << endl; return 0; } 37. (Assume the user enters 36720152. Use a calculator.) #include #include using namespace std; int main() { long seconds; double minutes, hours, days, months, years; cout << "Enter the number of seconds that have\n"; cout << "elapsed since some time in the past and\n"; cout << "I will tell you how many minutes, hours,\n"; cout << "days, months, and years have passed: "; cin >> seconds; 142 Chapter 3 Expressions and Interactivity minutes = seconds / 60; hours = minutes / 60; days = hours / 24; years = days / 365; months = years * 12; cout << setprecision(4) << fixed << showpoint << right; cout << "Minutes: " << setw(6) << minutes << endl; cout << "Hours: " << setw(6) << hours << endl; cout << "Days: " << setw(6) << days << endl; cout << "Months: " << setw(6) << months << endl; cout << "Years: " << setw(6) << years << endl; return 0; } VideoNote Solving the Stadium Seating Problem Programming Challenges 1. Miles per Gallon Write a program that calculates a car’s gas mileage. The program should ask the user to enter the number of gallons of gas the car can hold and the number of miles it can be driven on a full tank. It should then display the number of miles that may be driven per gallon of gas. 2. Stadium Seating There are three seating categories at a stadium. For a softball game, Class A seats cost $15, Class B seats cost $12, and Class C seats cost $9. Write a program that asks how many tickets for each class of seats were sold, then displays the amount of income generated from ticket sales. Format your dollar amount in fixed-point notation, with two decimal places of precision, and be sure the decimal point is always displayed. 3. Test Average Write a program that asks for five test scores. The program should calculate the average test score and display it. The number displayed should be formatted in fixed-point notation, with one decimal point of precision. 4. Average Rainfall Write a program that calculates the average rainfall for three months. The program should ask the user to enter the name of each month, such as June or July, and the amount of rain (in inches) that fell each month. The program should display a message similar to the following: The average rainfall for June, July, and August is 6.72 inches. 5. Male and Female Percentages Write a program that asks the user for the number of males and the number of females registered in a class. The program should display the percentage of males and females in the class. Hint: Suppose there are 8 males and 12 females in a class. There are 20 students in the class. The percentage of males can be calculated as 8 ÷ 20 = 0.4, or 40%. The percentage of females can be calculated as 12 ÷ 20 = 0.6, or 60%. Programming Challenges 143 6. Ingredient Adjuster A cookie recipe calls for the following ingredients: • 1.5 cups of sugar • 1 cup of butter • 2.75 cups of flour The recipe produces 48 cookies with this amount of the ingredients. Write a program that asks the user how many cookies he or she wants to make, and then displays the number of cups of each ingredient needed for the specified number of cookies. 7. Box Office A movie theater only keeps a percentage of the revenue earned from ticket sales. The remainder goes to the movie distributor. Write a program that calculates a theater’s gross and net box office profit for a night. The program should ask for the name of the movie, and how many adult and child tickets were sold. (The price of an adult ticket is $10.00 and a child’s ticket is $6.00.) It should display a report similar to Movie Name: Adult Tickets Sold: Child Tickets Sold: Gross Box Office Profit: Net Box Office Profit: Amount Paid to Distributor: “Wheels of Fury” 382 127 $ 4582.00 $ 916.40 $ 3665.60 N OTE: Assume the theater keeps 20 percent of the gross box office profit. 8. How Many Widgets? The Yukon Widget Company manufactures widgets that weigh 12.5 pounds each. Write a program that calculates how many widgets are stacked on a pallet, based on the total weight of the pallet. The program should ask the user how much the pallet weighs by itself and with the widgets stacked on it. It should then calculate and display the number of widgets stacked on the pallet. 9. How Many Calories? A bag of cookies holds 30 cookies. The calorie information on the bag claims that there are 10 “servings” in the bag and that a serving equals 300 calories. Write a program that asks the user to input how many cookies he or she actually ate and then reports how many total calories were consumed. 10. How Much Insurance? Many financial experts advise that property owners should insure their homes or buildings for at least 80 percent of the amount it would cost to replace the structure. Write a program that asks the user to enter the replacement cost of a building and then displays the minimum amount of insurance he or she should buy for the property. 11. Automobile Costs Write a program that asks the user to enter the monthly costs for the following expenses incurred from operating his or her automobile: loan payment, insurance, gas, oil, tires, and maintenance. The program should then display the total monthly cost of these expenses, and the total annual cost of these expenses. 144 Chapter 3 Expressions and Interactivity 12. Celsius to Fahrenheit Write a program that converts Celsius temperatures to Fahrenheit temperatures. The formula is F ϭ 95C ϩ 32 F is the Fahrenheit temperature, and C is the Celsius temperature. 13. Currency Write a program that will convert U.S. dollar amounts to Japanese yen and to euros, storing the conversion factors in the constants YEN_PER_DOLLAR and EUROS_PER_ DOLLAR. To get the most up-to-date exchange rates, search the Internet using the term “currency exchange rate”. If you cannot find the most recent exchange rates, use the following: 1 Dollar = 98.93 Yen 1 Dollar = 0.74 Euros Format your currency amounts in fixed-point notation, with two decimal places of precision, and be sure the decimal point is always displayed. 14. Monthly Sales Tax A retail company must file a monthly sales tax report listing the sales for the month and the amount of sales tax collected. Write a program that asks for the month, the year, and the total amount collected at the cash register (that is, sales plus sales tax). Assume the state sales tax is 4 percent and the county sales tax is 2 percent. If the total amount collected is known and the total sales tax is 6 percent, the amount of product sales may be calculated as: S ϭ T 1.06 S is the product sales and T is the total income (product sales plus sales tax). The program should display a report similar to Month: October -------------------Total Collected: Sales: County Sales Tax: State Sales Tax: Total Sales Tax: $ 26572.89 $ 25068.76 $ 501.38 $ 1002.75 $ 1504.13 15. Property Tax A county collects property taxes on the assessment value of property, which is 60 percent of the property’s actual value. If an acre of land is valued at $10,000, its assessment value is $6,000. The property tax is then 75¢ for each $100 of the assessment value. The tax for the acre assessed at $6,000 will be $45. Write a program that asks for the actual value of a piece of property and displays the assessment value and property tax. 16. Senior Citizen Property Tax Madison County provides a $5,000 homeowner exemption for its senior citizens. For example, if a senior’s house is valued at $158,000 its assessed value would be $94,800, Programming Challenges 145 as explained above. However, he would only pay tax on $89,800. At last year’s tax rate of $2.64 for each $100 of assessed value, the property tax would be $2,370.72. In addition to the tax break, senior citizens are allowed to pay their property tax in four equal payments. The quarterly payment due on this property would be $592.68. Write a program that asks the user to input the actual value of a piece of property and the current tax rate for each $100 of assessed value. The program should then calculate and report how much annual property tax a senior homeowner will be charged for this property and what the quarterly tax bill will be. 17. Math Tutor Write a program that can be used as a math tutor for a young student. The program should display two random numbers to be added, such as 247 ϩ129 The program should then pause while the student works on the problem. When the student is ready to check the answer, he or she can press a key and the program will display the correct solution: 247 ϩ129 376 18. Interest Earned Assuming there are no deposits other than the original investment, the balance in a savings account after one year may be calculated as Amount = Principal * (1 + Rate)T T Principal is the balance in the savings account, Rate is the interest rate, and T is the number of times the interest is compounded during a year (T is 4 if the interest is compounded quarterly). Write a program that asks for the principal, the interest rate, and the number of times the interest is compounded. It should display a report similar to Interest Rate: Times Compounded: Principal: Interest: Amount in Savings: 4.25% 12 $ 1000.00 $ 43.34 $ 1043.34 19. Monthly Payments The monthly payment on a loan may be calculated by the following formula: Rate * (1 + Rate)N Payment = ((1 + Rate)N - 1) * L Rate is the monthly interest rate, which is the annual interest rate divided by 12. (12% annual interest would be 1 percent monthly interest.) N is the number of payments, and 146 Chapter 3 Expressions and Interactivity L is the amount of the loan. Write a program that asks for these values and displays a report similar to Loan Amount: Monthly Interest Rate: Number of Payments: Monthly Payment: Amount Paid Back: Interest Paid: $ 10000.00 1% 36 $ 332.14 $ 11957.15 $ 1957.15 20. Pizza Pi Joe’s Pizza Palace needs a program to calculate the number of slices a pizza of any size can be divided into. The program should perform the following steps: A) Ask the user for the diameter of the pizza in inches. B) Calculate the number of slices that may be taken from a pizza of that size. C) Display a message telling the number of slices. To calculate the number of slices that may be taken from the pizza, you must know the following facts: • Each slice should have an area of 14.125 inches. • To calculate the number of slices, simply divide the area of the pizza by 14.125. • The area of the pizza is calculated with this formula: Area ϭ πr2 N OTE: π is the Greek letter pi. 3.14159 can be used as its value. The variable r is the radius of the pizza. Divide the diameter by 2 to get the radius. Make sure the output of the program displays the number of slices in fixed point notation, rounded to one decimal place of precision. Use a named constant for pi. 21. How Many Pizzas? Modify the program you wrote in Programming Challenge 18 (Pizza Pi) so that it reports the number of pizzas you need to buy for a party if each person attending is expected to eat an average of four slices. The program should ask the user for the number of people who will be at the party and for the diameter of the pizzas to be ordered. It should then calculate and display the number of pizzas to purchase. 22. Angle Calculator Write a program that asks the user for an angle, entered in radians. The program should then display the sine, cosine, and tangent of the angle. (Use the sin, cos, and tan library functions to determine these values.) The output should be displayed in fixed-point notation, rounded to four decimal places of precision. 23. Stock Transaction Program Last month Joe purchased some stock in Acme Software, Inc. Here are the details of the purchase: • The number of shares that Joe purchased was 1,000. • When Joe purchased the stock, he paid $45.50 per share. • Joe paid his stockbroker a commission that amounted to 2% of the amount he paid for the stock. Programming Challenges 147 Two weeks later Joe sold the stock. Here are the details of the sale: • The number of shares that Joe sold was 1,000. • He sold the stock for $56.90 per share. • He paid his stockbroker another commission that amounted to 2% of the amount he received for the stock. Write a program that displays the following information: • The amount of money Joe paid for the stock. • The amount of commission Joe paid his broker when he bought the stock. • The amount that Joe sold the stock for. • The amount of commission Joe paid his broker when he sold the stock. • Display the amount of profit that Joe made after selling his stock and paying the two commissions to his broker. (If the amount of profit that your program displays is a negative number, then Joe lost money on the transaction.) 24. Word Game Write a program that plays a word game with the user. The program should ask the user to enter the following: • His or her name • His or her age • The name of a city • The name of a college • A profession • A type of animal • A pet’s name After the user has entered these items, the program should display the following story, inserting the user’s input into the appropriate locations: There once was a person named NAME who lived in CITY. At the age of AGE, NAME went to college at COLLEGE. NAME graduated and went to work as a PROFESSION. Then, NAME adopted a(n) ANIMAL named PETNAME. They both lived happily ever after! This page intentionally left blank CHAPTER 4 Making Decisions TOPICS 4.1 Relational Operators 4.2 The if Statement 4.3 Expanding the if Statement 4.4 The if/else Statement 4.5 Nested if Statements 4.6 The if/else if Statement 4.7 Flags 4.8 Logical Operators 4.9 Checking Numeric Ranges with Logical Operators 4.10 Menus 4.11 Focus on Software Engineering: Validating User Input 4.12 Comparing Characters and Strings 4.13 The Conditional Operator 4.14 The switch Statement 4.15 More About Blocks and Variable Scope 4.1 Relational Operators CONCEPT: Relational operators allow you to compare numeric and char values and determine whether one is greater than, less than, equal to, or not equal to another. So far, the programs you have written follow this simple scheme: • Gather input from the user. • Perform one or more calculations. • Display the results on the screen. Computers are good at performing calculations, but they are also quite adept at comparing values to determine if one is greater than, less than, or equal to the other. These types of operations are valuable for tasks such as examining sales figures, determining profit and loss, checking a number to ensure it is within an acceptable range, and validating the input given by a user. Numeric data is compared in C++ by using relational operators. Each relational operator determines whether a specific relationship exists between two values. For example, 149 150 Chapter 4 Making Decisions the greater-than operator (>) determines if a value is greater than another. The equality operator (==) determines if two values are equal. Table 4-1 lists all of C++’s relational operators. Table 4-1 Relational Operators > < >= <= == != Meaning Greater than Less than Greater than or equal to Less than or equal to Equal to Not equal to All of the relational operators are binary, which means they use two operands. Here is an example of an expression using the greater-than operator: x>y This expression is called a relational expression. It is used to determine whether x is greater than y. The following expression determines whether x is less than y: xy x= y x <= y x == y x != y What the Expression Means Is x greater than y? Is x less than y? Is x greater than or equal to y? Is x less than or equal to y? Is x equal to y? Is x equal to y? N OTE: All the relational operators have left-to-right associativity. Recall that associativity is the order in which an operator works with its operands. The Value of a Relationship So, how are relational expressions used in a program? Remember, all expressions have a value. Relational expressions are also known as Boolean expressions, which means their value can only be true or false. If x is greater than y, the expression x > y will be true, while the expression y == x will be false. 4.1 Relational Operators 151 The == operator determines whether the operand on its left is equal to the operand on its right. If both operands have the same value, the expression is true. Assuming that a is 4, the following expression is true: a == 4 But the following is false: a == 2 W A R N I N G ! Notice the equality operator is two = symbols together. Don't confuse this operator with the assignment operator, which is one = symbol. The == operator determines whether a variable is equal to another value, but the = operator assigns the value on the operator's right to the variable on its left. There will be more about this later in the chapter. A couple of the relational operators actually test for two relationships. The >= operator determines whether the operand on its left is greater than or equal to the operand on the right. Assuming that a is 4, b is 6, and c is 4, both of the following expressions are true: b >= a a >= c But the following is false: a >= 5 The <= operator determines whether the operand on its left is less than or equal to the operand on its right. Once again, assuming that a is 4, b is 6, and c is 4, both of the following expressions are true: a <= c b <= 10 But the following is false: b <= a The last relational operator is !=, which is the not-equal operator. It determines whether the operand on its left is not equal to the operand on its right, which is the opposite of the == operator. As before, assuming a is 4, b is 6, and c is 4, both of the following expressions are true: a != b b != c These expressions are true because a is not equal to b and b is not equal to c. But the following expression is false because a is equal to c: a != c Table 4-3 shows other relational expressions and their true or false values. 152 Chapter 4 Making Decisions Table 4-3 (Assume x is 10 and y is 7.) Expression Value xy True, because x is greater than y. x >= y True, because x is greater than or equal to y. x <= y False, because x is not less than or equal to y. y != x True, because y is not equal to x. What Is Truth? The question “What is truth?” is one you would expect to find in a philosophy book, not a C++ programming text. It’s a good question for us to consider, though. If a relational expression can be either true or false, how are those values represented internally in a program? How does a computer store true in memory? How does it store false? As you saw in Program 2-17, those two abstract states are converted to numbers. In C++, relational expressions represent true states with the number 1 and false states with the number 0. N OTE: As you will see later in this chapter, 1 is not the only value regarded as true. To illustrate this more fully, look at Program 4-1. Program 4-1 1 // This program displays the values of true and false states. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 bool trueValue, falseValue; 8 int x = 5, y = 10; 9 10 trueValue = x < y; 11 falseValue = y == x; 12 13 cout << "True is " << trueValue << endl; 14 cout << "False is " << falseValue << endl; 15 return 0; 16 } Program Output True is 1 False is 0 4.1 Relational Operators 153 Let’s examine the statements containing the relational expressions, in lines 10 and 11, a little closer: trueValue = x < y; falseValue = y == x; These statements may seem odd because they are assigning the value of a comparison to a variable. In line 10 the variable trueValue is being assigned the result of x < y. Since x is less than y, the expression is true, and the variable trueValue is assigned the value 1. In line 11 the expression y == x is false, so the variable falseValue is set to 0. Table 4-4 shows examples of other statements using relational expressions and their outcomes. NOTE: Relational expressions have a higher precedence than the assignment operator. In the statement z = x < y; the expression x < y is evaluated first, and then its value is assigned to z. Table 4-4 (Assume x is 10, y is 7, and z, a, and b are ints or bools) Statement Outcome z=x y); a = x >= y; cout << (x <= y); b = y != x; z is assigned 0 because x is not less than y. Displays 1 because x is greater than y. a is assigned 1 because x is greater than or equal to y. Displays 0 because x is not less than or equal to y. b is assigned 1 because y is not equal to x. When writing statements such as these, it sometimes helps to enclose the relational expression in parentheses, such as: trueValue = (x < y); falseValue = (y == x); As interesting as relational expressions are, we’ve only scratched the surface of how to use them. In this chapter’s remaining sections you will see how to get the most from relational expressions by using them in statements that take action based on the results of the comparison. Checkpoint 4.1 Assuming x is 5, y is 6, and z is 8, indicate by circling the T or F whether each of the following relational expressions is true or false: A) x == 5 TF B) 7 <= (x + 2) TF C) z < 4 TF D) (2 + x) != y TF E) z != 4 TF F) x >= 9 TF G) x <= (y * 2) T F 154 Chapter 4 Making Decisions 4.2 Indicate whether the following statements about relational expressions are correct or incorrect. A) x <= y is the same as y > x. B) x != y is the same as y >= x. C) x >= y is the same as y <= x. 4.3 Answer the following questions with a yes or no. A) If it is true that x > y and it is also true that x < z, does that mean y < z is true? B) If it is true that x >= y and it is also true that z == x, does that mean that z == y is true? C) If it is true that x != y and it is also true that x != z, does that mean that z != y is true? 4.4 What will the following program display? #include using namespace std; int main () { int a = 0, b = 2, x = 4, y = 0; cout << (a == b) << endl; cout << (a != y) << endl; cout << (b <= x) << endl; cout << (y > a) << endl; return 0; } 4.2 The if Statement VideoNote The if Statement CONCEPT: The if statement can cause other statements to execute only under certain conditions. You might think of the statements in a procedural program as individual steps taken as you are walking down a road. To reach the destination, you must start at the beginning and take each step, one after the other, until you reach the destination. The programs you have written so far are like a “path” of execution for the program to follow. The type of code in Figure 4-1 is called a sequence structure because the statements are executed in sequence, without branching off in another direction. Programs often need more than one path of execution, however. Many algorithms require a program to execute some statements only under certain circumstances. This can be accomplished with a decision structure. In a decision structure’s simplest form, a specific action is taken only when a specific condition exists. If the condition does not exist, the action is not performed. The flowchart in Figure 4-2 shows the logic of a decision structure. The diamond symbol represents a yes/no Figure 4-1 Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 4.2 The if Statement 155 // A program to calculate the area of a rectangle #include using namespace std; int main() { double length, width, area; cout << "Enter the length of the rectangle: "; cin >> length; cout << "Enter the width of the rectangle: "; cin >> width; area = length * width; cout << "The area is: " << area << endl return 0; } question or a true/false condition. If the answer to the question is yes (or if the condition is true), the program flow follows one path, which leads to an action being performed. If the answer to the question is no (or the condition is false), the program flow follows another path, which skips the action. Figure 4-2 Is it cold outside? No Yes Wear a coat. In the flowchart, the action “Wear a coat” is performed only when it is cold outside. If it is not cold outside, the action is skipped. The action is conditionally executed because it is performed only when a certain condition (cold outside) exists. Figure 4-3 shows a more elaborate flowchart, where three actions are taken only when it is cold outside. We perform mental tests like these every day. Here are some other examples: If the car is low on gas, stop at a service station and get gas. If it’s raining outside, go inside. If you’re hungry, get something to eat. 156 Chapter 4 Making Decisions Figure 4-3 Is it cold outside? No Yes Wear a coat. Wear a hat. Wear gloves. One way to code a decision structure in C++ is with the if statement. Here is the general format of the if statement: if (expression) statement; The if statement is simple in the way it works: If the value of the expression inside the parentheses is true, the very next statement is executed. Otherwise, it is skipped. The statement is conditionally executed because it only executes under the condition that the expression in the parentheses is true. Program 4-2 shows an example of an if statement. The user enters three test scores, and the program calculates their average. If the average is greater than 95, the program congratulates the user on obtaining a high score. Program 4-2 1 // This program averages three test scores 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int HIGH_SCORE = 95; // A high score is 95 or greater 9 int score1, score2, score3; // To hold three test scores 10 double average; // TO hold the average score 11 4.2 The if Statement 157 12 // Get the three test scores. 13 cout << "Enter 3 test scores and I will average them: "; 14 cin >> score1 >> score2 >> score3; 15 16 // Calculate and display the average score. 17 average = (score1 + score2 + score3) / 3.0; 18 cout << fixed << showpoint << setprecision(1); 19 cout << "Your average is " << average << endl; 20 21 // If the average is a high score, congratulate the user. 22 if (average > HIGH_SCORE) 23 cout << "Congratulations! That's a high score!\n"; 24 return 0; 25 } Program Output with Example Input Shown in Bold Enter 3 test scores and I will average them: 80 90 70 [Enter] Your average is 80.0 Program Output with Different Example Input Shown in Bold Enter 3 test scores and I will average them: 100 100 100 [Enter] Your average is 100.0 Congratulations! That's a high score! Lines 22 and 23 cause the congratulatory message to be printed: if (average > HIGH_SCORE) cout << "Congratulations! That's a high score!\n"; The cout statement in line 23 is executed only if the average is greater than 95, the value of the HIGH_SCORE constant. If the average is not greater than 95, the cout statement is skipped. Figure 4-4 shows the logic of this if statement. Figure 4-4 average > HIGH_SCORE True False Display "Congratulations! That's a high score!" Table 4-5 shows other examples of if statements and their outcomes. 158 Chapter 4 Making Decisions Table 4-5 Statement if (hours > 40) overTime = true; if (value > 32) cout << "Invalid number\n"; if (overTime == true) payRate *= 2; Outcome Assigns true to the bool variable overTime only if hours is greater than 40 Displays the message “Invalid number” only if value is greater than 32 Multiplies payRate by 2 only if overTime is equal to true Be Careful with Semicolons Semicolons do not mark the end of a line, but the end of a complete C++ statement. The if statement isn’t complete without the conditionally executed statement that comes after it. So, you must not put a semicolon after the if (expression) portion of an if statement. No semicolon goes here. if (expression) statement; Semicolon goes here. If you inadvertently put a semicolon after the if part, the compiler will assume you are placing a null statement there. The null statement is an empty statement that does nothing. This will prematurely terminate the if statement, which disconnects it from the statement that follows it. The statement following the if will always execute, as shown in Program 4-3. Program 4-3 1 // This program demonstrates how a misplaced semicolon 2 // prematurely terminates an if statement. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int x = 0, y = 10; 9 10 cout << "x is " << x << " and y is " << y << endl; 11 if (x > y); // Error! Misplaced semicolon 12 cout << "x is greater than y\n"; //This is always executed. 13 return 0; 14 } Program Output x is 0 and y is 10 x is greater than y 4.2 The if Statement 159 Programming Style and the if Statement Even though if statements usually span more than one line, they are technically one long statement. For instance, the following if statements are identical except in style: if (a >= 100) cout << "The number is out of range.\n"; if (a >= 100) cout << "The number is out of range.\n"; In both the examples above, the compiler considers the if part and the cout statement as one unit, with a semicolon properly placed at the end. Indention and spacing are for the human readers of a program, not the compiler. Here are two important style rules you should adopt for writing if statements: • The conditionally executed statement should appear on the line after the if statement. • The conditionally executed statement should be indented one “level” from the if statement. N OTE: In most editors, each time you press the tab key, you are indenting one level. By indenting the conditionally executed statement you are causing it to stand out visually. This is so you can tell at a glance what part of the program the if statement executes. This is a standard way of writing if statements and is the method you should use. N OTE: Indentation and spacing are for the human readers of a program, not the compiler. Even though the cout statement following the if statement in Program 4-3 is indented, the semicolon still terminates the if statement. Comparing Floating-Point Numbers Because of the way that floating-point numbers are stored in memory, rounding errors sometimes occur. This is because some fractional numbers cannot be exactly represented using binary. So, you should be careful when using the equality operator (==) to compare floating point numbers. For example, Program 4-4 uses two double variables, a and b. Both variables are initialized to the value 1.5. Then, the value 0.0000000000000001 is added to a. This should make a’s contents different than b’s contents. Because of a roundoff error, however, the two variables are still the same. Program 4-4 1 // This program demonstrates how floating-point 2 // round-off errors can make equality operations unreliable. 3 #include 4 using namespace std; 5 6 int main() 7{ (program continues) 160 Chapter 4 Making Decisions Program 4-4 (continued) 8 9 10 11 12 13 14 15 16 17 18 } double a = 1.5; double b = 1.5; // a is 1.5. // b is 1.5. a += 0.0000000000000001; // Add a little to a. if (a == b) cout << "Both a and b are the same.\n"; else cout << "a and b are not the same.\n"; return 0; Program Output Both a and b are the same. To prevent round-off errors from causing this type of problem, you should stick with greater-than and less-than comparisons with floating-point numbers. And Now Back to Truth Now that you’ve gotten your feet wet with relational expressions and if statements, let’s look at the subject of truth again. You have seen that a relational expression has the value 1 when it is true and 0 when false. In the world of the if statement, however, the concept of truth is expanded. 0 is still false, but all values other than 0 are considered true. This means that any value, even a negative number, represents true as long as it is not 0. Just as in real life, truth is a complicated thing. Here is a summary of the rules you have seen so far: • When a relational expression is true it has the value 1. • When a relational expression is false it has the value 0. • Any expression that has the value 0 is considered false by the if statement. This includes the bool value false, which is equivalent to 0. • Any expression that has any value other than 0 is considered true by the if statement. This includes the bool value true, which is equivalent to 1. The fact that the if statement considers any nonzero value as true opens many possibilities. Relational expressions are not the only conditions that may be tested. For example, the following is a legal if statement in C++: if (value) cout << "It is True!"; The if statement above does not test a relational expression, but rather the contents of a variable. If the variable, value, contains any number other than 0, the message “It is True!” will be displayed. If value is set to 0, however, the cout statement will be skipped. Here is another example: if (x + y) cout << "It is True!"; 4.2 The if Statement 161 In this statement the sum of x and y is tested like any other value in an if statement: 0 is false and all other values are true. You may also use the return value of function calls as conditional expressions. Here is an example that uses the pow function: if (pow(a, b)) cout << "It is True!"; This if statement uses the pow function to raise a to the power of b. If the result is anything other than 0, the cout statement is executed. This is a powerful programming technique that you will learn more about in Chapter 6. Don’t Confuse == With = Earlier you saw a warning not to confuse the equality operator (==) with the assignment operator (=), as in the following statement: if (x = 2) //Caution here! cout << "It is True!"; The statement above does not determine whether x is equal to 2, it assigns x the value 2! Furthermore, the cout statement will always be executed because the expression x = 2 is always true. This occurs because the value of an assignment expression is the value being assigned to the variable on the left side of the = operator. That means the value of the expression x = 2 is 2. Since 2 is a nonzero value, it is interpreted as a true condition by the if statement. Program 4-5 is a version of Program 4-2 that attempts to test for a perfect average of 100. The = operator, however, was mistakenly used in the if statement. Program 4-5 1 // This program averages 3 test scores. The if statement 2 // uses the = operator, but the == operator was intended. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int score1, score2, score3; // To hold three test scores 10 double average; // TO hold the average score 11 12 // Get the three test scores. 13 cout << "Enter 3 test scores and I will average them: "; 14 cin >> score1 >> score2 >> score3; 15 16 // Calculate and display the average score. 17 average = (score1 + score2 + score3) / 3.0; 18 cout << fixed << showpoint << setprecision(1); 19 cout << "Your average is " << average << endl; 20 (program continues) 162 Chapter 4 Making Decisions Program 4-5 (continued) 21 22 23 24 25 26 } // Our intention is to congratulate the user // for having a perfect score. But, this doesn't work. if (average = 100) // WRONG! This is an assignment! cout << "Congratulations! That's a perfect score!\n"; return 0; Program Output with Example Input Shown in Bold Enter three test scores and I will average them: 80 90 70 [Enter] Your average is 80.0 Congratulations! That's a perfect score! Regardless of the average score, this program will print the message congratulating the user on a perfect score. Checkpoint 4.5 Write an if statement that performs the following logic: if the variable x is equal to 20, then assign 0 to the variable y. 4.6 Write an if statement that performs the following logic: if the variable price is greater than 500, then assign 0.2 to the variable discountRate. 4.7 Write an if statement that multiplies payRate by 1.5 if hours is greater than 40. 4.8 TRUE or FALSE: Both of the following if statements perform the same operation. if (sales > 10000) commissionRate = 0.15; if (sales > 10000) commissionRate = 0.15; 4.9 TRUE or FALSE: Both of the following if statements perform the same operation. if (calls == 20) rate *= 0.5; if (calls = 20) rate *= 0.5; 4.3 Expanding the if Statement CONCEPT: The if statement can conditionally execute a block of statements enclosed in braces. What if you want an if statement to conditionally execute a group of statements, not just one line? For instance, what if the test averaging program needed to use several cout 4.3 Expanding the if Statement 163 statements when a high score was reached? The answer is to enclose all of the conditionally executed statements inside a set of braces. Here is the format: if (expression) { statement; statement; // Place as many statements here as necessary. } Program 4-6, another modification of the test-averaging program, demonstrates this type of if statement. Program 4-6 1 // This program averages 3 test scores. 2 // It demonstrates an if statement executing 3 // a block of statements. 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 const int HIGH_SCORE = 95; // A high score is 95 or greater 11 int score1, score2, score3; // To hold three test scores 12 double average; // TO hold the average score 13 14 // Get the three test scores. 15 cout << "Enter 3 test scores and I will average them: "; 16 cin >> score1 >> score2 >> score3; 17 18 // Calculate and display the average score. 19 average = (score1 + score2 + score3) / 3.0; 20 cout << fixed << showpoint << setprecision(1); 21 cout << "Your average is " << average << endl; 22 23 // If the average is high, congratulate the user. 24 if (average > HIGH_SCORE) 25 { 26 cout << "Congratulations!\n"; 27 cout << "That's a high score.\n"; 28 cout << "You deserve a pat on the back!\n"; 29 } 30 return 0; 31 } (program output continues) 164 Chapter 4 Making Decisions Program 4-6 (continued) Program Output with Example Input Shown in Bold Enter 3 test scores and I will average them: 100 100 100 [Enter] Your average is 100.0 Congratulations! That's a high score. You deserve a pat on the back! Program Output with Different Example Input Shown in Bold Enter 3 test scores and I will average them: 80 90 70 [Enter] Your average is 80.0 Program 4-6 prints a more elaborate message when the average score is greater than 95. The if statement was expanded to execute three cout statements when highScore is set to true. Enclosing a group of statements inside a set of braces creates a block of code. The if statement will execute all the statements in the block, in the order they appear, only when average is greater than 95. Otherwise, the block will be skipped. Notice all the statements inside the braces are indented. As before, this visually separates the statements from lines that are not indented, making it more obvious they are part of the if statement. N OTE: Anytime your program has a block of code, all the statements inside the braces should be indented. Don’t Forget the Braces! If you intend to conditionally execute a block of statements with an if statement, don’t forget the braces. Remember, without a set of braces, the if statement only executes the very next statement. Program 4-7 shows the test-averaging program with the braces inadvertently left out of the if statement’s block. Program 4-7 1 // This program averages 3 test scores. The braces 2 // were inadvertently left out of the if statement. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 const int HIGH_SCORE = 95; // A high score is 95 or greater 10 int score1, score2, score3; // To hold three test scores 11 double average; // To hold the average score 12 4.3 Expanding the if Statement 165 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 } // Get the three test scores. cout << "Enter 3 test scores and I will average them: "; cin >> score1 >> score2 >> score3; // Calculate and display the average score. average = (score1 + score2 + score3) / 3.0; cout << fixed << showpoint << setprecision(1); cout << "Your average is " << average << endl; // ERROR! This if statement is missing its braces! if (average > HIGH_SCORE) cout << "Congratulations!\n"; cout << "That's a high score.\n"; cout << "You deserve a pat on the back!\n"; return 0; Program Output with Example Input Shown in Bold Enter 3 test scores and I will average them: 80 90 70 [Enter] Your average is 80 That's a high score. You deserve a pat on the back! The cout statements in lines 25 and 26 are always executed, even when average is not greater than 95. Because the braces have been removed, the if statement only controls execution of line 24. This is illustrated in Figure 4-5. Figure 4-5 These statements are always executed. Only this statement is conditionally executed. if (average > HIGH_SCORE) cout << "Congratulations!\n"; cout << "That's a high score.\n"; cout << "You deserve a pat on the back!\n"; Checkpoint 4.10 Write an if statement that performs the following logic: if the variable sales is greater than 50,000, then assign 0.25 to the commissionRate variable, and assign 250 to the bonus variable. 4.11 The following code segment is syntactically correct, but it appears to contain a logic error. Can you find the error? if (interestRate > .07) cout << "This account earns a $10 bonus.\n"; balance += 10.0; 166 Chapter 4 Making Decisions 4.4 The if/else Statement CONCEPT: The if/else statement will execute one group of statements if the expression is true, or another group of statements if the expression is false. VideoNote The if/else statement The if/else statement is an expansion of the if statement. Here is its format: if (expression) statement or block else statement or block As with the if statement, an expression is evaluated. If the expression is true, a statement or block of statements is executed. If the expression is false, however, a separate group of statements is executed. Program 4-8 uses the if/else statement along with the modulus operator to determine if a number is odd or even. Program 4-8 1 // This program uses the modulus operator to determine 2 // if a number is odd or even. If the number is evenly divisible 3 // by 2, it is an even number. A remainder indicates it is odd. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int number; 10 11 cout << "Enter an integer and I will tell you if it\n"; 12 cout << "is odd or even. "; 13 cin >> number; 14 if (number % 2 == 0) 15 cout << number << " is even.\n"; 16 else 17 cout << number << " is odd.\n"; 18 return 0; 19 } Program Output with Example Input Shown in Bold Enter an integer and I will tell you if it is odd or even. 17 [Enter] 17 is odd. The else part at the end of the if statement specifies a statement that is to be executed when the expression is false. When number % 2 does not equal 0, a message is printed indicating the number is odd. Note that the program will only take one of the two paths in the if/else statement. If you think of the statements in a computer program as steps 4.4 The if/else Statement 167 taken down a road, consider the if/else statement as a fork in the road. Instead of being a momentary detour, like an if statement, the if/else statement causes program execution to follow one of two exclusive paths. The flowchart in Figure 4-6 shows the logic of this if/else statement. Figure 4-6 True Indicate that the number is even. number % 2 == 0 False Indicate that the number is odd. Notice the programming style used to construct the if/else statement. The word else is at the same level of indention as if. The statement whose execution is controlled by else is indented one level. This visually depicts the two paths of execution that may be followed. Like the if part, the else part controls a single statement. If you wish to control more than one statement with the else part, create a block by writing the lines inside a set of braces. Program 4-9 shows this as a way of handling a classic programming problem: division by zero. Division by zero is mathematically impossible to perform, and it normally causes a program to crash. This means the program will prematurely stop running, sometimes with an error message. Program 4-9 shows a way to test the value of a divisor before the division takes place. Program 4-9 1 // This program asks the user for two numbers, num1 and num2. 2 // num1 is divided by num2 and the result is displayed. 3 // Before the division operation, however, num2 is tested 4 // for the value 0. If it contains 0, the division does not 5 // take place. 6 #include 7 using namespace std; 8 9 int main() 10 { 11 double num1, num2, quotient; 12 13 // Get the first number. 14 cout << "Enter a number: "; 15 cin >> num1; 16 17 // Get the second number. 18 cout << "Enter another number: "; (program continues) 168 Chapter 4 Making Decisions Program 4-9 (continued) 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 } cin >> num2; // If num2 is not zero, perform the division. if (num2 == 0) { cout << "Division by zero is not possible.\n"; cout << "Please run the program again and enter\n"; cout << "a number other than zero.\n"; } else { quotient = num1 / num2; cout << "The quotient of " << num1 << " divided by "; cout << num2 << " is " << quotient << ".\n"; } return 0; Program Output with Example Input Shown in Bold Enter a number: 10 [Enter] Enter another number: 0 [Enter] Division by zero is not possible. Please run the program again and enter a number other than zero. The value of num2 is tested in line 22 before the division is performed. If the user enters 0, the lines controlled by the if part execute, displaying a message that indicates that the program cannot perform a division by zero. Otherwise, the else part takes control, which divides num1 by num2 and displays the result. Checkpoint 4.12 TRUE or FALSE: The following if/else statements cause the same output to display. A) if (x > y) cout << "x is the greater.\n"; else cout << "x is not the greater.\n"; B) if (y <= x) cout << "x is not the greater.\n"; else cout << "x is the greater.\n"; 4.13 Write an if/else statement that assigns 1 to x if y is equal to 100. Otherwise it should assign 0 to x. 4.14 Write an if/else statement that assigns 0.10 to commissionRate unless sales is greater than or equal to 50000.00, in which case it assigns 0.20 to commissionRate. 4.5 Nested if Statements 169 4.5 Nested if Statements CONCEPT: To test more than one condition, an if statement can be nested inside another if statement. Sometimes an if statement must be nested inside another if statement. For example, consider a banking program that determines whether a bank customer qualifies for a special, low interest rate on a loan. To qualify, two conditions must exist: (1) the customer must be currently employed, and (2) the customer must have recently graduated from college (in the past two years). Figure 4-7 shows a flowchart for an algorithm that could be used in such a program. Figure 4-7 If we follow the flow of execution in the flowchart, we see that the expression employed == 'Y' is tested. If this expression is false, there is no need to perform further tests; we know that the customer does not qualify for the special interest rate. If the expression is true, however, we need to test the second condition. This is done with a nested decision structure that tests the expression recentGrad == 'Y'. If this expression is true, then the customer qualifies for the special interest rate. If this expression is false, then the customer does not qualify. Program 4-10 shows the code for the complete program. 170 Chapter 4 Making Decisions Program 4-10 1 // This program demonstrates the nested if statement. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 char employed, // Currently employed, Y or N 8 recentGrad; // Recent graduate, Y or N 9 10 // Is the user employed and a recent graduate? 11 cout << "Answer the following questions\n"; 12 cout << "with either Y for Yes or "; 13 cout << "N for No.\n"; 14 cout << "Are you employed? "; 15 cin >> employed; 16 cout << "Have you graduated from college "; 17 cout << "in the past two years? "; 18 cin >> recentGrad; 19 20 // Determine the user's loan qualifications. 21 if (employed == 'Y') 22 { 23 if (recentGrad == 'Y') //Nested if 24 { 25 cout << "You qualify for the special "; 26 cout << "interest rate.\n"; 27 } 28 } 29 return 0; 30 } Program Output with Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? Y [Enter] Have you graduated from college in the past two years? Y [Enter] You qualify for the special interest rate. Program Output with Different Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? Y [Enter] Have you graduated from college in the past two years? N [Enter] Look at the if statement that begins in line 21. It tests the expression employed == 'Y'. If this expression is true, the if statement that begins in line 23 is executed. Otherwise the program jumps to the return statement in line 29 and the program ends. Notice in the second sample execution of Program 4-10 that the program output does not inform the user whether he or she qualifies for the special interest rate. If the user enters an ‘N’ (or any character other than ‘Y’) for employed or recentGrad, the program does not 4.5 Nested if Statements 171 print a message letting the user know that he or she does not qualify. An else statement should be able to remedy this, as illustrated by Program 4-11. Program 4-11 1 // This program demonstrates the nested if statement. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 char employed, // Currently employed, Y or N 8 recentGrad; // Recent graduate, Y or N 9 10 // Is the user employed and a recent graduate? 11 cout << "Answer the following questions\n"; 12 cout << "with either Y for Yes or "; 13 cout << "N for No.\n"; 14 cout << "Are you employed? "; 15 cin >> employed; 16 cout << "Have you graduated from college "; 17 cout << "in the past two years? "; 18 cin >> recentGrad; 19 20 // Determine the user's loan qualifications. 21 if (employed == 'Y') 22 { 23 if (recentGrad == 'Y') // Nested if 24 { 25 cout << "You qualify for the special "; 26 cout << "interest rate.\n"; 27 } 28 else // Not a recent grad, but employed 29 { 30 cout << "You must have graduated from "; 31 cout << "college in the past two\n"; 32 cout << "years to qualify.\n"; 33 } 34 } 35 else // Not employed 36 { 37 cout << "You must be employed to qualify.\n"; 38 } 39 return 0; 40 } Program Output with Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? N [Enter] Have you graduated from college in the past two years? Y [Enter] You must be employed to qualify. (program output continues) 172 Chapter 4 Making Decisions Program 4-11 (continued) Program Output with Different Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? Y [Enter] Have you graduated from college in the past two years? N [Enter] You must have graduated from college in the past two years to qualify. Program Output with Different Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? Y [Enter] Have you graduated from college in the past two years? Y [Enter] You qualify for the special interest rate. In this version of the program, both if statements have else clauses that inform the user why he or she does not qualify for the special interest rate. Programming Style and Nested Decision Structures For readability and easier debugging, it’s important to use proper alignment and indentation in a set of nested if statements. This makes it easier to see which actions are performed by each part of the decision structure. For example, the following code is functionally equivalent to lines 21 through 38 in Program 4-11. Although this code is logically correct, it is very difficult to read and would be very difficult to debug because it is not properly indented. if (employed == 'Y') { if (recentGrad == 'Y') // Nested if { cout << "You qualify for the special "; cout << "interest rate.\n"; } else // Not a recent grad, but employed { cout << "You must have graduated from "; cout << "college in the past two\n"; cout << "years to qualify.\n"; } } else // Not employed { cout << "You must be employed to qualify.\n"; } Don’t write code like this! Proper indentation and alignment also makes it easier to see which if and else clauses belong together, as shown in Figure 4-8. Figure 4-8 4.5 Nested if Statements 173 Testing a Series of Conditions In the previous example you saw how a program can use nested decision structures to test more than one condition. It is not uncommon for a program to have a series of conditions to test and then perform an action depending on which condition is true. One way to accomplish this is to have a decision structure with numerous other decision structures nested inside it. For example, consider the program presented in the following In the Spotlight section. In the Spotlight: Multiple Nested Decision Structures Dr. Suarez teaches a literature class and uses the following 10-point grading scale for all of his exams: Test Score Grade 90 and above A 80–89 B 70–79 C 60–69 D Below 60 F He has asked you to write a program that will allow a student to enter a test score and then display the grade for that score. Here is the algorithm that you will use: Ask the user to enter a test score. Determine the grade in the following manner: If the score is greater than or equal to 90, then the grade is A. Otherwise, if the score is greater than or equal to 80, then the grade is B. Otherwise, if the score is greater than or equal to 70, then the grade is C. Otherwise, if the score is greater than or equal to 60, then the grade is D. Otherwise, the grade is F. 174 Chapter 4 Making Decisions You decide that the process of determining the grade will require several nested decisions structures, as shown in Figure 4-9. Program 4-12 shows the code for the complete program. The code for the nested decision structures is in lines 17 through 45. Figure 4-9 Nested decision structure to determine a grade False Display "Your grade is F." False score >= 90 True False score >= 80 True Display "Your grade is A." False score >= 70 True Display "Your grade is B." score >= 60 True Display "Your grade is C." Display "Your grade is D." Program 4-12 1 // This program uses nested if/else statements to assign a 2 // letter grade (A, B, C, D, or F) to a numeric test score. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Constants for grade thresholds 9 const int A_SCORE = 90, 10 B_SCORE = 80, 11 C_SCORE = 70, 12 D_SCORE = 60; 13 14 int testScore; // To hold a numeric test score 15 16 // Get the numeric test score. 17 cout << "Enter your numeric test score and I will\n"; 4.5 Nested if Statements 175 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 } cout << "tell you the letter grade you earned: "; cin >> testScore; // Determine the letter grade. if (testScore >= A_SCORE) { cout << "Your grade is A.\n"; } else { if (testScore >= B_SCORE) { cout << "Your grade is B.\n"; } else { if (testScore >= C_SCORE) { cout << "Your grade is C.\n"; } else { if (testScore >= D_SCORE) { cout << "Your grade is D.\n"; } else { cout << "Your grade is F.\n"; } } } } return 0; Program Output with Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: 78 [Enter] Your grade is C. Program Output with Different Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: 84 [Enter] Your grade is B. Checkpoint 4.15 If you executed the following code, what would it display if the user enters 5? What if the user enters 15? What if the user enters 30? What if the user enters −1? 176 Chapter 4 Making Decisions int number; cout << "Enter a number: "; cin >> number; if (number > 0) { cout << "Zero\n"; if (number > 10) { cout << "Ten\n"; if (number > 20) { cout << "Twenty\n"; } } } 4.6 The if/else if Statement CONCEPT: The if/else if statement tests a series of conditions. It is often simpler to test a series of conditions with the if/else if statement than with a set of nested if/else statements. VideoNote The if/else if Statement Even though Program 4-12 is a simple example, the logic of the nested decision structure is fairly complex. In C++, and many other languages, you can alternatively test a series of conditions using the if/else if statement. The if/else if statement makes certain types of nested decision logic simpler to write. Here is the general format of the if/else if statement: if (expression_1) { statement statement etc. } 6 If expression_1 is true these statements are executed, and the rest of the structure is ignored. else if (expression_2) { statement statement etc. 6 } Insert as many else if clauses as necessary Otherwise, if expression_2 is true these statements are executed, and the rest of the structure is ignored. else { statement statement etc. } 6 These statements are executed if none of the expressions above are true. 4.6 The if/else if Statement 177 When the statement executes, expression_1 is tested. If expression_1 is true, the block of statements that immediately follows is executed, and the rest of the structure is ignored. If expression_1 is false, however, the program jumps to the very next else if clause and tests expression_2. If it is true, the block of statements that immediately follows is executed, and then the rest of the structure is ignored. This process continues, from the top of the structure to the bottom, until one of the expressions is found to be true. If none of the expressions are true, the last else clause takes over, and the block of statements immediately following it is executed. The last else clause, which does not have an if statement following it, is referred to as the trailing else. The trailing else is optional, but in most cases you will use it. N OTE: The general format shows braces surrounding each block of conditionally executed statements. As with other forms of the if statement, the braces are required only when more than one statement is conditionally executed. Program 4-13 shows an example of the if/else if statement. This program is a modification of Program 4-12, which appears in the previous In the Spotlight section. Program 4-13 1 // This program uses an if/else if statement to assign a 2 // letter grade (A, B, C, D, or F) to a numeric test score. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Constants for grade thresholds 9 const int A_SCORE = 90, 10 B_SCORE = 80, 11 C_SCORE = 70, 12 D_SCORE = 60; 13 14 int testScore; // To hold a numeric test score 15 16 // Get the numeric test score. 17 cout << "Enter your numeric test score and I will\n" 18 << "tell you the letter grade you earned: "; 19 cin >> testScore; 20 21 // Determine the letter grade. 22 if (testScore >= A_SCORE) 23 cout << "Your grade is A.\n"; 24 else if (testScore >= B_SCORE) 25 cout << "Your grade is B.\n"; 26 else if (testScore >= C_SCORE) 27 cout << "Your grade is C.\n"; 28 else if (testScore >= D_SCORE) 29 cout << "Your grade is D.\n"; 30 else 31 cout << "Your grade is F.\n"; (program continues) 178 Chapter 4 Making Decisions Program 4-13 (continued) 32 33 34 } return 0; Program Output with Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: 78 [Enter] Your grade is C. Program Output with Different Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: 84 [Enter] Your grade is B. Let’s analyze how the if/else if statement in lines 22 through 31 works. First, the expression testScore >= A_SCORE is tested in line 22: ➔ if (testScore >= A_SCORE) cout << "Your grade is A.\n"; else if (testScore >= B_SCORE) cout << "Your grade is B.\n"; else if (testScore >= C_SCORE) cout << "Your grade is C.\n"; else if (testScore >= D_SCORE) cout << "Your grade is D.\n"; else cout << "Your grade is F.\n"; If testScore is greater than or equal to 90, the message "Your grade is A.\n" is displayed and the rest of the if/else if statement is skipped. If testScore is not greater than or equal to 90, the else clause in line 24 takes over and causes the next if statement to be executed: if (testScore >= A_SCORE) cout << "Your grade is A.\n"; ➔ else if (testScore >= B_SCORE) cout << "Your grade is B.\n"; else if (testScore >= C_SCORE) cout << "Your grade is C.\n"; else if (testScore >= D_SCORE) cout << "Your grade is D.\n"; else cout << "Your grade is F.\n"; The first if statement handles all of the grades greater than or equal to 90, so when this if statement executes, testScore will have a value of 89 or less. If testScore is greater than or equal to 80, the message "Your grade is B.\n" is displayed and the rest of the if/else if statement is skipped. This chain of events continues until one of the expressions is found to be true, or the last else clause at the end of the statement is encountered. Notice the alignment and indentation that is used with the if/else if statement: The starting if clause, the else if clauses, and the trailing else clause are all aligned, and the conditionally executed statements are indented. 4.6 The if/else if Statement 179 Using the Trailing else To Catch Errors The trailing else clause, which appears at the end of the if/else if statement, is optional, but in many situations you will use it to catch errors. For example, Program 4-13 will assign a grade to any number that is entered as the test score, including negative numbers. If a negative test score is entered, however, the user has probably made a mistake. We can modify the code as shown in Program 4-14 so the trailing else clause catches any test score that is less then 0 and displays an error message. Program 4-14 1 // This program uses an if/else if statement to assign a 2 // letter grade (A, B, C, D, or F) to a numeric test score. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Constants for grade thresholds 9 const int A_SCORE = 90, 10 B_SCORE = 80, 11 C_SCORE = 70, 12 D_SCORE = 60; 13 14 int testScore; // To hold a numeric test score 15 16 // Get the numeric test score. 17 cout << "Enter your numeric test score and I will\n" 18 << "tell you the letter grade you earned: "; 19 cin >> testScore; 20 21 // Determine the letter grade. 22 if (testScore >= A_SCORE) 23 cout << "Your grade is A.\n"; 24 else if (testScore >= B_SCORE) 25 cout << "Your grade is B.\n"; 26 else if (testScore >= C_SCORE) 27 cout << "Your grade is C.\n"; 28 else if (testScore >= D_SCORE) 29 cout << "Your grade is D.\n"; 30 else if (testScore >= 0) 31 cout << "Your grade is F.\n"; 32 else 33 cout << "Invalid test score.\n"; 34 35 return 0; 36 } Program Output with Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: −1 [Enter] Invalid test score. 180 Chapter 4 Making Decisions The if/else if Statement Compared to a Nested Decision Structure You never have to use the if/else if statement because its logic can be coded with nested if/else statements. However, a long series of nested if/else statements has two particular disadvantages when you are debugging code: • The code can grow complex and become difficult to understand. • Because indenting is important in nested statements, a long series of nested if/else statements can become too long to be displayed on the computer screen without horizontal scrolling. Also, long statements tend to “wrap around” when printed on paper, making the code even more difficult to read. The logic of an if/else if statement is usually easier to follow than that of a long series of nested if/else statements. And, because all of the clauses are aligned in an if/else if statement, the lengths of the lines in the statement tend to be shorter. Checkpoint 4.16 What will the following code display? int funny = 7, serious = 15; funny = serious % 2; if (funny != 1) { funny = 0; serious = 0; } else if (funny == 2) { funny = 10; serious = 10; } else { funny = 1; serious = 1; } cout << funny << "" << serious << endl; 4.17 The following code is used in a bookstore program to determine how many discount coupons a customer gets. Complete the table that appears after the program. int numBooks, numCoupons; cout << "How many books are being purchased? "; cin >> numBooks; if (numBooks < 1) numCoupons = 0; else if (numBooks < 3) numCoupons = 1; else if (numBooks < 5) numCoupons = 2; 4.7 Flags 181 else numCoupons = 3; cout << "The number of coupons to give is " << numCoupons << endl; If the customer purchases this many books 1 3 4 5 10 This many coupons are given. 4.7 Flags CONCEPT: A flag is a Boolean or integer variable that signals when a condition exists. A flag is typically a bool variable that signals when some condition exists in the program. When the flag variable is set to false, it indicates that the condition does not exist. When the flag variable is set to true, it means the condition does exist. For example, suppose a program that calculates sales commissions has a bool variable, defined and initialized as shown here: bool salesQuotaMet = false; In the program, the salesQuotaMet variable is used as a flag to indicate whether a salesperson has met the sales quota. When we define the variable, we initialize it with false because we do not yet know if the salesperson has met the sales quota. Assuming a variable named sales holds the amount of sales, code similar to the following might be used to set the value of the salesQuotaMet variable: if (sales >= QUOTA_AMOUNT) salesQuotaMet = true; else salesQuotaMet = false; As a result of this code, the salesQuotaMet variable can be used as a flag to indicate whether the sales quota has been met. Later in the program we might test the flag in the following way: if (salesQuotaMet) cout << "You have met your sales quota!\n"; This code displays You have met your sales quota! if the bool variable salesQuotaMet is true. Notice that we did not have to use the == operator to explicitly compare the salesQuotaMet variable with the value true. This code is equivalent to the following: if (salesQuotaMet == true) cout << "You have met your sales quota!\n"; 182 Chapter 4 Making Decisions Integer Flags Integer variables may also be used as flags. This is because in C++ the value 0 is considered false, and any nonzero value is considered true. In the sales commission program previously described, we could define the salesQuotaMet variable with the following statement: int salesQuotaMet = 0; // 0 means false. As before, we initialize the variable with 0 because we do not yet know if the sales quota has been met. After the sales have been calculated, we can use code similar to the following to set the value of the salesQuotaMet variable: if (sales >= QUOTA_AMOUNT) salesQuotaMet = 1; else salesQuotaMet = 0; Later in the program we might test the flag in the following way: if (salesQuotaMet) cout << "You have met your sales quota!\n"; 4.8 Logical Operators CONCEPT: Logical operators connect two or more relational expressions into one or reverse the logic of an expression. In the previous section you saw how a program tests two conditions with two if statements. In this section you will see how to use logical operators to combine two or more relational expressions into one. Table 4-6 lists C++’s logical operators. Table 4-6 Operator Meaning Effect && AND Connects two expressions into one. Both expressions must be true for the overall expression to be true. || OR Connects two expressions into one. One or both expressions must be true for the overall expression to be true. It is only necessary for one to be true, and it does not matter which. ! NOT The ! operator reverses the "truth" of an expression. It makes a true expression false, and a false expression true. The && Operator The && operator is known as the logical AND operator. It takes two expressions as operands and creates an expression that is true only when both sub-expressions are true. Here is an example of an if statement that uses the && operator: if (temperature < 20 && minutes > 12) cout << "The temperature is in the danger zone."; In the statement above the two relational expressions are combined into a single expression. The cout statement will only be executed if temperature is less than 20 AND minutes is greater than 12. If either relational test is false, the entire expression is false, and the cout statement is not executed. 4.8 Logical Operators 183 T I P : You must provide complete expressions on both sides of the && operator. For example, the following is not correct because the condition on the right side of the && operator is not a complete expression. temperature > 0 && < 100 The expression must be rewritten as temperature > 0 && temperature < 100 Table 4-7 shows a truth table for the && operator. The truth table lists all the possible combinations of values that two expressions may have, and the resulting value returned by the && operator connecting the two expressions. Table 4-7 Expression true && false false && true false && false true && true Value of Expression false (0) false (0) false (0) true (1) As the table shows, both sub-expressions must be true for the && operator to return a true value. N O T E : If the sub-expression on the left side of an && operator is false, the expression on the right side will not be checked. Since the entire expression is false if only one of the sub-expressions is false, it would waste CPU time to check the remaining expression. This is called short circuit evaluation. The && operator can be used to simplify programs that otherwise would use nested if statements. Program 4-15 performs a similar operation as Program 4-11, which qualifies a bank customer for a special interest rate. This program uses a logical operator. Program 4-15 1 // This program demonstrates the && logical operator. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 char employed, // Currently employed, Y or N 8 recentGrad; // Recent graduate, Y or N 9 10 // Is the user employed and a recent graduate? 11 cout << "Answer the following questions\n"; 12 cout << "with either Y for Yes or N for No.\n"; 13 (program continues) 184 Chapter 4 Making Decisions Program 4-15 (continued) 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 } cout << "Are you employed? "; cin >> employed; cout << "Have you graduated from college " << "in the past two years? "; cin >> recentGrad; // Determine the user's loan qualifications. if (employed == 'Y' && recentGrad == 'Y') { cout << "You qualify for the special " << "interest rate.\n"; } else { cout << "You must be employed and have\n" << "graduated from college in the\n" << "past two years to qualify.\n"; } return 0; Program Output with Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? Y [Enter] Have you graduated from college in the past two years? N [Enter] You must be employed and have graduated from college in the past two years to qualify. Program Output with Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? N [Enter] Have you graduated from college in the past two years? Y [Enter] You must be employed and have graduated from college in the past two years to qualify. Program Output with Example Input Shown in Bold Answer the following questions with either Y for Yes or N for No. Are you employed? Y [Enter] Have you graduated from college in the past two years? Y [Enter] You qualify for the special interest rate. The message “You qualify for the special interest rate” is displayed only when both the expressions employed == 'Y' and recentGrad == 'Y' are true. If either of these is false, the message “You must be employed and have graduated from college in the past two years to qualify.” is printed. 4.8 Logical Operators 185 NOTE: Although it is similar, Program 4-15 is not the logical equivalent of Program 4-11. For example, Program 4-15 doesn’t display the message “You must be employed to qualify.” The || Operator The || operator is known as the logical OR operator. It takes two expressions as operands and creates an expression that is true when either of the sub-expressions are true. Here is an example of an if statement that uses the || operator: if (temperature < 20 || temperature > 100) cout << "The temperature is in the danger zone."; The cout statement will be executed if temperature is less than 20 OR temperature is greater than 100. If either relational test is true, the entire expression is true and the cout statement is executed. T I P : You must provide complete expressions on both sides of the || operator. For example, the following is not correct because the condition on the right side of the || operator is not a complete expression. temperature < 0 || > 100 The expression must be rewritten as temperature < 0 || temperature > 100 Table 4-8 shows a truth table for the || operator. Table 4-8 Expression true || false false || true false || false true || true Value of the Expression true (1) true (1) false (0) true (1) All it takes for an OR expression to be true is for one of the sub-expressions to be true. It doesn’t matter if the other sub-expression is false or true. N O T E : The || operator also performs short circuit evaluation. If the sub-expression on the left side of an || operator is true, the expression on the right side will not be checked. Since it’s only necessary for one of the sub-expressions to be true, it would waste CPU time to check the remaining expression. Program 4-16 performs different tests to qualify a person for a loan. This one determines if the customer earns at least $35,000 per year, or has been employed for more than five years. 186 Chapter 4 Making Decisions Program 4-16 1 // This program demonstrates the logical || operator. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Constants for minimum income and years 8 const double MIN_INCOME = 35000.0; 9 const int MIN_YEARS = 5; 10 11 double income; // Annual income 12 int years; // Years at the current job 13 14 // Get the annual income 15 cout << "What is your annual income? "; 16 cin >> income; 17 18 // Get the number of years at the current job. 19 cout << "How many years have you worked at " 20 << "your current job? "; 21 cin >> years; 22 23 // Determine the user's loan qualifications. 24 if (income >= MIN_INCOME || years > MIN_YEARS) 25 cout << "You qualify.\n"; 26 else 27 { 28 cout << "You must earn at least $" 29 << MIN_INCOME << " or have been " 30 << "employed more than " << MIN_YEARS 31 << " years.\n"; 32 } 33 return 0; 34 } Program Output with Example Input Shown in Bold What is your annual income? 40000 [Enter] How many years have you worked at your current job? 2 [Enter] You qualify. Program Output with Example Input Shown in Bold What is your annual income? 20000 [Enter] How many years have you worked at your current job? 7 [Enter] You qualify. Program Output with Example Input Shown in Bold What is your annual income? 30000 [Enter] How many years have you worked at your current job? 3 [Enter] You must earn at least $35000 or have been employed more than 5 years. 4.8 Logical Operators 187 The message “You qualify\n.” is displayed when either or both the expressions income >= 35000 or years > 5 are true. If both of these are false, the disqualifying message is printed. The ! Operator The ! operator performs a logical NOT operation. It takes an operand and reverses its truth or falsehood. In other words, if the expression is true, the ! operator returns false, and if the expression is false, it returns true. Here is an if statement using the ! operator: if (!(temperature > 100)) cout << "You are below the maximum temperature.\n"; First, the expression (temperature > 100) is tested to be true or false. Then the ! operator is applied to that value. If the expression (temperature > 100) is true, the ! operator returns false. If it is false, the ! operator returns true. In the example, it is equivalent to asking “is the temperature not greater than 100?” Table 4-9 shows a truth table for the ! operator. Table 4-9 Expression !true !false Value of the Expression false (0) true (1) Program 4-17 performs the same task as Program 4-16. The if statement, however, uses the ! operator to determine if the user does not make at least $35,000 or has not been on the job more than five years. Program 4-17 1 // This program demonstrates the logical ! operator. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Constants for minimum income and years 8 const double MIN_INCOME = 35000.0; 9 const int MIN_YEARS = 5; 10 11 double income; // Annual income 12 int years; // Years at the current job 13 14 // Get the annual income 15 cout << "What is your annual income? "; 16 cin >> income; 17 18 // Get the number of years at the current job. 19 cout << "How many years have you worked at " 20 << "your current job? "; (program continues) 188 Chapter 4 Making Decisions Program 4-17 (continued) 21 22 23 24 25 26 27 28 29 30 31 32 33 34 } cin >> years; // Determine the user's loan qualifications. if (!(income >= MIN_INCOME || years > MIN_YEARS)) { cout << "You must earn at least $" << MIN_INCOME << " or have been " << "employed more than " << MIN_YEARS << " years.\n"; } else cout << "You qualify.\n"; return 0; The output of Program 4-17 is the same as Program 4-16. Precedence and Associativity of Logical Operators Table 4-10 shows the precedence of C++’s logical operators, from highest to lowest. Table 4-10 Logical Operators in Order of Precedence ! && || The ! operator has a higher precedence than many of the C++ operators. To avoid an error, you should always enclose its operand in parentheses unless you intend to apply it to a variable or a simple expression with no other operators. For example, consider the following expressions: !(x > 2) !x > 2 The first expression applies the ! operator to the expression x > 2. It is asking, “Is x not greater than 2?” The second expression, however, applies the ! operator to x only. It is asking, “Is the logical negation of x greater than 2?” Suppose x is set to 5. Since 5 is nonzero, it would be considered true, so the ! operator would reverse it to false, which is 0. The > operator would then determine if 0 is greater than 2. To avoid a catastrophe like this, always use parentheses! The && and || operators rank lower in precedence than the relational operators, so precedence problems are less likely to occur. If you feel unsure, however, it doesn’t hurt to use parentheses anyway. (a > b) && (x < y) is the same as a > b && x < y (x == y) || (b > a) is the same as x == y || b > a 4.9 Checking Numeric Ranges with Logical Operators 189 The logical operators have left-to-right associativity. In the following expression, a < b is evaluated before y == z. a < b || y == z In the following expression, y == z is evaluated first, however, because the && operator has higher precedence than ||. a < b || y == z && m > j The expression is equivalent to (a < b) || ((y == z) && (m > j)) 4.9 Checking Numeric Ranges with Logical Operators CONCEPT: Logical operators are effective for determining whether a number is in or out of a range. When determining whether a number is inside a numeric range, it’s best to use the && operator. For example, the following if statement checks the value in x to determine whether it is in the range of 20 through 40: if (x >= 20 && x <= 40) cout << x << " is in the acceptable range.\n"; The expression in the if statement will be true only when x is both greater than or equal to 20 AND less than or equal to 40. x must be within the range of 20 through 40 for this expression to be true. When determining whether a number is outside a range, the || operator is best to use. The following statement determines whether x is outside the range of 20 to 40: if (x < 20 || x > 40) cout << x << " is outside the acceptable range.\n"; It’s important not to get the logic of these logical operators confused. For example, the following if statement would never test true: if (x < 20 && x > 40) cout << x << " is outside the acceptable range.\n"; Obviously, x cannot be less than 20 and at the same time greater than 40. N O T E : C++ does not allow you to check numeric ranges with expressions such as 5 < x < 20. Instead, you must use a logical operator to connect two relational expressions, as previously discussed. Checkpoint 4.18 The following truth table shows various combinations of the values true and false connected by a logical operator. Complete the table by indicating if the result of such a combination is TRUE or FALSE. 190 Chapter 4 Making Decisions Logical Expression true && false true && true false && true false && false true || false true || true false || true false || false !true !false Result (true or false) 4.19 4.20 4.21 Assume the variables a = 2, b = 4, and c = 6. Indicate by circling the T or F if each of the following conditions is true or false: a == 4 || b > 2 6 <= c && a > 3 1 != b && c != 3 a >= -1 || a <= b !(a > 2) TF TF TF TF TF Write an if statement that prints the message “The number is valid” if the variable speed is within the range 0 through 200. Write an if statement that prints the message “The number is not valid” if the variable speed is outside the range 0 through 200. 4.10 Menus CONCEPT: You can use nested if/else statements or the if/else if statement to create menu-driven programs. A menu-driven program allows the user to determine the course of action by selecting it from a list of actions. A menu is a screen displaying a set of choices the user selects from. For example, a program that manages a mailing list might give you the following menu: 1. Add a name to the list. 2. Remove a name from the list. 3. Change a name in the list. 4. Print the list. 5. Quit the program. The user selects one of the operations by entering its number. Entering 4, for example, causes the mailing list to be printed, and entering 5 causes the program to end. Nested if/else statements or an if/else if structure can be used to set up such a menu. After the user enters a number, the program compares the number with the available selections and executes the statements that perform that operation. Program 4-18 calculates the charges for membership in a health club. The club has three membership packages to choose from: standard adult membership, child membership, and 4.10 Menus 191 senior citizen membership. The program presents a menu that allows the user to choose the desired package and then calculates the cost of the membership. Program 4-18 1 // This program displays a menu and asks the user to make a 2 // selection. An if/else if statement determines which item 3 // the user has chosen. 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 int choice; // To hold a menu choice 11 int months; // To hold the number of months 12 double charges; // To hold the monthly charges 13 14 // Constants for membership rates 15 const double ADULT = 40.0, 16 SENIOR = 30.0, 17 CHILD = 20.0; 18 19 // Constants for menu choices 20 const int ADULT_CHOICE = 1, 21 CHILD_CHOICE = 2, 22 SENIOR_CHOICE = 3, 23 QUIT_CHOICE = 4; 24 25 // Display the menu and get a choice. 26 cout << "\t\tHealth Club Membership Menu\n\n" 27 << "1. Standard Adult Membership\n" 28 << "2. Child Membership\n" 29 << "3. Senior Citizen Membership\n" 30 << "4. Quit the Program\n\n" 31 << "Enter your choice: "; 32 cin >> choice; 33 34 // Set the numeric output formatting. 35 cout << fixed << showpoint << setprecision(2); 36 37 // Respond to the user's menu selection. 38 if (choice == ADULT_CHOICE) 39 { 40 cout << "For how many months? "; 41 cin >> months; 42 charges = months * ADULT; 43 cout << "The total charges are $" << charges << endl; 44 } 45 else if (choice == CHILD_CHOICE) 46 { 47 cout << "For how many months? "; (program continues) 192 Chapter 4 Making Decisions Program 4-18 (continued) 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 } cin >> months; charges = months * CHILD; cout << "The total charges are $" << charges << endl; } else if (choice == SENIOR_CHOICE) { cout << "For how many months? "; cin >> months; charges = months * SENIOR; cout << "The total charges are $" << charges << endl; } else if (choice == QUIT_CHOICE) { cout << "Program ending.\n"; } else { cout << "The valid choices are 1 through 4. Run the\n" << "program again and select one of those.\n"; } return 0; Program Output with Example Input Shown in Bold Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 3 [Enter] For how many months? 6 [Enter] The total charges are $180.00 Let’s take a closer look at the program: • Lines 10–12 define the following variables: • The choice variable will hold the user’s menu choice. • The months variable will hold the number of months of health club membership. • The charges variable will hold the total charges. • Lines 15–17 define named constants for the monthly membership rates for adult, senior citizen, and child memberships. • Lines 20–23 define named constants for the menu choices. • Lines 26–32 display the menu and get the user’s choice. • Line 35 sets the numeric output formatting for floating point numbers. • Lines 38–67 is an if/else if statement that determines the user’s menu choice in the following manner: • If the user selected 1 from the menu (adult membership), the statements in lines 40–43 are executed. 4.11 Focus on Software Engineering: Validating User Input 193 • Otherwise, if the user selected 2 from the menu (child membership), the statements in lines 47–50 are executed. • Otherwise, if the user selected 3 from the menu (senior citizen membership), the statements in lines 54–57 are executed. • Otherwise, if the user selected 4 from the menu (quit the program), the statement in line 61 is executed. • If the user entered any choice other than 1, 2, 3, or 4, the else clause in lines 63–67 executes, displaying an error message. 4.11 Focus on Software Engineering: Validating User Input CONCEPT: As long as the user of a program enters bad input, the program will produce bad output. Programs should be written to filter out bad input. Perhaps the most famous saying of the computer world is “Garbage in, garbage out.” The integrity of a program’s output is only as good as its input, so you should try to make sure garbage does not go into your programs. Input validation is the process of inspecting data given to a program by the user and determining if it is valid. A good program should give clear instructions about the kind of input that is acceptable and not assume the user has followed those instructions. Here are just a few examples of input validations performed by programs: • Numbers are checked to ensure they are within a range of possible values. For example, there are 168 hours in a week. It is not possible for a person to be at work longer than 168 hours in one week. • Values are checked for their “reasonableness.” Although it might be possible for a person to be at work for 168 hours per week, it is not probable. • Items selected from a menu or other sets of choices are checked to ensure they are available options. • Variables are checked for values that might cause problems, such as division by zero. Program 4-19 is a test scoring program that rejects any test score less than 0 or greater than 100. Program 4-19 1 // This test scoring program does not accept test scores 2 // that are less than 0 or greater than 100. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Constants for grade thresholds 9 const int A_SCORE = 90, (program continues) 194 Chapter 4 Making Decisions Program 4-19 (continued) 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 } B_SCORE = 80, C_SCORE = 70, D_SCORE = 60, MIN_SCORE = 0, MAX_SCORE = 100; // Minimum valid score // Maximum valid score int testScore; // To hold a numeric test score // Get the numeric test score. cout << "Enter your numeric test score and I will\n" << "tell you the letter grade you earned: "; cin >> testScore; // Validate the input and determine the grade. if (testScore >= MIN_SCORE && testScore <= MAX_SCORE) { // Determine the letter grade. if (testScore >= A_SCORE) cout << "Your grade is A.\n"; else if (testScore >= B_SCORE) cout << "Your grade is B.\n"; else if (testScore >= C_SCORE) cout << "Your grade is C.\n"; else if (testScore >= D_SCORE) cout << "Your grade is D.\n"; else cout << "Your grade is F.\n"; } else { // An invalid score was entered. cout << "That is an invalid score. Run the program\n" << "again and enter a value in the range of\n" << MIN_SCORE << " through " << MAX_SCORE << ".\n"; } return 0; Program Output with Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: −1 [Enter] That is an invalid score. Run the program again and enter a value in the range of 0 through 100. Program Output with Example Input Shown in Bold Enter your numeric test score and I will tell you the letter grade you earned: 81 [Enter] Your grade is B. 4.12 Comparing Characters and Strings 195 4.12 Comparing Characters and Strings CONCEPT: Relational operators can also be used to compare characters and string objects. Earlier in this chapter you learned to use relational operators to compare numeric values. They can also be used to compare characters and string objects. Comparing Characters As you learned in Chapter 3, characters are actually stored in memory as integers. On most systems, this integer is the ASCII value of the character. For example, the letter ‘A’ is represented by the number 65, the letter ‘B’ is represented by the number 66, and so on. Table 4-11 shows the ASCII numbers that correspond to some of the commonly used characters. Table 4-11 ASCII Values of Commonly Used Characters Character ASCII Value ‘0’ – ‘9’ ‘A’ – ‘Z’ ‘a’ – ‘z’ blank period 48 – 57 65 – 90 97 – 122 32 46 Notice that every character, even the blank, has an ASCII code associated with it. Notice also that the ASCII code of a character representing a digit, such as '1' or '2', is not the same as the value of the digit itself. A complete table showing the ASCII values for all characters can be found in Appendix B. When two characters are compared, it is actually their ASCII values that are being compared. 'A' < 'B' because the ASCII value of 'A' (65) is less than the ASCII value of 'B' (66). Likewise '1' < '2' because the ASCII value of '1' (49) is less than the ASCII value of '2' (50). However, as shown in Table 4-11, lowercase letters have higher ASCII codes than uppercase letters, so 'a' > 'Z'. Program 4-20 shows how characters can be compared with relational operators. Program 4-20 1 // This program demonstrates how characters can be 2 // compared with the relational operators. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char ch; 9 (program continues) 196 Chapter 4 Making Decisions Program 4-20 (continued) 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 } // Get a character from the user. cout << "Enter a digit or a letter: "; ch = cin.get(); // Determine what the user entered. if (ch >= '0' && ch <= '9') cout << "You entered a digit.\n"; else if (ch >= 'A' && ch <= 'Z') cout << "You entered an uppercase letter.\n"; else if (ch >= 'a' && ch <= 'z') cout << "You entered a lowercase letter.\n"; else cout << "That is not a digit or a letter.\n"; return 0; Program Output with Example Input Shown in Bold Enter a digit or a letter: t [Enter] You entered a lowercase letter. Program Output with Example Input Shown in Bold Enter a digit or a letter: V [Enter] You entered an uppercase letter. Program Output with Example Input Shown in Bold Enter a digit or a letter: 5 [Enter] You entered a digit. Program Output with Example Input Shown in Bold Enter a digit or a letter: & [Enter] That is not a digit or a letter. Comparing string Objects string objects can also be compared with relational operators. As with individual characters, when two string objects are compared, it is actually the ASCII value of the characters making up the strings that are being compared. For example, assume the following definitions exist in a program: string str1 = "ABC"; string str2 = "XYZ"; The string object str1 is considered less than the string object str2 because the characters "ABC" alphabetically precede (have lower ASCII values than) the characters "XYZ". So, the following if statement will cause the message "str1 is less than str2." to be displayed on the screen. if (str1 < str2) cout << "str1 is less than str2."; 4.12 Comparing Characters and Strings 197 One by one, each character in the first operand is compared with the character in the corresponding position in the second operand. If all the characters in both string objects match, the two strings are equal. Other relationships can be determined if two characters in corresponding positions do not match. The first operand is less than the second operand if the first mismatched character in the first operand is less than its counterpart in the second operand. Likewise, the first operand is greater than the second operand if the first mismatched character in the first operand is greater than its counterpart in the second operand. For example, assume a program has the following definitions: string name1 = "Mary"; string name2 = "Mark"; The value in name1, "Mary", is greater than the value in name2, "Mark". This is because the first three characters in name1 have the same ASCII values as the first three characters in name2, but the 'y' in the fourth position of "Mary" has a greater ASCII value than the 'k' in the corresponding position of "Mark". Any of the relational operators can be used to compare two string objects. Here are some of the valid comparisons of name1 and name2. name1 > name2 // true name1 <= name2 // false name1 != name2 // true string objects can also, of course, be compared to string literals: name1 < "Mary Jane" // true Program 4-21 further demonstrates how relational operators can be used with string objects. Program 4-21 1 // This program uses relational operators to compare a string 2 // entered by the user with valid stereo part numbers. 3 #include 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 const double PRICE_A = 249.0, 11 PRICE_B = 299.0; 12 13 string partNum; // Holds a stereo part number 14 15 // Display available parts and get the user's selection 16 cout << "The stereo part numbers are:\n" 17 << "Boom Box: part number S-29A \n" 18 << "Shelf Model: part number S-29B \n" 19 << "Enter the part number of the stereo you\n" (program continues) 198 Chapter 4 Making Decisions Program 4-21 (continued) 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 } << "wish to purchase: "; cin >> partNum; // Set the numeric output formatting cout << fixed << showpoint << setprecision(2); // Determine and display the correct price if (partNum == "S-29A") cout << "The price is $" << PRICE_A << endl; else if (partNum == "S-29B") cout << "The price is $" << PRICE_B << endl; else cout << partNum << " is not a valid part number.\n"; return 0; Program Output with Example Input Shown in Bold The stereo part numbers are: Boom Box: part number S-29A Shelf Model: part number S-29B Enter the part number of the stereo you wish to purchase: S-29A [Enter] The price is $249.00 Checkpoint 4.22 Indicate whether each of the following relational expressions is true or false. Refer to the ASCII table in Appendix B if necessary. A) 'a' < 'z' B) 'a' == 'A' C) '5' < '7' D) 'a' < 'A' E) '1' == 1 F) '1' == 49 4.23 Indicate whether each of the following relational expressions is true or false. Refer to the ASCII table in Appendix B if necessary. A) "Bill" == "BILL" B) "Bill" < "BILL" C) "Bill" < "Bob" D) "189" > "23" E) "189" > "Bill" F) "Mary" < "MaryEllen" G) "MaryEllen" < "Mary Ellen" 4.13 The Conditional Operator 199 4.13 The Conditional Operator CONCEPT: You can use the conditional operator to create short expressions that work like if/else statements. The conditional operator is powerful and unique. It provides a shorthand method of expressing a simple if/else statement. The operator consists of the question-mark (?) and the colon (:). Its format is: expression ? expression : expression; Here is an example of a statement using the conditional operator: x < 0 ? y = 10 : z = 20; The statement above is called a conditional expression and consists of three sub-expressions separated by the ? and : symbols. The expressions are x < 0, y = 10, and z = 20, as illustrated here: x < 0 ? y = 10 : z = 20; N OTE: Since it takes three operands, the conditional operator is considered a ternary operator. The conditional expression above performs the same operation as the following if/else statement: if (x < 0) y = 10; else z = 20; The part of the conditional expression that comes before the question mark is the expression to be tested. It’s like the expression in the parentheses of an if statement. If the expression is true, the part of the statement between the ? and the : is executed. Otherwise, the part after the : is executed. Figure 4-10 illustrates the roles played by the three sub-expressions. Figure 4-10 1st Expression: Expression to be tested. 3rd Expression: Executes if the 1st expression is false. x < 0 ? y = 10 : z = 20; 2nd Expression: Executes if the 1st expression is true. 200 Chapter 4 Making Decisions If it helps, you can put parentheses around the sub-expressions, as in the following: (x < 0) ? (y = 10) : (z = 20); Using the Value of a Conditional Expression Remember, in C++ all expressions have a value, and this includes the conditional expression. If the first sub-expression is true, the value of the conditional expression is the value of the second sub-expression. Otherwise it is the value of the third sub-expression. Here is an example of an assignment statement using the value of a conditional expression: a = x > 100 ? 0 : 1; The value assigned to a will be either 0 or 1, depending upon whether x is greater than 100. This statement could be expressed as the following if/else statement: if (x > 100) a = 0; else a = 1; Program 4-22 can be used to help a consultant calculate her charges. Her rate is $50.00 per hour, but her minimum charge is for five hours. The conditional operator is used in a statement that ensures the number of hours does not go below five. Program 4-22 1 // This program calculates a consultant's charges at $50 2 // per hour, for a minimum of 5 hours. The ?: operator 3 // adjusts hours to 5 if less than 5 hours were worked. 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 const double PAY_RATE = 50.0; // Hourly pay rate 11 const int MIN_HOURS = 5; // Minimum billable hours 12 double hours, // Hours worked 13 charges; // Total charges 14 15 // Get the hours worked. 16 cout << "How many hours were worked? "; 17 cin >> hours; 18 19 // Determine the hours to charge for. 20 hours = hours < MIN_HOURS ? MIN_HOURS : hours; 21 22 // Calculate and display the charges. 23 charges = PAY_RATE * hours; 24 cout << fixed << showpoint << setprecision(2) 25 << "The charges are $" << charges << endl; 26 return 0; 27 } 4.13 The Conditional Operator 201 Program Output with Example Input Shown in Bold How many hours were worked? 10 [Enter] The charges are $500.00 Program Output with Example Input Shown in Bold How many hours were worked? 2 [Enter] The charges are $250.00 Notice that in line 11 a constant named MIN_HOURS is defined to represent the minimum number of hours, which is 5. Here is the statement in line 20, with the conditional expression: hours = hours < MIN_HOURS ? MIN_HOURS : hours; If the value in hours is less than 5, then 5 is stored in hours. Otherwise hours is assigned the value it already has. The hours variable will not have a value less than 5 when it is used in the next statement, which calculates the consultant’s charges. As you can see, the conditional operator gives you the ability to pack decision-making power into a concise line of code. With a little imagination it can be applied to many other programming problems. For instance, consider the following statement: cout << "Your grade is: " << (score < 60 ? "Fail." : "Pass."); If you were to use an if/else statement, the statement above would be written as follows: if (score < 60) cout << "Your grade is: Fail."; else cout << "Your grade is: Pass."; N O T E : The parentheses are placed around the conditional expression because the << operator has higher precedence than the ?: operator. Without the parentheses, just the value of the expression score < 60 would be sent to cout. Checkpoint 4.24 Rewrite the following if/else statements as conditional expressions: A) if (x > y) z = 1; else z = 20; B) if (temp > 45) population = base * 10; else population = base * 2; C) if (hours > 40) wages *= 1.5; else wages *= 1; D) if (result >= 0) cout << "The result is positive\n"; else cout << "The result is negative.\n"; 202 Chapter 4 Making Decisions 4.25 4.26 The following statements use conditional expressions. Rewrite each with an if/ else statement. A) j = k > 90 ? 57 : 12; B) factor = x >= 10 ? y * 22 : y * 35; C) total += count == 1 ? sales : count * sales; D) cout << (((num % 2) == 0) ? "Even\n" : "Odd\n"); What will the following program display? #include using namespace std; int main() { const int UPPER = 8, LOWER = 2; int num1, num2, num3 = 12, num4 = 3; num1 = num3 < num4 ? UPPER : LOWER; num2 = num4 > UPPER ? num3 : LOWER; cout << num1 << " " << num2 << endl; return 0; } 4.14 The switch Statement CONCEPT: The switch statement lets the value of a variable or expression determine where the program will branch. A branch occurs when one part of a program causes another part to execute. The if/else if statement allows your program to branch into one of several possible paths. It performs a series of tests (usually relational) and branches when one of these tests is true. The switch statement is a similar mechanism. It, however, tests the value of an integer expression and then uses that value to determine which set of statements to branch to. Here is the format of the switch statement: switch (IntegerExpression) { case ConstantExpression: // place one or more // statements here case ConstantExpression: // place one or more // statements here // case statements may be repeated as many // times as necessary default: // place one or more // statements here } 4.14 The switch Statement 203 The first line of the statement starts with the word switch, followed by an integer expression inside parentheses. This can be either of the following: • a variable of any of the integer data types (including char) • an expression whose value is of any of the integer data types On the next line is the beginning of a block containing several case statements. Each case statement is formatted in the following manner: case ConstantExpression: // place one or more // statements here After the word case is a constant expression (which must be of an integer type), followed by a colon. The constant expression may be an integer literal or an integer named constant. The case statement marks the beginning of a section of statements. The program branches to these statements if the value of the switch expression matches that of the case expression. W A R N I N G ! The expression of each case statement in the block must be unique. NOTE: The expression following the word case must be an integer literal or constant. It cannot be a variable, and it cannot be an expression such as x < 22 or n == 50. An optional default section comes after all the case statements. The program branches to this section if none of the case expressions match the switch expression. So, it functions like a trailing else in an if/else if statement. Program 4-23 shows how a simple switch statement works. Program 4-23 1 // The switch statement in this program tells the user something 2 // he or she already knows: the data just entered! 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char choice; 9 10 cout << "Enter A, B, or C: "; 11 cin >> choice; 12 switch (choice) 13 { 14 case 'A': cout << "You entered A.\n"; 15 break; 16 case 'B': cout << "You entered B.\n"; 17 break; 18 case 'C': cout << "You entered C.\n"; 19 break; 20 default: cout << "You did not enter A, B, or C!\n"; 21 } 22 return 0; 23 } (program continues) 204 Chapter 4 Making Decisions Program 4-23 (continued) Program Output with Example Input Shown in Bold Enter A, B, or C: B [Enter] You entered B. Program Output with Example Input Shown in Bold Enter A, B, or C: F [Enter] You did not enter A, B, or C! The first case statement is case 'A':, the second is case 'B':, and the third is case 'C':. These statements mark where the program is to branch to if the variable choice contains the values 'A', 'B', or 'C'. (Remember, character variables and literals are considered integers.) The default section is branched to if the user enters anything other than A, B, or C. Notice the break statements that are in the case 'A', case 'B', and case 'C' sections. switch (choice) { case 'A':cout << "You entered A.\n"; break; case 'B':cout << "You entered B.\n"; break; case 'C':cout << "You entered C.\n"; break; default: cout << "You did not enter A, B, or C!\n"; } The case statements show the program where to start executing in the block and the break statements show the program where to stop. Without the break statements, the program would execute all of the lines from the matching case statement to the end of the block. N O T E : The default section (or the last case section, if there is no default) does not need a break statement. Some programmers prefer to put one there anyway, for consistency. Program 4-24 is a modification of Program 4-23, without the break statements. Program 4-24 1 // The switch statement in this program tells the user something 2 // he or she already knows: the data just entered! 3 #include 4 using namespace std; 5 6 int main() 7{ 8 char choice; 9 4.14 The switch Statement 205 10 11 12 13 14 15 16 17 18 19 20 21 22 } cout << "Enter A, B, or C: "; cin >> choice; // The following switch is // missing its break statements! switch (choice) { case 'A': cout << "You entered A.\n"; case 'B': cout << "You entered B.\n"; case 'C': cout << "You entered C.\n"; default: cout << "You did not enter A, B, or C!\n"; } return 0; Program Output with Example Input Shown in Bold Enter A, B, or C: A [Enter] You entered A. You entered B. You entered C. You did not enter A, B, or C! Program Output with Example Input Shown in Bold Enter A, B, or C: C [Enter] You entered C. You did not enter A, B, or C! Without the break statement, the program “falls through” all of the statements below the one with the matching case expression. Sometimes this is what you want. Program 4-25 lists the features of three TV models a customer may choose from. The Model 100 has remote control. The Model 200 has remote control and stereo sound. The Model 300 has remote control, stereo sound, and picture-in-a-picture capability. The program uses a switch statement with carefully omitted breaks to print the features of the selected model. Program 4-25 1 // This program is carefully constructed to use the "fall through" 2 // feature of the switch statement. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int modelNum; // Model number 9 10 // Get a model number from the user. 11 cout << "Our TVs come in three models:\n"; 12 cout << "The 100, 200, and 300. Which do you want? "; 13 cin >> modelNum; 14 (program continues) 206 Chapter 4 Making Decisions Program 4-25 (continued) 15 16 17 18 19 20 21 22 23 24 25 26 27 } // Display the model's features. cout << "That model has the following features:\n"; switch (modelNum) { case 300: cout << "\tPicture-in-a-picture.\n"; case 200: cout << "\tStereo sound.\n"; case 100: cout << "\tRemote control.\n"; break; default: cout << "You can only choose the 100,"; cout << "200, or 300.\n"; } return 0; Program Output with Example Input Shown in Bold Our TVs come in three models: The 100, 200, and 300. Which do you want? 100 [Enter] That model has the following features: Remote control. Program Output with Example Input Shown in Bold Our TVs come in three models: The 100, 200, and 300. Which do you want? 200 [Enter] That model has the following features: Stereo sound. Remote control. Program Output with Example Input Shown in Bold Our TVs come in three models: The 100, 200, and 300. Which do you want? 300 [Enter] That model has the following features: Picture-in-a-picture. Stereo sound. Remote control. Program Output with Example Input Shown in Bold Our TVs come in three models: The 100, 200, and 300. Which do you want? 500 [Enter] That model has the following features: You can only choose the 100, 200, or 300. Another example of how useful this “fall through” capability can be is when you want the program to branch to the same set of statements for multiple case expressions. For instance, Program 4-26 asks the user to select a grade of pet food. The available choices are A, B, and C. The switch statement will recognize either upper or lowercase letters. Program 4-26 1 // The switch statement in this program uses the "fall through" 2 // feature to catch both uppercase and lowercase letters entered 3 // by the user. 4 #include 4.14 The switch Statement 207 5 using namespace std; 6 7 int main() 8{ 9 char feedGrade; 10 11 // Get the desired grade of feed. 12 cout << "Our pet food is available in three grades:\n"; 13 cout << "A, B, and C. Which do you want pricing for? "; 14 cin >> feedGrade; 15 16 // Display the price. 17 switch(feedGrade) 18 { 19 case 'a': 20 case 'A': cout << "30 cents per pound.\n"; 21 break; 22 case 'b': 23 case 'B': cout << "20 cents per pound.\n"; 24 break; 25 case 'c': 26 case 'C': cout << "15 cents per pound.\n"; 27 break; 28 default: cout << "That is an invalid choice.\n"; 29 } 30 return 0; 31 } Program Output with Example Input Shown in Bold Our pet food is available in three grades: A, B, and C. Which do you want pricing for? b [Enter] 20 cents per pound. Program Output with Example Input Shown in Bold Our pet food is available in three grades: A, B, and C. Which do you want pricing for? B [Enter] 20 cents per pound. When the user enters 'a' the corresponding case has no statements associated with it, so the program falls through to the next case, which corresponds with 'A'. case 'a': case 'A': cout << "30 cents per pound.\n"; break; The same technique is used for 'b' and 'c'. Using switch in Menu Systems The switch statement is a natural mechanism for building menu systems. Recall that Program 4-18 gives a menu to select which health club package the user wishes to purchase. The program uses if/else if statements to determine which package the user has selected and displays the calculated charges. Program 4-27 is a modification of that program, using a switch statement instead of if/else if. 208 Chapter 4 Making Decisions Program 4-27 1 // This program uses a switch statement to determine 2 // the item selected from a menu. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int choice; // To hold a menu choice 10 int months; // To hold the number of months 11 double charges; // To hold the monthly charges 12 13 // Constants for membership rates 14 const double ADULT = 40.0, 15 CHILD = 20.0, 16 SENIOR = 30.0; 17 18 // Constants for menu choices 19 const int ADULT_CHOICE = 1, 20 CHILD_CHOICE = 2, 21 SENIOR_CHOICE = 3, 22 QUIT_CHOICE = 4; 23 24 // Display the menu and get a choice. 25 cout << "\t\tHealth Club Membership Menu\n\n" 26 << "1. Standard Adult Membership\n" 27 << "2. Child Membership\n" 28 << "3. Senior Citizen Membership\n" 29 << "4. Quit the Program\n\n" 30 << "Enter your choice: "; 31 cin >> choice; 32 33 // Set the numeric output formatting. 34 cout << fixed << showpoint << setprecision(2); 35 36 // Respond to the user's menu selection. 37 switch (choice) 38 { 39 case ADULT_CHOICE: 40 cout << "For how many months? "; 41 cin >> months; 42 charges = months * ADULT; 43 cout << "The total charges are $" << charges << endl; 44 break; 45 46 case CHILD_CHOICE: 47 cout << "For how many months? "; 48 cin >> months; 49 charges = months * CHILD; 50 cout << "The total charges are $" << charges << endl; 51 break; 52 4.14 The switch Statement 209 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 } case SENIOR_CHOICE: cout << "For how many months? "; cin >> months; charges = months * SENIOR; cout << "The total charges are $" << charges << endl; break; case QUIT_CHOICE: cout << "Program ending.\n"; break; default: cout << "The valid choices are 1 through 4. Run the\n" << "program again and select one of those.\n"; } return 0; Program Output with Example Input Shown in Bold Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 2 [Enter] For how many months? 6 [Enter] The total charges are $120.00 Checkpoint 4.27 Explain why you cannot convert the following if/else if statement into a switch statement. if (temp == 100) x = 0; else if (population > 1000) x = 1; else if (rate < .1) x = −1; 4.28 What is wrong with the following switch statement? switch (temp) { case temp < 0 : cout << "Temp is negative.\n"; break; case temp == 0: cout << "Temp is zero.\n"; break; case temp > 0 : cout << "Temp is positive.\n"; break; } 210 Chapter 4 Making Decisions 4.29 4.30 4.31 What will the following program display? #include using namespace std; int main() { int funny = 7, serious = 15; funny = serious * 2; switch (funny) { case 0 : cout << "That is funny.\n"; break; case 30: cout << "That is serious.\n"; break; case 32: cout << "That is seriously funny.\n"; break; default: cout << funny << endl; } return 0; } Complete the following program skeleton by writing a switch statement that displays “one” if the user has entered 1, “two” if the user has entered 2, and “three” if the user has entered 3. If a number other than 1, 2, or 3 is entered, the program should display an error message. #include using namespace std; int main() { int userNum; cout << "Enter one of the numbers 1, 2, or 3: "; cin >> userNum; // // Write the switch statement here. // return 0; } Rewrite the following program. Use a switch statement instead of the if/else if statement. #include using namespace std; int main() { int selection; cout << "Which formula do you want to see?\n\n"; cout << "1. Area of a circle\n"; cout << "2. Area of a rectangle\n"; cout << "3. Area of a cylinder\n" cout << "4. None of them!\n"; cin >> selection; if (selection == 1) cout << "Pi times radius squared\n"; 4.15 More About Blocks and Variable Scope 211 else if (selection == 2) cout << "Length times width\n"; else if (selection == 3) cout << "Pi times radius squared times height\n"; else if (selection == 4) cout << "Well okay then, good bye!\n"; else cout << "Not good with numbers, eh?\n"; return 0; } 4.15 More About Blocks and Variable Scope CONCEPT: The scope of a variable is limited to the block in which it is defined. C++ allows you to create variables almost anywhere in a program. Program 4-28 is a modification of Program 4-17, which determines if the user qualifies for a loan. The definitions of the variables income and years have been moved to later points in the program. Program 4-28 1 // This program demonstrates late variable definition 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Constants for minimum income and years 8 const double MIN_INCOME = 35000.0; 9 const int MIN_YEARS = 5; 10 11 // Get the annual income. 12 cout << "What is your annual income? "; 13 double income; // Variable definition 14 cin >> income; 15 16 // Get the number of years at the current job. 17 cout << "How many years have you worked at " 18 << "your current job? "; 19 int years; // Variable definition 20 cin >> years; 21 22 // Determine the user's loan qualifications. 23 if (income >= MIN_INCOME || years > MIN_YEARS) 24 cout << "You qualify.\n"; 25 else 26 { 27 cout << "You must earn at least $" 28 << MIN_INCOME << " or have been " 29 << "employed more than " << MIN_YEARS 30 << " years.\n"; 31 } 32 return 0; 33 } 212 Chapter 4 Making Decisions It is a common practice to define all of a function’s variables at the top of the function. Sometimes, especially in longer programs, it’s a good idea to define variables near the part of the program where they are used. This makes the purpose of the variable more evident. Recall from Chapter 2 that the scope of a variable is defined as the part of the program where the variable may be used. In Program 4-28, the scope of the income variable is the part of the program in lines 13 through 32. The scope of the years variable is the part of the program in lines 19 through 32. The variables income and years are defined inside function main’s braces. Variables defined inside a set of braces have local scope or block scope. They may only be used in the part of the program between their definition and the block’s closing brace. You may define variables inside any block. For example, look at Program 4-29. This version of the loan program has the variable years defined inside the block of the if statement. The scope of years is the part of the program in lines 21 through 31. Program 4-29 1 // This program demonstrates a variable defined in an inner block. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Constants for minimum income and years 8 const double MIN_INCOME = 35000.0; 9 const int MIN_YEARS = 5; 10 11 // Get the annual income. 12 cout << "What is your annual income? "; 13 double income; // Variable definition 14 cin >> income; 15 16 if (income >= MIN_INCOME) 17 { 18 // Get the number of years at the current job. 19 cout << "How many years have you worked at " 20 << "your current job? "; 21 int years; // Variable definition 22 cin >> years; 23 24 if (years > MIN_YEARS) 25 cout << "You qualify.\n"; 26 else 27 { 28 cout << "You must have been employed for\n" 29 << "more than " << MIN_YEARS 30 << " years to qualify.\n"; 31 } 32 } 33 else 4.15 More About Blocks and Variable Scope 213 34 35 36 37 38 39 } { cout << "You must earn at least $" << MIN_INCOME << " to qualify.\n"; } return 0; Notice the scope of years is only within the block where it is defined. The variable is not visible before its definition or after the closing brace of the block. This is true of any variable defined inside a set of braces. N OTE: When a program is running and it enters the section of code that constitutes a variable’s scope, it is said that the variable comes into scope. This simply means the variable is now visible and the program may reference it. Likewise, when a variable leaves scope, it may no longer be used. Variables with the Same Name When a block is nested inside another block, a variable defined in the inner block may have the same name as a variable defined in the outer block. As long as the variable in the inner block is visible, however, the variable in the outer block will be hidden. This is illustrated by Program 4-30. Program 4-30 1 // This program uses two variables with the name number. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Define a variable named number. 8 int number; 9 10 cout << "Enter a number greater than 0: "; 11 cin >> number; 12 if (number > 0) 13 { 14 int number; // Another variable named number. 15 cout << "Now enter another number: "; 16 cin >> number; 17 cout << "The second number you entered was " 18 << number << endl; 19 } 20 cout << "Your first number was " << number << endl; 21 return 0; 22 } Program Output with Example Input Shown in Bold Enter a number greater than 0: 2 [Enter] Now enter another number: 7 [Enter] The second number you entered was 7 Your first number was 2 214 Chapter 4 Making Decisions Program 4-30 has two separate variables named number. The cin and cout statements in the inner block (belonging to the if statement) can only work with the number variable defined in that block. As soon as the program leaves that block, the inner number goes out of scope, revealing the outer number variable. W ARN IN G ! Although it’s perfectly acceptable to define variables inside nested blocks, you should avoid giving them the same names as variables in the outer blocks. It’s too easy to confuse one variable with another. Case Study: See the Sales Commission Case Study on this book’s companion Web site at www.pearsonhighered.com/gaddis. Review Questions and Exercises Short Answer 1. Describe the difference between the if/else if statement and a series of if statements. 2. In an if/else if statement, what is the purpose of a trailing else? 3. What is a flag and how does it work? 4. Can an if statement test expressions other than relational expressions? Explain. 5. Briefly describe how the && operator works. 6. Briefly describe how the || operator works. 7. Why are the relational operators called relational? 8. Why do most programmers indent the conditionally executed statements in a decision structure? Fill-in-the-Blank 9. An expression using the greater-than, less-than, greater-than-or-equal to, less-than-orequal-to, equal, or not-equal to operator is called a(n) __________ expression. 10. A relational expression is either __________ or __________. 11. The value of a relational expression is 0 if the expression is __________ or 1 if the expression is __________. 12. The if statement regards an expression with the value 0 as __________. 13. The if statement regards an expression with a nonzero value as __________. 14. For an if statement to conditionally execute a group of statements, the statements must be enclosed in a set of __________. 15. In an if/else statement, the if part executes its statement or block if the expression is __________, and the else part executes its statement or block if the expression is __________. 16. The trailing else in an if/else if statement has a similar purpose as the __________ section of a switch statement. Review Questions and Exercises 215 17. The if/else if statement is actually a form of the __________ if statement. 18. If the sub-expression on the left of the __________ logical operator is false, the right sub-expression is not checked. 19. If the sub-expression on the left of the __________ logical operator is true, the right sub-expression is not checked. 20. The __________ logical operator has higher precedence than the other logical operators. 21. The logical operators have __________ associativity. 22. The __________ logical operator works best when testing a number to determine if it is within a range. 23. The __________ logical operator works best when testing a number to determine if it is outside a range. 24. A variable with __________ scope is only visible when the program is executing in the block containing the variable’s definition. 25. You use the __________ operator to determine whether one string object is greater then another string object. 26. An expression using the conditional operator is called a(n) __________ expression. 27. The expression that is tested by a switch statement must have a(n) __________ value. 28. The expression following a case statement must be a(n) __________ __________. 29. A program will “fall through” a case section if it is missing the __________ statement. 30. What value will be stored in the variable t after each of the following statements executes? A) t = (12 > 1); __________ B) t = (2 < 0); __________ C) t = (5 == (3 * 2)); __________ D) t = (5 == 5); __________ Algorithm Workbench 31. Write an if statement that assigns 100 to x when y is equal to 0. 32. Write an if/else statement that assigns 0 to x when y is equal to 10. Otherwise it should assign 1 to x. 33. Using the following chart, write an if/else if statement that assigns .10, .15, or .20 to commission, depending on the value in sales. Sales Up to $10,000 $10,000 to $15,000 Over $15,000 Commission Rate 10% 15% 20% 34. Write an if statement that sets the variable hours to 10 when the flag variable minimum is set. 35. Write nested if statements that perform the following tests: If amount1 is greater than 10 and amount2 is less than 100, display the greater of the two. 216 Chapter 4 Making Decisions 36. Write an if statement that prints the message “The number is valid” if the variable grade is within the range 0 through 100. 37. Write an if statement that prints the message “The number is valid” if the variable temperature is within the range −50 through 150. 38. Write an if statement that prints the message “The number is not valid” if the variable hours is outside the range 0 through 80. 39. Assume str1 and str2 are string objects that have been initialized with different values. Write an if/else statement that compares the two objects and displays the one that is alphabetically greatest. 40. Convert the following if/else if statement into a switch statement: if (choice == 1) { cout << fixed << showpoint << setprecision(2); } else if (choice == 2 || choice == 3) { cout << fixed << showpoint << setprecision(4); } else if (choice == 4) { cout << fixed << showpoint << setprecision(6); } else { cout << fixed << showpoint << setprecision(8); } 41. Match the conditional expression with the if/else statement that performs the same operation. A) q = x < y ? a + b : x * 2; B) q = x < y ? x * 2 : a + b; C) x < y ? q = 0 : q = 1; ____ if (x < y) q = 0; else q = 1; ____ if (x < y) q = a + b; else q = x * 2; ____ if (x < y) q = x * 2; else q = a + b; True or False 42. T F The = operator and the == operator perform the same operation when used in a Boolean expression. Review Questions and Exercises 217 43. T 44. T 45. T 46. T 47. T 48. T 49. T 50. T 51. T 52. T 53. T F A variable defined in an inner block may not have the same name as a variable defined in the outer block. F A conditionally executed statement should be indented one level from the if statement. F All lines in a block should be indented one level. F It’s safe to assume that all uninitialized variables automatically start with 0 as their value. F When an if statement is nested in the if part of another statement, the only time the inner if is executed is when the expression of the outer if is true. F When an if statement is nested in the else part of another statement, as in an if/else if, the only time the inner if is executed is when the expression of the outer if is true. F The scope of a variable is limited to the block in which it is defined. F You can use the relational operators to compare string objects. F x != y is the same as (x > y || x < y) F y < x is the same as x >= y F x >= y is the same as (x > y && x = y) Assume the variables x = 5, y = 6, and z = 8. Indicate by circling the T or F whether each of the following conditions is true or false: 54. T F x == 5 || y > 3 55. T F 7 <= x && z > 4 56. T F 2 != y && z != 4 57. T F x >= 0 || x <= y Find the Errors Each of the following programs has errors. Find as many as you can. 58. // This program averages 3 test scores. // It uses the variable perfectScore as a flag. include using namespace std; int main() { cout << "Enter your 3 test scores and I will "; << "average them:"; int score1, score2, score3, cin >> score1 >> score2 >> score3; double average; average = (score1 + score2 + score3) / 3.0; if (average = 100); perfectScore = true; // Set the flag variable cout << "Your average is " << average << endl; bool perfectScore; if (perfectScore); { cout << "Congratulations!\n"; cout << "That's a perfect score.\n"; cout << "You deserve a pat on the back!\n"; return 0; } 218 Chapter 4 Making Decisions 59. // This program divides a user-supplied number by another // user-supplied number. It checks for division by zero. #include using namespace std; int main() { double num1, num2, quotient; cout << "Enter a number: "; cin >> num1; cout << "Enter another number: "; cin >> num2; if (num2 == 0) cout << "Division by zero is not possible.\n"; cout << "Please run the program again "; cout << "and enter a number besides zero.\n"; else quotient = num1 / num2; cout << "The quotient of " << num1 << cout << " divided by " << num2 << " is "; cout << quotient << endl; return 0; } 60. // This program uses an if/else if statement to assign a // letter grade (A, B, C, D, or F) to a numeric test score. #include using namespace std; int main() { int testScore; cout << "Enter your test score and I will tell you\n"; cout << "the letter grade you earned: "; cin >> testScore; if (testScore < 60) cout << "Your grade is F.\n"; else if (testScore < 70) cout << "Your grade is D.\n"; else if (testScore < 80) cout << "Your grade is C.\n"; else if (testScore < 90) cout << "Your grade is B.\n"; else cout << "That is not a valid score.\n"; else if (testScore <= 100) cout << "Your grade is A.\n"; return 0; } Review Questions and Exercises 219 61. // This program uses a switch-case statement to assign a // letter grade (A, B, C, D, or F) to a numeric test score. #include using namespace std; int main() { double testScore; cout << "Enter your test score and I will tell you\n"; cout << "the letter grade you earned: "; cin >> testScore; switch (testScore) { case (testScore < 60.0): cout << "Your grade is F.\n"; break; case (testScore < 70.0): cout << "Your grade is D.\n"; break; case (testScore < 80.0): cout << "Your grade is C.\n"; break; case (testScore < 90.0): cout << "Your grade is B.\n"; break; case (testScore <= 100.0): cout << "Your grade is A.\n"; break; default: cout << "That score isn't valid\n"; return 0; } 62. The following statement should determine if x is not greater than 20. What is wrong with it? if (!x > 20) 63. The following statement should determine if count is within the range of 0 through 100. What is wrong with it? if (count >= 0 || count <= 100) 64. The following statement should determine if count is outside the range of 0 through 100. What is wrong with it? if (count < 0 && count > 100) 65. The following statement should assign 0 to z if a is less than 10, otherwise it should assign 7 to z. What is wrong with it? z = (a < 10) : 0 ? 7; 220 Chapter 4 Making Decisions Programming Challenges 1. Minimum/Maximum Write a program that asks the user to enter two numbers. The program should use the conditional operator to determine which number is the smaller and which is the larger. 2. Roman Numeral Converter Write a program that asks the user to enter a number within the range of 1 through 10. Use a switch statement to display the Roman numeral version of that number. Input Validation: Do not accept a number less than 1 or greater than 10. 3. Magic Dates The date June 10, 1960 is special because when we write it in the following format, the month times the day equals the year. 6/10/60 Write a program that asks the user to enter a month (in numeric form), a day, and a two-digit year. The program should then determine whether the month times the day is equal to the year. If so, it should display a message saying the date is magic. Otherwise it should display a message saying the date is not magic. 4. Areas of Rectangles The area of a rectangle is the rectangle’s length times its width. Write a program that asks for the length and width of two rectangles. The program should tell the user which rectangle has the greater area, or if the areas are the same. 5. Body Mass Index Write a program that calculates and displays a person’s body mass index (BMI). The BMI is often used to determine whether a person with a sedentary lifestyle is overweight or underweight for his or her height. A person’s BMI is calculated with the following formula: BMI ϭ weight ϫ 703 / height2 where weight is measured in pounds and height is measured in inches. The program should display a message indicating whether the person has optimal weight, is underweight, or is overweight. A sedentary person’s weight is considered to be optimal if his or her BMI is between 18.5 and 25. If the BMI is less than 18.5, the person is considered to be underweight. If the BMI value is greater than 25, the person is considered to be overweight. 6. Mass and Weight Scientists measure an object’s mass in kilograms and its weight in newtons. If you know the amount of mass that an object has, you can calculate its weight, in newtons, with the following formula: Weight ϭ mass ϫ 9.8 Write a program that asks the user to enter an object’s mass, and then calculates and displays its weight. If the object weighs more than 1,000 newtons, display a message indicating that it is too heavy. If the object weighs less than 10 newtons, display a message indicating that the object is too light. Programming Challenges 221 7. Time Calculator Write a program that asks the user to enter a number of seconds. VideoNote Solving the Time Calculator Problem • There are 60 seconds in a minute. If the number of seconds entered by the user is greater than or equal to 60, the program should display the number of minutes in that many seconds. • There are 3,600 seconds in an hour. If the number of seconds entered by the user is greater than or equal to 3,600, the program should display the number of hours in that many seconds. • There are 86,400 seconds in a day. If the number of seconds entered by the user is greater than or equal to 86,400, the program should display the number of days in that many seconds. 8. Color Mixer The colors red, blue, and yellow are known as the primary colors because they cannot be made by mixing other colors. When you mix two primary colors, you get a secondary color, as shown here: When you mix red and blue, you get purple. When you mix red and yellow, you get orange. When you mix blue and yellow, you get green. Write a program that prompts the user to enter the names of two primary colors to mix. If the user enters anything other than “red,” “blue,” or “yellow,” the program should display an error message. Otherwise, the program should display the name of the secondary color that results by mixing two primary colors. 9. Change for a Dollar Game Create a change-counting game that gets the user to enter the number of coins required to make exactly one dollar. The program should ask the user to enter the number of pennies, nickels, dimes, and quarters. If the total value of the coins entered is equal to one dollar, the program should congratulate the user for winning the game. Otherwise, the program should display a message indicating whether the amount entered was more than or less than one dollar. 10. Days in a Month Write a program that asks the user to enter the month (letting the user enter an integer in the range of 1 through 12) and the year. The program should then display the number of days in that month. Use the following criteria to identify leap years: 1. Determine whether the year is divisible by 100. If it is, then it is a leap year if and only if it is divisible by 400. For example, 2000 is a leap year but 2100 is not. 2. If the year is not divisible by 100, then it is a leap year if and if only it is divisible by 4. For example, 2008 is a leap year but 2009 is not. Here is a sample run of the program: Enter a month (1-12): 2 [Enter] Enter a year: 2008 [Enter] 29 days 222 Chapter 4 Making Decisions 11. Math Tutor This is a modification of Programming Challenge 17 from Chapter 3. Write a program that can be used as a math tutor for a young student. The program should display two random numbers that are to be added, such as: 247 ϩ 129 The program should wait for the student to enter the answer. If the answer is correct, a message of congratulations should be printed. If the answer is incorrect, a message should be printed showing the correct answer. 12. Software Sales A software company sells a package that retails for $99. Quantity discounts are given according to the following table. Quantity 10–19 20–49 50–99 100 or more Discount 20% 30% 40% 50% Write a program that asks for the number of units sold and computes the total cost of the purchase. Input Validation: Make sure the number of units is greater than 0. 13. Book Club Points Serendipity Booksellers has a book club that awards points to its customers based on the number of books purchased each month. The points are awarded as follows: • If a customer purchases 0 books, he or she earns 0 points. • If a customer purchases 1 book, he or she earns 5 points. • If a customer purchases 2 books, he or she earns 15 points. • If a customer purchases 3 books, he or she earns 30 points. • If a customer purchases 4 or more books, he or she earns 60 points. Write a program that asks the user to enter the number of books that he or she has purchased this month and then displays the number of points awarded. 14. Bank Charges A bank charges $10 per month plus the following check fees for a commercial checking account: $.10 each for fewer than 20 checks $.08 each for 20–39 checks $.06 each for 40–59 checks $.04 each for 60 or more checks The bank also charges an extra $15 if the balance of the account falls below $400 (before any check fees are applied). Write a program that asks for the beginning balance and the number of checks written. Compute and display the bank’s service fees for the month. Input Validation: Do not accept a negative value for the number of checks written. If a negative value is given for the beginning balance, display an urgent message indicating the account is overdrawn. Programming Challenges 223 15. Shipping Charges The Fast Freight Shipping Company charges the following rates: Weight of Package (in Kilograms) 2 kg or less Over 2 kg but not more than 6 kg Over 6 kg but not more than 10 kg Over 10 kg but not more than 20 kg Rate per 500 Miles Shipped $1.10 $2.20 $3.70 $4.80 Write a program that asks for the weight of the package and the distance it is to be shipped, and then displays the charges. Input Validation: Do not accept values of 0 or less for the weight of the package. Do not accept weights of more than 20 kg (this is the maximum weight the company will ship). Do not accept distances of less than 10 miles or more than 3,000 miles. These are the company’s minimum and maximum shipping distances. 16. Running the Race Write a program that asks for the names of three runners and the time it took each of them to finish a race. The program should display who came in first, second, and third place. Input Validation: Only accept positive numbers for the times. 17. Personal Best Write a program that asks for the name of a pole vaulter and the dates and vault heights (in meters) of the athlete’s three best vaults. It should then report, in order of height (best first), the date on which each vault was made and its height. Input Validation: Only accept values between 2.0 and 5.0 for the heights. 18. Fat Gram Calculator Write a program that asks for the number of calories and fat grams in a food. The program should display the percentage of calories that come from fat. If the calories from fat are less than 30% of the total calories of the food, it should also display a message indicating that the food is low in fat. One gram of fat has 9 calories, so Calories from fat ϭ fat grams * 9 The percentage of calories from fat can be calculated as Calories from fat Ϭ total calories Input Validation: Make sure the number of calories and fat grams are not less than 0. Also, the number of calories from fat cannot be greater than the total number of calories. If that happens, display an error message indicating that either the calories or fat grams were incorrectly entered. 19. Spectral Analysis If a scientist knows the wavelength of an electromagnetic wave, he or she can determine what type of radiation it is. Write a program that asks for the wavelength of an electromagnetic wave in meters and then displays what that wave is according to the 224 Chapter 4 Making Decisions chart below. (For example, a wave with a wavelength of 1E-10 meters would be an X-ray.) 1 × 10–2 1 × 10–3 7 × 10–7 4 × 10–7 1 × 10–8 1 × 10–11 Radio Waves Microwaves Infrared Visible Light Ultraviolet X Rays Gamma Rays 20. The Speed of Sound The following table shows the approximate speed of sound in air, water, and steel. Medium Air Water Steel Speed 1,100 feet per second 4,900 feet per second 16,400 feet per second Write a program that displays a menu allowing the user to select air, water, or steel. After the user has made a selection, he or she should be asked to enter the distance a sound wave will travel in the selected medium. The program will then display the amount of time it will take. (Round the answer to four decimal places.) Input Validation: Check that the user has selected one of the available choices from the menu. Do not accept distances less than 0. 21. The Speed of Sound in Gases When sound travels through a gas, its speed depends primarily on the density of the medium. The less dense the medium, the faster the speed will be. The following table shows the approximate speed of sound at 0 degrees centigrade, measured in meters per second, when traveling through carbon dioxide, air, helium, and hydrogen. Medium Carbon Dioxide Air Helium Hydrogen Speed (Meters per Second) 258.0 331.5 972.0 1,270.0 Write a program that displays a menu allowing the user to select one of these four gases. After a selection has been made, the user should enter the number of seconds it took for the sound to travel in this medium from its source to the location at which it was detected. The program should then report how far away (in meters) the source of the sound was from the detection location. Input Validation: Check that the user has selected one of the available menu choices. Do not accept times less than 0 seconds or more than 30 seconds. 22. Freezing and Boiling Points The following table lists the freezing and boiling points of several substances. Write a program that asks the user to enter a temperature and then shows all the substances that will freeze at that temperature and all that will boil at that temperature. For example, if the user enters −20 the program should report that water will freeze and oxygen will boil at that temperature. Programming Challenges 225 Substance Ethyl alcohol Mercury Oxygen Water Freezing Point (°F) -173 -38 -362 32 Boiling Point (°F) 172 676 -306 212 23. Geometry Calculator Write a program that displays the following menu: Geometry Calculator 1. Calculate the Area of a Circle 2. Calculate the Area of a Rectangle 3. Calculate the Area of a Triangle 4. Quit Enter your choice (1-4): If the user enters 1, the program should ask for the radius of the circle and then display its area. Use the following formula: area ϭ πr2 Use 3.14159 for π and the radius of the circle for r. If the user enters 2, the program should ask for the length and width of the rectangle and then display the rectangle’s area. Use the following formula: area = length * width If the user enters 3 the program should ask for the length of the triangle’s base and its height, and then display its area. Use the following formula: area = base * height * .5 If the user enters 4, the program should end. Input Validation: Display an error message if the user enters a number outside the range of 1 through 4 when selecting an item from the menu. Do not accept negative values for the circle’s radius, the rectangle’s length or width, or the triangle’s base or height. 24. Long-Distance Calls A long-distance carrier charges the following rates for telephone calls: Starting Time of Call 00:00–06:59 07:00–19:00 19:01–23:59 Rate per Minute 0.05 0.45 0.20 Write a program that asks for the starting time and the number of minutes of the call, and displays the charges. The program should ask for the time to be entered as a floatingpoint number in the form HH.MM. For example, 07:00 hours will be entered as 07.00, and 16:28 hours will be entered as 16.28. Input Validation: The program should not accept times that are greater than 23:59. Also, no number whose last two digits are greater than 59 should be accepted. Hint: Assuming num is a floating-point variable, the following expression will give you its fractional part: num − static_cast(num) 226 Chapter 4 Making Decisions 25. Mobile Service Provider A mobile phone service provider has three different subscription packages for its customers: Package A: For $39.99 per month 450 minutes are provided. Additional minutes are $0.45 per minute. Package B: For $59.99 per month 900 minutes are provided. Additional minutes are $0.40 per minute. Package C: For $69.99 per month unlimited minutes provided. Write a program that calculates a customer’s monthly bill. It should ask which package the customer has purchased and how many minutes were used. It should then display the total amount due. Input Validation: Be sure the user only selects package A, B, or C. 26. Mobile Service Provider, Part 2 Modify the Program in Programming Challenge 25 so that it also displays how much money Package A customers would save if they purchased packages B or C, and how much money Package B customers would save if they purchased Package C. If there would be no savings, no message should be printed. 27. Mobile Service Provider, Part 3 Months with 30 days have 720 hours, and months with 31 days have 744 hours. February, with 28 days, has 672 hours. You can calculate the number of minutes in a month by multiplying its number of hours by 60. Enhance the input validation of the Mobile Service Provider program by asking the user for the month (by name), and validating that the number of minutes entered is not more than the maximum for the entire month. Here is a table of the months, their days, and number of hours in each. Month January February March April May June July August September October November December Days 31 28 31 30 31 30 31 31 30 31 30 31 Hours 744 672 744 720 744 720 744 744 720 744 720 744 CHAPTER 5 Loops and Files TOPICS 5.1 The Increment and Decrement Operators 5.2 Introduction to Loops: The while Loop 5.3 Using the while Loop for Input Validation 5.4 Counters 5.5 The do-while Loop 5.6 The for Loop 5.7 Keeping a Running Total 5.8 Sentinels 5.9 Focus on Software Engineering: Deciding Which Loop to Use 5.10 Nested Loops 5.11 Using Files for Data Storage 5.12 Optional Topics: Breaking and Continuing a Loop 5.1 The Increment and Decrement Operators CONCEPT: ++ and −− are operators that add and subtract 1 from their operands. To increment a value means to increase it by one, and to decrement a value means to decrease it by one. Both of the following statements increment the variable num: num = num + 1; num += 1; And num is decremented in both of the following statements: num = num − 1; num −= 1; C++ provides a set of simple unary operators designed just for incrementing and decrementing variables. The increment operator is ++, and the decrement operator is −−. The following statement uses the ++ operator to increment num: num++; And the following statement decrements num: num−−; 227 228 Chapter 5 Loops and Files N O T E : The expression num++ is pronounced “num plus plus,” and num−− is pronounced “num minus minus.” Our examples so far show the increment and decrement operators used in postfix mode, which means the operator is placed after the variable. The operators also work in prefix mode, where the operator is placed before the variable name: ++num; −−num; In both postfix and prefix mode, these operators add 1 to or subtract 1 from their operand. Program 5-1 shows how they work. Program 5-1 1 // This program demonstrates the ++ and −− operators. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int num = 4; // num starts out with 4. 8 9 // Display the value in num. 10 cout << "The variable num is " << num << endl; 11 cout << "I will now increment num.\n\n"; 12 13 // Use postfix ++ to increment num. 14 num++; 15 cout << "Now the variable num is " << num << endl; 16 cout << "I will increment num again.\n\n"; 17 18 // Use prefix ++ to increment num. 19 ++num; 20 cout << "Now the variable num is " << num << endl; 21 cout << "I will now decrement num.\n\n"; 22 23 // Use postfix −− to decrement num. 24 num−−; 25 cout << "Now the variable num is " << num << endl; 26 cout << "I will decrement num again.\n\n"; 27 28 // Use prefix −− to increment num. 29 −−num; 30 cout << "Now the variable num is " << num << endl; 31 return 0; 32 } Program Output The variable num is 4 I will now increment num. 5.1 The Increment and Decrement Operators 229 Now the variable num is 5 I will increment num again. Now the variable num is 6 I will now decrement num. Now the variable num is 5 I will decrement num again. Now the variable num is 4 The Difference Between Postfix and Prefix Modes In the simple statements used in Program 5-1, it doesn’t matter if the increment or decrement operator is used in postfix or prefix mode. The difference is important, however, when these operators are used in statements that do more than just incrementing or decrementing. For example, look at the following lines: num = 4; cout << num++; This cout statement is doing two things: (1) displaying the value of num, and (2) incrementing num. But which happens first? cout will display a different value if num is incremented first than if num is incremented last. The answer depends on the mode of the increment operator. Postfix mode causes the increment to happen after the value of the variable is used in the expression. In the example, cout will display 4, then num will be incremented to 5. Prefix mode, however, causes the increment to happen first. In the following statements, num will be incremented to 5, then cout will display 5: num = 4; cout << ++num; Program 5-2 illustrates these dynamics further: Program 5-2 1 // This program demonstrates the prefix and postfix 2 // modes of the increment and decrement operators. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int num = 4; 9 10 cout << num << endl; // Displays 4 11 cout << num++ << endl; // Displays 4, then adds 1 to num 12 cout << num << endl; // Displays 5 13 cout << ++num << endl; // Adds 1 to num, then displays 6 14 cout << endl; // Displays a blank line 15 (program continues) 230 Chapter 5 Loops and Files Program 5-2 (continued) 16 17 18 19 20 21 22 } cout << num << endl; // Displays 6 cout << num−− << endl; // Displays 6, then subtracts 1 from num cout << num << endl; // Displays 5 cout << −−num << endl; // Subtracts 1 from num, then displays 4 return 0; Program Output 4 4 5 6 6 6 5 4 Let’s analyze the statements in this program. In line 8, num is initialized with the value 4, so the cout statement in line 10 displays 4. Then, line 11 sends the expression num++ to cout. Because the ++ operator is used in postfix mode, the value 4 is first sent to cout, and then 1 is added to num, making its value 5. When line 12 executes, num will hold the value 5, so 5 is displayed. Then, line 13 sends the expression ++num to cout. Because the ++ operator is used in prefix mode, 1 is first added to num (making it 6), and then the value 6 is sent to cout. This same sequence of events happens in lines 16 through 19, except the −− operator is used. For another example, look at the following code: int x = 1; int y y = x++; // Postfix increment The first statement defines the variable x (initialized with the value 1), and the second statement defines the variable y. The third statement does two things: • It assigns the value of x to the variable y. • The variable x is incremented. The value that will be stored in y depends on when the increment takes place. Because the ++ operator is used in postfix mode, it acts after the assignment takes place. So, this code will store 1 in y. After the code has executed, x will contain 2. Let’s look at the same code, but with the ++ operator used in prefix mode: int x = 1; int y; y = ++x; // Prefix increment 5.1 The Increment and Decrement Operators 231 In the third statement, the ++ operator is used in prefix mode, so it acts on the variable x before the assignment takes place. So, this code will store 2 in y. After the code has executed, x will also contain 2. Using ++ and −− in Mathematical Expressions The increment and decrement operators can also be used on variables in mathematical expressions. Consider the following program segment: a = 2; b = 5; c = a * b++; cout << a << " " << b << " " << c; In the statement c = a * b++, c is assigned the value of a times b, which is 10. The variable b is then incremented. The cout statement will display 2 6 10 If the statement were changed to read c = a * ++b; the variable b would be incremented before it was multiplied by a. In this case c would be assigned the value of 2 times 6, so the cout statement would display 2 6 12 You can pack a lot of action into a single statement using the increment and decrement operators, but don’t get too tricky with them. You might be tempted to try something like the following, thinking that c will be assigned 11: a = 2; b = 5; c = ++(a * b); // Error! But this assignment statement simply will not work because the operand of the increment and decrement operators must be an lvalue. Recall from Chapter 2 that an lvalue identifies a place in memory whose contents may be changed. The increment and decrement operators usually have variables for their operands, but generally speaking, anything that can go on the left side of an = operator is legal. Using ++ and −− in Relational Expressions Sometimes you will see code where the ++ and −− operators are used in relational expressions. Just as in mathematical expressions, the difference between postfix and prefix mode is critical. Consider the following program segment: x = 10; if (x++ > 10) cout << "x is greater than 10.\n"; Two operations are happening in this if statement: (1) The value in x is tested to determine if it is greater than 10, and (2) x is incremented. Because the increment operator is used in postfix mode, the comparison happens first. Since 10 is not greater than 10, the cout 232 Chapter 5 Loops and Files statement won’t execute. If the mode of the increment operator is changed, however, the if statement will compare 11 to 10, and the cout statement will execute: x = 10; if (++x > 10) cout << "x is greater than 10.\n"; Checkpoint 5.1 What will the following program segments display? A) x = 2; y = x++; cout << x << y; B) x = 2; y = ++x; cout << x << y; C) x = 2; y = 4; cout << x++ << −−y; D) x = 2; y = 2 * x++; cout << x << y; E) x = 99; if (x++ < 100) cout "It is true!\n"; else cout << "It is false!\n"; F) x = 0; if (++x) cout << "It is true!\n"; else cout << "It is false!\n"; 5.2 Introduction to Loops: The while Loop VideoNote The while Loop CONCEPT: A loop is part of a program that repeats. Chapter 4 introduced the concept of control structures, which direct the flow of a program. A loop is a control structure that causes a statement or group of statements to repeat. C++ has three looping control structures: the while loop, the do-while loop, and the for loop. The difference between these structures is how they control the repetition. The while Loop The while loop has two important parts: (1) an expression that is tested for a true or false value, and (2) a statement or block that is repeated as long as the expression is true. Figure 5-1 shows the logic of a while loop. Figure 5-1 5.2 Introduction to Loops: The while Loop 233 True Expression Statement(s) False Here is the general format of the while loop: while (expression) statement; In the general format, expression is any expression that can be evaluated as true or false, and statement is any valid C++ statement. The first line shown in the format is sometimes called the loop header. It consists of the key word while followed by an expression enclosed in parentheses. Here’s how the loop works: the expression is tested, and if it is true, the statement is executed. Then, the expression is tested again. If it is true, the statement is executed. This cycle repeats until the expression is false. The statement that is repeated is known as the body of the loop. It is also considered a conditionally executed statement, because it is executed only under the condition that the expression is true. Notice there is no semicolon after the expression in parentheses. Like the if statement, the while loop is not complete without the statement that follows it. If you wish the while loop to repeat a block of statements, its format is: while (expression) { statement; statement; // Place as many statements here // as necessary. } The while loop works like an if statement that executes over and over. As long as the expression inside the parentheses is true, the conditionally executed statement or block will repeat. Program 5-3 uses the while loop to print “Hello” five times. 234 Chapter 5 Loops and Files Program 5-3 1 // This program demonstrates a simple while loop. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int number = 0; 8 9 while (number < 5) 10 { 11 cout << "Hello\n"; 12 number++; 13 } 14 cout << "That's all!\n"; 15 return 0; 16 } Program Output Hello Hello Hello Hello Hello That's all! Let’s take a closer look at this program. In line 7 an integer variable, number, is defined and initialized with the value 0. In line 9 the while loop begins with this statement: while (number < 5) This statement tests the variable number to determine whether it is less than 5. If it is, then the statements in the body of the loop (lines 11 and 12) are executed: cout << "Hello\n"; number++; The statement in line 11 prints the word “Hello.” The statement in line 12 uses the increment operator to add one to number. This is the last statement in the body of the loop, so after it executes, the loop starts over. It tests the expression number < 5 again, and if it is true, the statements in the body of the loop are executed again. This cycle repeats until the expression number < 5 is false. This is illustrated in Figure 5-2. Each repetition of a loop is known as an iteration. This loop will perform five iterations because the variable number is initialized with the value 0, and it is incremented each time the body of the loop is executed. When the expression number < 5 is tested and found to be false, the loop will terminate and the program will resume execution at the statement that immediately follows the loop. Figure 5-3 shows the logic of this loop. Figure 5-2 Figure 5-3 5.2 Introduction to Loops: The while Loop 235 Test this expression. If the expression is true, while (number < 5) perform these statements. { cout << "Hello\n"; number++; } After executing the body of the loop, start over. number <5 True Print "Hello" False Add 1 to number In this example, the number variable is referred to as the loop control variable because it controls the number of times that the loop iterates. The while Loop Is a Pretest Loop The while loop is known as a pretest loop, which means it tests its expression before each iteration. Notice the variable definition in line 7 of Program 5-3: int number = 0; The number variable is initialized with the value 0. If number had been initialized with the value 5 or greater, as shown in the following program segment, the loop would never execute: int number = 6; while (number < 5) { cout << "Hello\n"; number++; } An important characteristic of the while loop is that the loop will never iterate if the test expression is false to start with. If you want to be sure that a while loop executes the first time, you must initialize the relevant data in such a way that the test expression starts out as true. 236 Chapter 5 Loops and Files Infinite Loops In all but rare cases, loops must contain within themselves a way to terminate. This means that something inside the loop must eventually make the test expression false. The loop in Program 5-3 stops when the expression number < 5 is false. If a loop does not have a way of stopping, it is called an infinite loop. An infinite loop continues to repeat until the program is interrupted. Here is an example of an infinite loop: int number = 0; while (number < 5) { cout << "Hello\n"; } This is an infinite loop because it does not contain a statement that changes the value of the number variable. Each time the expression number < 5 is tested, number will contain the value 0. It’s also possible to create an infinite loop by accidentally placing a semicolon after the first line of the while loop. Here is an example: int number = 0; while (number < 5); // This semicolon is an ERROR! { cout << "Hello\n"; number++; } The semicolon at the end of the first line is assumed to be a null statement and disconnects the while statement from the block that comes after it. To the compiler, this loop looks like: while (number < 5); This while loop will forever execute the null statement, which does nothing. The program will appear to have “gone into space” because there is nothing to display screen output or show activity. Don’t Forget the Braces with a Block of Statements If you write a loop that conditionally executes a block of statements, don’t forget to enclose all of the statements in a set of braces. If the braces are accidentally left out, the while statement conditionally executes only the very next statement. For example, look at the following code. int number = 0; // This loop is missing its braces! while (number < 5) cout << "Hello\n"; number++; In this code the number++ statement is not in the body of the loop. Because the braces are missing, the while statement only executes the statement that immediately follows it. This loop will execute infinitely because there is no code in its body that changes the number variable. 5.2 Introduction to Loops: The while Loop 237 Another common pitfall with loops is accidentally using the = operator when you intend to use the == operator. The following is an infinite loop because the test expression assigns 1 to remainder each time it is evaluated instead of testing whether remainder is equal to 1. while (remainder = 1) // Error: Notice the assignment { cout << "Enter a number: "; cin >> num; remainder = num % 2; } Remember, any nonzero value is evaluated as true. Programming Style and the while Loop It’s possible to create loops that look like this: while (number < 5) { cout << "Hello\n"; number++; } Avoid this style of programming. The programming style you should use with the while loop is similar to that of the if statement: • If there is only one statement repeated by the loop, it should appear on the line after the while statement and be indented one additional level. • If the loop repeats a block, each line inside the braces should be indented. This programming style should visually set the body of the loop apart from the surrounding code. In general, you’ll find a similar style being used with the other types of loops presented in this chapter. In the Spotlight: Designing a Program with a while Loop A project currently underway at Chemical Labs, Inc. requires that a substance be continually heated in a vat. A technician must check the substance’s temperature every 15 minutes. If the substance’s temperature does not exceed 102.5 degrees Celsius, then the technician does nothing. However, if the temperature is greater than 102.5 degrees Celsius, the technician must turn down the vat’s thermostat, wait 5 minutes, and check the temperature again. The technician repeats these steps until the temperature does not exceed 102.5 degrees Celsius. The director of engineering has asked you to write a program that guides the technician through this process. Here is the algorithm: 1. Prompt the user to enter the substance’s temperature. 2. Repeat the following steps as long as the temperature is greater than 102.5 degrees Celsius: a. Tell the technician to turn down the thermostat, wait 5 minutes, and check the tem- perature again. b. Prompt the user to enter the substance’s temperature. 3. After the loop finishes, tell the technician that the temperature is acceptable and to check it again in 15 minutes. 238 Chapter 5 Loops and Files After reviewing this algorithm, you realize that steps 2a and 2b should not be performed if the test condition (temperature is greater than 102.5) is false to begin with. The while loop will work well in this situation, because it will not execute even once if its condition is false. Program 5-4 shows the code for the program. Program 5-4 1 // This program assists a technician in the process 2 // of checking a substance's temperature. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const double MAX_TEMP = 102.5; // Maximum temperature 9 double temperature; // To hold the temperature 10 11 // Get the current temperature. 12 cout << "Enter the substance's Celsius temperature: "; 13 cin >> temperature; 14 15 // As long as necessary, instruct the technician 16 // to adjust the thermostat. 17 while (temperature > MAX_TEMP) 18 { 19 cout << "The temperature is too high. Turn the\n"; 20 cout << "thermostat down and wait 5 minutes.\n"; 21 cout << "Then take the Celsius temperature again\n"; 22 cout << "and enter it here: "; 23 cin >> temperature; 24 } 25 26 // Remind the technician to check the temperature 27 // again in 15 minutes. 28 cout << "The temperature is acceptable.\n"; 29 cout << "Check it again in 15 minutes.\n"; 30 31 return 0; 32 } Program Output with Example Input Shown in Bold Enter the substance's Celsius temperature: 104.7 [Enter] The temperature is too high. Turn the thermostat down and wait 5 minutes. Then take the Celsius temperature again and enter it here: 103.2 [Enter] The temperature is too high. Turn the thermostat down and wait 5 minutes. Then take the Celsius temperature again and enter it here: 102.1 [Enter] The temperature is acceptable. Check it again in 15 minutes. 5.3 Using the while Loop for Input Validation 239 5.3 Using the while Loop for Input Validation CONCEPT: The while loop can be used to create input routines that repeat until acceptable data is entered. Perhaps the most famous saying of the computer industry is “garbage in, garbage out.” The integrity of a program’s output is only as good as its input, so you should try to make sure garbage does not go into your programs. Input validation is the process of inspecting data given to a program by the user and determining if it is valid. A good program should give clear instructions about the kind of input that is acceptable and not assume the user has followed those instructions. The while loop is especially useful for validating input. If an invalid value is entered, a loop can require that the user reenter it as many times as necessary. For example, the following loop asks for a number in the range of 1 through 100: cout << "Enter a number in the range 1-100: "; cin >> number; while (number < 1 || number > 100) { cout << "ERROR: Enter a value in the range 1-100: "; cin >> number; } This code first allows the user to enter a number. This takes place just before the loop. If the input is valid, the loop will not execute. If the input is invalid, however, the loop will display an error message and require the user to enter another number. The loop will continue to execute until the user enters a valid number. The general logic of performing input validation is shown in Figure 5-4. Figure 5-4 Read the first value. Is the Yes value Display an invalid? error message. No Read another value. 240 Chapter 5 Loops and Files The read operation that takes place just before the loop is called a priming read. It provides the first value for the loop to test. Subsequent values are obtained by the loop. Program 5-5 calculates the number of soccer teams a youth league may create, based on a given number of players and a maximum number of players per team. The program uses while loops (in lines 25 through 34 and lines 41 through 46) to validate the user’s input. Program 5-5 1 // This program calculates the number of soccer teams 2 // that a youth league may create from the number of 3 // available players. Input validation is demonstrated 4 // with while loops. 5 #include 6 using namespace std; 7 8 int main() 9{ 10 // Constants for minimum and maximum players 11 const int MIN_PLAYERS = 9, 12 MAX_PLAYERS = 15; 13 14 // Variables 15 int players, // Number of available players 16 teamPlayers, // Number of desired players per team 17 numTeams, // Number of teams 18 leftOver; // Number of players left over 19 20 // Get the number of players per team. 21 cout << "How many players do you wish per team? "; 22 cin >> teamPlayers; 23 24 // Validate the input. 25 while (teamPlayers < MIN_PLAYERS || teamPlayers > MAX_PLAYERS) 26 { 27 // Explain the error. 28 cout << "You should have at least " << MIN_PLAYERS 29 << " but no more than " << MAX_PLAYERS << " per team.\n"; 30 31 // Get the input again. 32 cout << "How many players do you wish per team? "; 33 cin >> teamPlayers; 34 } 35 36 // Get the number of players available. 37 cout << "How many players are available? "; 38 cin >> players; 39 40 // Validate the input. 41 while (players <= 0) 42 { 43 // Get the input again. 44 cout << "Please enter 0 or greater: "; 45 cin >> players; 46 } 5.4 Counters 241 47 48 49 50 51 52 53 54 55 56 57 58 } // Calculate the number of teams. numTeams = players / teamPlayers; // Calculate the number of leftover players. leftOver = players % teamPlayers; // Display the results. cout << "There will be " << numTeams << " teams with " << leftOver << " players left over.\n"; return 0; Program Output with Example Input Shown in Bold How many players do you wish per team? 4 [Enter] You should have at least 9 but no more than 15 per team. How many players do you wish per team? 12 [Enter] How many players are available? −142 [Enter] Please enter 0 or greater: 142 [Enter] There will be 11 teams with 10 players left over. Checkpoint 5.2 Write an input validation loop that asks the user to enter a number in the range of 10 through 25. 5.3 Write an input validation loop that asks the user to enter ‘Y’, ‘y’, ‘N’, or ‘n’. 5.4 Write an input validation loop that asks the user to enter “Yes” or “No”. 5.4 Counters CONCEPT: A counter is a variable that is regularly incremented or decremented each time a loop iterates. Sometimes it’s important for a program to control or keep track of the number of iterations a loop performs. For example, Program 5-6 displays a table consisting of the numbers 1 through 10 and their squares, so its loop must iterate 10 times. Program 5-6 1 // This program displays a list of numbers and 2 // their squares. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int MIN_NUMBER = 1, // Starting number to square 9 MAX_NUMBER = 10; // Maximum number to square 10 (program continues) 242 Chapter 5 Loops and Files Program 5-6 (continued) 11 12 13 14 15 16 17 18 19 20 21 } int num = MIN_NUMBER; // Counter cout << "Number Number Squared\n"; cout << "-------------------------\n"; while (num <= MAX_NUMBER) { cout << num << "\t\t" << (num * num) << endl; num++; //Increment the counter. } return 0; Program Output Number Number Squared --------------------- 1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100 In Program 5-6, the variable num, which starts at 1, is incremented each time through the loop. When num reaches 11 the loop stops. num is used as a counter variable, which means it is regularly incremented in each iteration of the loop. In essence, num keeps count of the number of iterations the loop has performed. N O T E : It’s important that num be properly initialized. Remember, variables defined inside a function have no guaranteed starting value. 5.5 The do-while Loop CONCEPT: The do-while loop is a posttest loop, which means its expression is tested after each iteration. The do-while loop looks something like an inverted while loop. Here is the do-while loop’s format when the body of the loop contains only a single statement: do statement; while (expression); 5.5 The do-while Loop 243 Here is the format of the do-while loop when the body of the loop contains multiple statements: do { statement; statement; // Place as many statements here // as necessary. } while (expression); N O T E : The do-while loop must be terminated with a semicolon. The do-while loop is a posttest loop. This means it does not test its expression until it has completed an iteration. As a result, the do-while loop always performs at least one iteration, even if the expression is false to begin with. This differs from the behavior of a while loop, which you will recall is a pretest loop. For example, in the following while loop the cout statement will not execute at all: int x = 1; while (x < 0) cout << x << endl; But the cout statement in the following do-while loop will execute once because the do-while loop does not evaluate the expression x < 0 until the end of the iteration. int x = 1; do cout << x << endl; while (x < 0); Figure 5-5 illustrates the logic of the do-while loop. Figure 5-5 Statement(s) True Expression False You should use the do-while loop when you want to make sure the loop executes at least once. For example, Program 5-7 averages a series of three test scores for a student. After the 244 Chapter 5 Loops and Files average is displayed, it asks the user if he or she wants to average another set of test scores. The program repeats as long as the user enters Y for yes. Program 5-7 1 // This program averages 3 test scores. It repeats as 2 // many times as the user wishes. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int score1, score2, score3; // Three scores 9 double average; // Average score 10 char again; // To hold Y or N input 11 12 do 13 { 14 // Get three scores. 15 cout << "Enter 3 scores and I will average them: "; 16 cin >> score1 >> score2 >> score3; 17 18 // Calculate and display the average. 19 average = (score1 + score2 + score3) / 3.0; 20 cout << "The average is " << average << ".\n"; 21 22 // Does the user want to average another set? 23 cout << "Do you want to average another set? (Y/N) "; 24 cin >> again; 25 } while (again == 'Y' || again == 'y'); 26 return 0; 27 } Program Output with Example Input Shown in Bold Enter 3 scores and I will average them: 80 90 70 [Enter] The average is 80. Do you want to average another set? (Y/N) y [Enter] Enter 3 scores and I will average them: 60 75 88 [Enter] The average is 74.3333. Do you want to average another set? (Y/N) n [Enter] When this program was written, the programmer had no way of knowing the number of times the loop would iterate. This is because the loop asks the user if he or she wants to repeat the process. This type of loop is known as a user-controlled loop, because it allows the user to decide the number of iterations. Using do-while with Menus The do-while loop is a good choice for repeating a menu. Recall Program 4-27, which displayed a menu of health club packages. Program 5-8 is a modification of that program, which uses a do-while loop to repeat the program until the user selects item 4 from the menu. 5.5 The do-while Loop 245 Program 5-8 1 // This program displays a menu and asks the user to make a 2 // selection. A do-while loop repeats the program until the 3 // user selects item 4 from the menu. 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 // Constants for menu choices 11 const int ADULT_CHOICE = 1, 12 CHILD_CHOICE = 2, 13 SENIOR_CHOICE = 3, 14 QUIT_CHOICE = 4; 15 16 // Constants for membership rates 17 const double ADULT = 40.0, 18 CHILD = 20.0, 19 SENIOR = 30.0; 20 21 // Variables 22 int choice; // Menu choice 23 int months; // Number of months 24 double charges; // Monthly charges 25 26 // Set up numeric output formatting. 27 cout << fixed << showpoint << setprecision(2); 28 29 do 30 { 31 // Display the menu. 32 cout << "\n\t\tHealth Club Membership Menu\n\n" 33 << "1. Standard Adult Membership\n" 34 << "2. Child Membership\n" 35 << "3. Senior Citizen Membership\n" 36 << "4. Quit the Program\n\n" 37 << "Enter your choice: "; 38 cin >> choice; 39 40 // Validate the menu selection. 41 while (choice < ADULT_CHOICE || choice > QUIT_CHOICE) 42 { 43 cout << "Please enter a valid menu choice: "; 44 cin >> choice; 45 } 46 47 // Process the user's choice. 48 if (choice != QUIT_CHOICE) 49 { 50 // Get the number of months. 51 cout << "For how many months? "; (program continues) 246 Chapter 5 Loops and Files Program 5-8 (continued) 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 } cin >> months; // Respond to the user's menu selection. switch (choice) { case ADULT_CHOICE: charges = months * ADULT; break; case CHILD_CHOICE: charges = months * CHILD; break; case SENIOR_CHOICE: charges = months * SENIOR; } // Display the monthly charges. cout << "The total charges are $" << charges << endl; } } while (choice != QUIT_CHOICE); return 0; Program Output with Example Input Shown in Bold Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 1 [Enter] For how many months? 12 [Enter] The total charges are $480.00 Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 4 [Enter] Program ending. Checkpoint 5.5 What will the following program segments display? A) int count = 10; do { cout << "Hello World\n"; count++; } while (count < 1); B) int v = 10; do cout << v << end1; while (v < 5); C) int count = 0, number = 0, limit = 4; do { number += 2; count++; } while (count < limit); cout << number << " " << count << endl; 5.6 The for Loop 247 5.6 The for Loop VideoNote The for Loop CONCEPT: The for loop is ideal for performing a known number of iterations. In general, there are two categories of loops: conditional loops and count-controlled loops. A conditional loop executes as long as a particular condition exists. For example, an input validation loop executes as long as the input value is invalid. When you write a conditional loop, you have no way of knowing the number of times it will iterate. Sometimes you know the exact number of iterations that a loop must perform. A loop that repeats a specific number of times is known as a count-controlled loop. For example, if a loop asks the user to enter the sales amounts for each month in the year, it will iterate twelve times. In essence, the loop counts to twelve and asks the user to enter a sales amount each time it makes a count. A count-controlled loop must possess three elements: 1. It must initialize a counter variable to a starting value. 2. It must test the counter variable by comparing it to a maximum value. When the counter variable reaches its maximum value, the loop terminates. 3. It must update the counter variable during each iteration. This is usually done by incrementing the variable. Count-controlled loops are so common that C++ provides a type of loop specifically for them. It is known as the for loop. The for loop is specifically designed to initialize, test, and update a counter variable. Here is the format of the for loop when it is used to repeat a single statement: for (initialization; test; update) statement; The format of the for loop when it is used to repeat a block is for (initialization; test; update) { statement; statement; // Place as many statements here // as necessary. } 248 Chapter 5 Loops and Files The first line of the for loop is the loop header. After the key word for, there are three expressions inside the parentheses, separated by semicolons. (Notice there is not a semicolon after the third expression.) The first expression is the initialization expression. It is normally used to initialize a counter variable to its starting value. This is the first action performed by the loop, and it is only done once. The second expression is the test expression. This is an expression that controls the execution of the loop. As long as this expression is true, the body of the for loop will repeat. The for loop is a pretest loop, so it evaluates the test expression before each iteration. The third expression is the update expression. It executes at the end of each iteration. Typically, this is a statement that increments the loop’s counter variable. Here is an example of a simple for loop that prints “Hello” five times: for (count = 0; count < 5; count++) cout << "Hello" << endl; In this loop, the initialization expression is count = 0, the test expression is count < 5, and the update expression is count++. The body of the loop has one statement, which is the cout statement. Figure 5-6 illustrates the sequence of events that takes place during the loop’s execution. Notice that Steps 2 through 4 are repeated as long as the test expression is true. Figure 5-6 Step 1: Perform the initialization expression. Step 2: Evaluate the test expression. If it is true, go to Step 3. Otherwise, terminate the loop. for (count = 0; count < 5; count++) cout << "Hello" << endl; Step 3: Execute the body of the loop. Step 4: Perform the update expression, then go back to Step 2. Figure 5-7 shows the loop’s logic in the form of a flowchart. Figure 5-7 Assign 0 to count count True <5 False cout statement Increment count 5.6 The for Loop 249 Notice how the counter variable, count, is used to control the number of times that the loop iterates. During the execution of the loop, this variable takes on the values 1 through 5, and when the test expression count < 5 is false, the loop terminates. Also notice that in this example the count variable is used only in the loop header, to control the number of loop iterations. It is not used for any other purpose. It is also possible to use the counter variable within the body of the loop. For example, look at the following code: for (number = 1; number <= 10; number++) cout << number << " " << endl; The counter variable in this loop is number. In addition to controlling the number of iterations, it is also used in the body of the loop. This loop will produce the following output: 1 2 3 4 5 6 7 8 9 10 As you can see, the loop displays the contents of the number variable during each iteration. Program 5-9 shows another example of a for loop that uses its counter variable within the body of the loop. This is yet another program that displays a table showing the numbers 1 through 10 and their squares. Program 5-9 1 // This program displays the numbers 1 through 10 and 2 // their squares. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int MIN_NUMBER = 1, // Starting value 9 MAX_NUMBER = 10; // Ending value 10 int num; 11 12 cout << "Number Number Squared\n"; 13 cout << "-------------------------\n"; 14 15 for (num = MIN_NUMBER; num <= MAX_NUMBER; num++) 16 cout << num << "\t\t" << (num * num) << endl; 17 18 return 0; 19 } Program Output Number Number Squared --------------------- 1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81 10 100 250 Chapter 5 Loops and Files Figure 5-8 illustrates the sequence of events performed by this for loop, and Figure 5-9 shows the logic of the loop as a flowchart. Figure 5-8 Step 1: Perform the initialization expression. Step 2: Evaluate the test expression. If it is true, go to Step 3. Otherwise, terminate the loop. Step 4: Perform the update expression, then go back to Step 2. for (num = MIN_NUMBER; num <= MAX_NUMBER; num++) cout << num << "\t\t" << (num * num) << endl; Step 3: Execute the body of the loop. Figure 5-9 Assign MIN_NUMBER to num num <= True Display num MAX_NUMBER and num * num False Increment num Using the for Loop Instead of while or do-while You should use the for loop instead of the while or do-while loop in any situation that clearly requires an initialization, uses a false condition to stop the loop, and requires an update to occur at the end of each loop iteration. Program 5-9 is a perfect example. It requires that the num variable be initialized to 1, it stops the loop when num is greater than 10, and it increments num at the end of each loop iteration. Recall that when we first introduced the idea of a counter variable we examined Program 5-6, which uses a while loop to display the table of numbers and their squares. Because the loop in that program requires an initialization, uses a false test expression to stop, and performs an increment at the end of each iteration, it can easily be converted to a for loop. Figure 5-10 shows how the while loop in Program 5-6 and the for loop in Program 5-9 each have initialization, test, and update expressions. Figure 5-10 5.6 The for Loop 251 Initialization expression Test expression int num = MIN_NUMBER; while (num <= MAX_NUMBER) { cout << num << "\t\t" << (num * num) << endl; num++; } Update expression Initialization expression Test expression Update expression for (num = MIN_NUMBER; num <= MAX_NUMBER; num++) cout << num << "\t\t" << (num * num) << endl; The for Loop Is a Pretest Loop Because the for loop tests its test expression before it performs an iteration, it is a pretest loop. It is possible to write a for loop in such a way that it will never iterate. Here is an example: for (count = 11; count <= 10; count++) cout << "Hello" << endl; Because the variable count is initialized to a value that makes the test expression false from the beginning, this loop terminates as soon as it begins. Avoid Modifying the Counter Variable in the Body of the for Loop Be careful not to place a statement that modifies the counter variable in the body of the for loop. All modifications of the counter variable should take place in the update expression, which is automatically executed at the end of each iteration. If a statement in the body of the loop also modifies the counter variable, the loop will probably not terminate when you expect it to. The following loop, for example, increments x twice for each iteration: for (x = 1; x <= 10; x++) { cout << x << endl; x++; // Wrong! } 252 Chapter 5 Loops and Files Other Forms of the Update Expression You are not limited to using increment statements in the update expression. Here is a loop that displays all the even numbers from 2 through 100 by adding 2 to its counter: for (num = 2; num <= 100; num += 2) cout << num << endl; And here is a loop that counts backward from 10 down to 0: for (num = 10; num >= 0; num--) cout << num << endl; Defining a Variable in the for Loop’s Initialization Expression Not only may the counter variable be initialized in the initialization expression, it may be defined there as well. The following code shows an example. This is a modified version of the loop in Program 5-9. for (int num = MIN_NUMBER; num <= MAX_NUMBER; num++) cout << num << "\t\t" << (num * num) << endl; In this loop, the num variable is both defined and initialized in the initialization expression. If the counter variable is used only in the loop, it makes sense to define it in the loop header. This makes the variable’s purpose more clear. When a variable is defined in the initialization expression of a for loop, the scope of the variable is limited to the loop. This means you cannot access the variable in statements outside the loop. For example, the following program segment will not compile because the last cout statement cannot access the variable count. for (int count = 1; count <= 10; count++) cout << count << endl; cout << "count is now " << count << endl; // ERROR! Creating a User Controlled for Loop Sometimes you want the user to determine the maximum value of the counter variable in a for loop, and therefore determine the number of times the loop iterates. For example, look at Program 5-10. This is another program that displays a list of numbers and their squares. Instead of displaying the numbers 1 through 10, this program allows the user to enter the minimum and maximum values to display. Program 5-10 1 // This program demonstrates a user controlled for loop. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int minNumber, // Starting number to square 8 maxNumber; // Maximum number to square 9 5.6 The for Loop 253 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 } // Get the minimum and maximum values to display. cout << "I will display a table of numbers and " << "their squares.\n" << "Enter the starting number: "; cin >> minNumber; cout << "Enter the ending number: "; cin >> maxNumber; // Display the table. cout << "Number Number Squared\n" << "-------------------------\n"; for (int num = minNumber; num <= maxNumber; num++) cout << num << "\t\t" << (num * num) << endl; return 0; Program Output with Example Input Shown in Bold I will display a table of numbers and their squares. Enter the starting number: 6 [Enter] Enter the ending number: 12 [Enter] Number Number Squared --------------------- 6 36 7 49 8 64 9 81 10 100 11 121 12 144 Before the loop, the code in lines 11 through 16 asks the user to enter the starting and ending numbers. These values are stored in the minNumber and maxNumber variables. These values are used in the for loop’s initialization and test expressions: for (int num = minNumber; num <= maxNumber; num++) In this loop, the num variable takes on the values from maxNumber through maxValue, and then the loop terminates. Using Multiple Statements in the Initialization and Update Expressions It is possible to execute more than one statement in the initialization expression and the update expression. When using multiple statements in either of these expressions, simply separate the statements with commas. For example, look at the loop in the following code, which has two statements in the initialization expression. 254 Chapter 5 Loops and Files int x, y; for (x = 1, y = 1; x <= 5; x++) { cout << x << " plus " << y << " equals " << (x + y) << endl; } This loop’s initialization expression is x = 1, y = 1 This initializes two variables, x and y. The output produced by this loop is 1 plus 1 equals 2 2 plus 1 equals 3 3 plus 1 equals 4 4 plus 1 equals 5 5 plus 1 equals 6 We can further modify the loop to execute two statements in the update expression. Here is an example: int x, y; for (x = 1, y = 1; x <= 5; x++, y++) { cout << x << " plus " << y << " equals " << (x + y) << endl; } The loop’s update expression is x++, y++ This update expression increments both the x and y variables. The output produced by this loop is 1 plus 1 equals 2 2 plus 2 equals 4 3 plus 3 equals 6 4 plus 4 equals 8 5 plus 5 equals 10 Connecting multiple statements with commas works well in the initialization and update expressions, but do not try to connect multiple expressions this way in the test expression. If you wish to combine multiple expressions in the test expression, you must use the && or || operators. Omitting the for Loop’s Expressions The initialization expression may be omitted from inside the for loop’s parentheses if it has already been performed or no initialization is needed. Here is an example of the loop in Program 5-10 with the initialization being performed prior to the loop: int num = 1; for ( ; num <= maxValue; num++) cout << num << "\t\t" << (num * num) << endl; 5.6 The for Loop 255 You may also omit the update expression if it is being performed elsewhere in the loop or if none is needed. Although this type of code is not recommended, the following for loop works just like a while loop: int num = 1; for ( ; num <= maxValue; ) { cout << num << "\t\t" << (num * num) << endl; num++; } You can even go so far as to omit all three expressions from the for loop’s parentheses. Be warned, however, that if you leave out the test expression, the loop has no built-in way of terminating. Here is an example: for ( ; ; ) cout << "Hello World\n"; Because this loop has no way of stopping, it will display "Hello World\n" forever (or until something interrupts the program). In the Spotlight: Designing a Count-Controlled Loop with the for Statement Your friend Amanda just inherited a European sports car from her uncle. Amanda lives in the United States, and she is afraid she will get a speeding ticket because the car’s speedometer indicates kilometers per hour. She has asked you to write a program that displays a table of speeds in kilometers per hour with their values converted to miles per hour. The formula for converting kilometers per hour to miles per hour is: MPH ϭ KPH * 0.6214 In the formula, MPH is the speed in miles per hour and KPH is the speed in kilometers per hour. The table that your program displays should show speeds from 60 kilometers per hour through 130 kilometers per hour, in increments of 10, along with their values converted to miles per hour. The table should look something like this: KPH 60 70 80 etc. . . . 130 MPH 37.3 43.5 49.7 80.8 After thinking about this table of values, you decide that you will write a for loop that uses a counter variable to hold the kilometer-per-hour speeds. The counter’s starting value will be 60, its ending value will be 130, and you will add 10 to the counter variable after each iteration. Inside the loop you will use the counter variable to calculate a speed in miles-perhour. Program 5-11 shows the code. 256 Chapter 5 Loops and Files Program 5-11 1 // This program converts the speeds 60 kph through 2 // 130 kph (in 10 kph increments) to mph. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 // Constants for the speeds 10 const int START_KPH = 60, // Starting speed 11 END_KPH = 130, // Ending speed 12 INCREMENT = 10; // Speed increment 13 14 // Constant for the conversion factor 15 const double CONVERSION_FACTOR = 0.6214; 16 17 // Variables 18 int kph; // To hold speeds in kph 19 double mph; // To hold speeds in mph 20 21 // Set the numeric output formatting. 22 cout << fixed << showpoint << setprecision(1); 23 24 // Display the table headings. 25 cout << "KPH\tMPH\n"; 26 cout << "---------------\n"; 27 28 // Display the speeds. 29 for (kph = START_KPH; kph <= END_KPH; kph += INCREMENT) 30 { 31 // Calculate mph 32 mph = kph * CONVERSION_FACTOR; 33 34 // Display the speeds in kph and mph. 35 cout << kph << "\t" << mph << endl; 36 37 } 38 return 0; 39 } Program Output KPH MPH --------------- 60 37.3 70 43.5 80 49.7 90 55.9 100 62.1 110 68.4 120 74.6 130 80.8 5.7 Keeping a Running Total 257 Checkpoint 5.6 Name the three expressions that appear inside the parentheses in the for loop’s header. 5.7 You want to write a for loop that displays “I love to program” 50 times. Assume that you will use a counter variable named count. A) What initialization expression will you use? B) What test expression will you use? C) What update expression will you use? D) Write the loop. 5.8 What will the following program segments display? A) for (int count = 0; count < 6; count++) cout << (count + count); B) for (int value = −5; value < 5; value++) cout << value; C) int x; for (x = 5; x <= 14; x += 3) cout << x << endl; cout << x << endl; 5.9 Write a for loop that displays your name 10 times. 5.10 Write a for loop that displays all of the odd numbers, 1 through 49. 5.11 Write a for loop that displays every fifth number, zero through 100. 5.7 Keeping a Running Total CONCEPT: A running total is a sum of numbers that accumulates with each iteration of a loop. The variable used to keep the running total is called an accumulator. Many programming tasks require you to calculate the total of a series of numbers. For example, suppose you are writing a program that calculates a business’s total sales for a week. The program would read the sales for each day as input and calculate the total of those numbers. Programs that calculate the total of a series of numbers typically use two elements: • A loop that reads each number in the series. • A variable that accumulates the total of the numbers as they are read. The variable that is used to accumulate the total of the numbers is called an accumulator. It is often said that the loop keeps a running total because it accumulates the total as it reads each number in the series. Figure 5-11 shows the general logic of a loop that calculates a running total. 258 Chapter 5 Loops and Files Figure 5-11 Logic for calculating a running total When the loop finishes, the accumulator will contain the total of the numbers that were read by the loop. Notice that the first step in the flowchart is to set the accumulator variable to 0. This is a critical step. Each time the loop reads a number, it adds it to the accumulator. If the accumulator starts with any value other than 0, it will not contain the correct total when the loop finishes. Let’s look at a program that calculates a running total. Program 5-12 calculates a company’s total sales over a period of time by taking daily sales figures as input and calculating a running total of them as they are gathered. Program 5-12 1 // This program takes daily sales figures over a period of time 2 // and calculates their total. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int days; // Number of days 10 double total = 0.0; // Accumulator, initialized with 0 11 12 // Get the number of days. 13 cout << "For how many days do you have sales figures? "; 5.7 Keeping a Running Total 259 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 } cin >> days; // Get the sales for each day and accumulate a total. for (int count = 1; count <= days; count++) { double sales; cout << "Enter the sales for day " << count << ": "; cin >> sales; total += sales; // Accumulate the running total. } // Display the total sales. cout << fixed << showpoint << setprecision(2); cout << "The total sales are $" << total << endl; return 0; Program Output with Example Input Shown in Bold For how many days do you have sales figures? 5 [Enter] Enter the sales for day 1: 489.32 [Enter] Enter the sales for day 2: 421.65 [Enter] Enter the sales for day 3: 497.89 [Enter] Enter the sales for day 4: 532.37 [Enter] Enter the sales for day 5: 506.92 [Enter] The total sales are $2448.15 Let’s take a closer look at this program. Line 9 defines the days variable, which will hold the number of days that we have sales figures for. Line 10 defines the total variable, which will hold the total sales. Because total is an accumulator, it is initialized with 0.0. In line 14 the user enters the number of days that he or she has sales figures for. The number is assigned to the days variable. Next, the for loop in lines 17 through 23 executes. In the loop’s initialization expression, in line 17, the variable count is defined and initialized with 1. The test expression specifies the loop will repeat as long as count is less than or equal to days. The update expression increments count by one at the end of each loop iteration. Line 19 defines a variable named sales. Because the variable is defined in the body of the loop, its scope is limited to the loop. During each loop iteration, the user enters the amount of sales for a specific day, which is assigned to the sales variable. This is done in line 21. Then, in line 22 the value of sales is added to the existing value in the total variable. (Note that line 22 does not assign sales to total, but adds sales to total. Put another way, this line increases total by the amount in sales.) Because total was initially assigned 0.0, after the first iteration of the loop, total will be set to the same value as sales. After each subsequent iteration, total will be increased by the amount in sales. After the loop has finished, total will contain the total of all the daily sales figures entered. Now it should be clear why we assigned 0.0 to total before the loop executed. If total started at any other value, the total would be incorrect. 260 Chapter 5 Loops and Files 5.8 Sentinels CONCEPT: A sentinel is a special value that marks the end of a list of values. Program 5-12, in the previous section, requires the user to know in advance the number of days he or she wishes to enter sales figures for. Sometimes the user has a list that is very long and doesn’t know how many items there are. In other cases, the user might be entering several lists, and it is impractical to require that every item in every list be counted. A technique that can be used in these situations is to ask the user to enter a sentinel at the end of the list. A sentinel is a special value that cannot be mistaken as a member of the list and signals that there are no more values to be entered. When the user enters the sentinel, the loop terminates. Program 5-13 calculates the total points earned by a soccer team over a series of games. It allows the user to enter the series of game points, then −1 to signal the end of the list. Program 5-13 1 // This program calculates the total number of points a 2 // soccer team has earned over a series of games. The user 3 // enters a series of point values, then −1 when finished. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int game = 1, // Game counter 10 points, // To hold a number of points 11 total = 0; // Accumulator 12 13 cout << "Enter the number of points your team has earned\n"; 14 cout << "so far in the season, then enter −1 when finished.\n\n"; 15 cout << "Enter the points for game " << game << ": "; 16 cin >> points; 17 18 while (points != −1) 19 { 20 total += points; 21 game++; 22 cout << "Enter the points for game " << game << ": "; 23 cin >> points; 24 } 25 cout << "\nThe total points are " << total << endl; 26 return 0; 27 } 5.9 Focus on Software Engineering: Deciding Which Loop to Use 261 Program Output with Example Input Shown in Bold Enter the number of points your team has earned so far in the season, then enter −1 when finished. Enter the points for game 1: 7 [Enter] Enter the points for game 2: 9 [Enter] Enter the points for game 3: 4 [Enter] Enter the points for game 4: 6 [Enter] Enter the points for game 5: 8 [Enter] Enter the points for game 6: −1 [Enter] The total points are 34 The value Ϫ1 was chosen for the sentinel in this program because it is not possible for a team to score negative points. Notice that this program performs a priming read in line 18 to get the first value. This makes it possible for the loop to immediately terminate if the user enters Ϫ1 as the first value. Also note that the sentinel value is not included in the running total. Checkpoint 5.12 Write a for loop that repeats seven times, asking the user to enter a number. The loop should also calculate the sum of the numbers entered. 5.13 In the following program segment, which variable is the counter variable and which is the accumulator? int a, x, y = 0; for (x = 0; x < 10; x++) { cout << "Enter a number: "; cin >> a; y += a; } cout << "The sum of those numbers is " << y << endl; 5.14 Why should you be careful when choosing a sentinel value? 5.15 How would you modify Program 5-13 so any negative value is a sentinel? 5.9 Focus on Software Engineering: Deciding Which Loop to Use CONCEPT: Although most repetitive algorithms can be written with any of the three types of loops, each works best in different situations. Each of the three C++ loops is ideal to use in different situations. Here’s a short summary of when each loop should be used. • The while loop. The while loop is a conditional loop, which means it repeats as long as a particular condition exists. It is also a pretest loop, so it is ideal in situations where you do not want the loop to iterate if the condition is false from the beginning. 262 Chapter 5 Loops and Files For example, validating input that has been read and reading lists of data terminated by a sentinel value are good applications of the while loop. • The do-while loop. The do-while loop is also a conditional loop. Unlike the while loop, however, do-while is a posttest loop. It is ideal in situations where you always want the loop to iterate at least once. The do-while loop is a good choice for repeating a menu. • The for loop. The for loop is a pretest loop that has built-in expressions for initializing, testing, and updating. These expressions make it very convenient to use a counter variable to control the number of iterations that the loop performs. The initialization expression can initialize the counter variable to a starting value, the test expression can test the counter variable to determine whether it holds the maximum value, and the update expression can increment the counter variable. The for loop is ideal in situations where the exact number of iterations is known. 5.10 Nested Loops CONCEPT: A loop that is inside another loop is called a nested loop. A nested loop is a loop that appears inside another loop. A clock is a good example of something that works like a nested loop. The second hand, minute hand, and hour hand all spin around the face of the clock. Each time the hour hand increments, the minute hand increments 60 times. Each time the minute hand increments, the second hand increments 60 times. Here is a program segment with a for loop that partially simulates a digital clock. It displays the seconds from 0 to 59: cout << fixed << right; cout.fill('0'); for (int seconds = 0; seconds < 60; seconds++) cout << setw(2) << seconds << endl; N O T E : The fill member function of cout changes the fill character, which is a space by default. In the program segment above, the fill function causes a zero to be printed in front of all single digit numbers. We can add a minutes variable and nest the loop above inside another loop that cycles through 60 minutes: cout << fixed << right; cout.fill('0'); for (int minutes = 0; minutes < 60; minutes++) { for (int seconds = 0; seconds < 60; seconds++) { cout << setw(2) << minutes << ":"; cout << setw(2) << seconds << endl; } } 5.10 Nested Loops 263 To make the simulated clock complete, another variable and loop can be added to count the hours: cout << fixed << right; cout.fill('0'); for (int hours = 0; hours < 24; hours++) { for (int minutes = 0; minutes < 60; minutes++) { for (int seconds = 0; seconds < 60; seconds++) { cout << setw(2) << hours << ":"; cout << setw(2) << minutes << ":"; cout << setw(2) << seconds << endl; } } } The output of the previous program segment follows: 00:00:00 00:00:01 00:00:02 . (The program will count through each second of 24 hours.) . . 23:59:59 The innermost loop will iterate 60 times for each iteration of the middle loop. The middle loop will iterate 60 times for each iteration of the outermost loop. When the outermost loop has iterated 24 times, the middle loop will have iterated 1,440 times and the innermost loop will have iterated 86,400 times! The simulated clock example brings up a few points about nested loops: • An inner loop goes through all of its iterations for each iteration of an outer loop. • Inner loops complete their iterations faster than outer loops. • To get the total number of iterations of a nested loop, multiply the number of itera- tions of all the loops. Program 5-14 is another test-averaging program. It asks the user for the number of students and the number of test scores per student. A nested inner loop, in lines 26 through 33, asks for all the test scores for one student, iterating once for each test score. The outer loop in lines 23 through 37 iterates once for each student. Program 5-14 1 // This program averages test scores. It asks the user for the 2 // number of students and the number of test scores per student. 3 #include 4 #include 5 using namespace std; (program continues) 264 Chapter 5 Loops and Files Program 5-14 (continued) 6 7 int main() 8{ 9 int numStudents, // Number of students 10 numTests; // Number of tests per student 11 double total, // Accumulator for total scores 12 average; // Average test score 13 14 // Set up numeric output formatting. 15 cout << fixed << showpoint << setprecision(1); 16 17 // Get the number of students. 18 cout << "This program averages test scores.\n"; 19 cout << "For how many students do you have scores? "; 20 cin >> numStudents; 21 22 // Get the number of test scores per student. 23 cout << "How many test scores does each student have? "; 24 cin >> numTests; 25 26 // Determine each student's average score. 27 for (int student = 1; student <= numStudents; student++) 28 { 29 total = 0; // Initialize the accumulator. 30 for (int test = 1; test <= numTests; test++) 31 { 32 double score; 33 cout << "Enter score " << test << " for "; 34 cout << "student " << student << ": "; 35 cin >> score; 36 total += score; 37 } 38 average = total / numTests; 39 cout << "The average score for student " << student; 40 cout << " is " << average << ".\n\n"; 41 } 42 return 0; 43 } Program Output with Example Input Shown in Bold This program averages test scores. For how many students do you have scores? 2 [Enter] How many test scores does each student have? 3 [Enter] Enter score 1 for student 1: 84 [Enter] Enter score 2 for student 1: 79 [Enter] Enter score 3 for student 1: 97 [Enter] The average score for student 1 is 86.7. Enter score 1 for student 2: 92 [Enter] Enter score 2 for student 2: 88 [Enter] Enter score 3 for student 2: 94 [Enter] The average score for student 2 is 91.3. 5.11 Using Files for Data Storage 265 5.11 Using Files for Data Storage CONCEPT: When a program needs to save data for later use, it writes the data in a file. The data can then be read from the file at a later time. The programs you have written so far require the user to reenter data each time the program runs, because data kept in variables and control properties is stored in RAM and disappears once the program stops running. If a program is to retain data between the times it runs, it must have a way of saving it. Data is saved in a file, which is usually stored on a computer’s disk. Once the data is saved in a file, it will remain there after the program stops running. Data that is stored in a file can be then retrieved and used at a later time. Most of the commercial software that you use on a day-to-day basis store data in files. The following are a few examples. • Word processors: Word processing programs are used to write letters, memos, reports, and other documents. The documents are then saved in files so they can be edited and printed. • Image editors: Image editing programs are used to draw graphics and edit images such as the ones that you take with a digital camera. The images that you create or edit with an image editor are saved in files. • Spreadsheets: Spreadsheet programs are used to work with numerical data. Numbers and mathematical formulas can be inserted into the rows and columns of the spreadsheet. The spreadsheet can then be saved in a file for use later. • Games: Many computer games keep data stored in files. For example, some games keep a list of player names with their scores stored in a file. These games typically display the players’ names in order of their scores, from highest to lowest. Some games also allow you to save your current game status in a file so you can quit the game and then resume playing it later without having to start from the beginning. • Web browsers: Sometimes when you visit a Web page, the browser stores a small file known as a cookie on your computer. Cookies typically contain information about the browsing session, such as the contents of a shopping cart. Programs that are used in daily business operations rely extensively on files. Payroll programs keep employee data in files, inventory programs keep data about a company’s products in files, accounting systems keep data about a company’s financial operations in files, and so on. Programmers usually refer to the process of saving data in a file as writing data to the file. When a piece of data is written to a file, it is copied from a variable in RAM to the file. This is illustrated in Figure 5-12. An output file is a file that data is written to. It is called an output file because the program stores output in it. The process of retrieving data from a file is known as reading data from the file. When a piece of data is read from a file, it is copied from the file into a variable in RAM. Figure 5-13 illustrates this process. An input file is a file that data is read from. It is called an input file because the program gets input from the file. 266 Chapter 5 Loops and Files Figure 5-12 Writing data to a file Variable payRate 18.65 Variable 7451Z employeeID Variable Cindy Chandler employeeName Data is copied from variables to the file. Cindy Chandler 7451Z 18.65 A file on the disk Figure 5-13 Reading data from a file Variable payRate 18.65 Variable 7451Z employeeID Variable Cindy Chandler employeeName Data is copied from the file to variables. Cindy Chandler 7451Z 18.65 A file on the disk This section discusses ways to create programs that write data to files and read data from files. When a file is used by a program, three steps must be taken. 1. Open the file—Opening a file creates a connection between the file and the program. Opening an output file usually creates the file on the disk and allows the program to write data to it. Opening an input file allows the program to read data from the file. 2. Process the file—Data is either written to the file (if it is an output file) or read from the file (if it is an input file). 3. Close the file—After the program is finished using the file, the file must be closed. Closing a file disconnects the file from the program. Types of Files In general, there are two types of files: text and binary. A text file contains data that has been encoded as text, using a scheme such as ASCII or Unicode. Even if the file contains numbers, those numbers are stored in the file as a series of characters. As a result, the file may be opened and viewed in a text editor such as Notepad. A binary file contains data 5.11 Using Files for Data Storage 267 that has not been converted to text. Thus, you cannot view the contents of a binary file with a text editor. In this chapter we work only with text files. In Chapter 12 you will learn to work with binary files. File Access Methods There are two general ways to access data stored in a file: sequential access and direct access. When you work with a sequential access file, you access data from the beginning of the file to the end of the file. If you want to read a piece of data that is stored at the very end of the file, you have to read all of the data that comes before it—you cannot jump directly to the desired data. This is similar to the way cassette tape players work. If you want to listen to the last song on a cassette tape, you have to either fast-forward over all of the songs that come before it or listen to them. There is no way to jump directly to a specific song. When you work with a random access file (also known as a direct access file), you can jump directly to any piece of data in the file without reading the data that comes before it. This is similar to the way a CD player or an MP3 player works. You can jump directly to any song that you want to listen to. This chapter focuses on sequential access files. Sequential access files are easy to work with, and you can use them to gain an understanding of basic file operations. In Chapter 12 you will learn to work with random access files. Filenames and File Stream Objects Files on a disk are identified by a filename. For example, when you create a document with a word processor and then save the document in a file, you have to specify a filename. When you use a utility such as Windows Explorer to examine the contents of your disk, you see a list of filenames. Figure 5-14 shows how three files named cat.jpg, notes.txt, and resume .doc might be represented in Windows Explorer. Figure 5-14 Three files Each operating system has its own rules for naming files. Many systems, including Windows, support the use of filename extensions, which are short sequences of characters that appear at the end of a filename preceded by a period (known as a “dot”). For example, the files depicted in Figure 5-14 have the extensions .jpg, .txt, and .doc. The extension usually indicates the type of data stored in the file. For example, the .jpg extension usually indicates that the file contains a graphic image that is compressed according to the JPEG image standard. The .txt extension usually indicates that the file contains text. The .doc extension usually indicates that the file contains a Microsoft Word document. In order for a program to work with a file on the computer’s disk, the program must create a file stream object in memory. A file stream object is an object that is associated with a specific file and provides a way for the program to work with that file. It is called a “stream” object because a file can be thought of as a stream of data. 268 Chapter 5 Loops and Files File stream objects work very much like the cin and cout objects. A stream of data may be sent to cout, which causes values to be displayed on the screen. A stream of data may be read from the keyboard by cin, and stored in variables. Likewise, streams of data may be sent to a file stream object, which writes the data to a file. When data is read from a file, the data flows from the file stream object that is associated with the file, into variables. Setting Up a Program for File Input/Output Just as cin and cout require the iostream file to be included in the program, C++ file access requires another header file. The file fstream contains all the declarations necessary for file operations. It is included with the following statement: #include The fstream header file defines the data types ofstream, ifstream, and fstream. Before a C++ program can work with a file, it must define an object of one of these data types. The object will be “linked” with an actual file on the computer’s disk, and the operations that may be performed on the file depend on which of these three data types you pick for the file stream object. Table 5-1 lists and describes the file stream data types. Table 5-1 File Stream Data Type ofstream ifstream fstream Description Output file stream. You create an object of this data type when you want to create a file and write data to it. Input file stream. You create an object of this data type when you want to open an existing file and read data from it. File stream. Objects of this data type can be used to open files for reading, writing, or both. N O T E : In this chapter we discuss only the ofstream and ifstream types. The fstream type is covered in Chapter 12. Creating a File Object and Opening a File Before data can be written to or read from a file, the following things must happen: • A file stream object must be created • The file must be opened and linked to the file stream object. The following code shows an example of opening a file for input (reading). ifstream inputFile; inputFile.open("Customers.txt"); The first statement defines an ifstream object named inputFile. The second statement calls the object’s open member function, passing the string "Customers.txt" as an argument. In this statement, the open member function opens the Customers.txt file and links it with the inputFile object. After this code executes, you will be able to use the inputFile object to read data from the Customers.txt file. 5.11 Using Files for Data Storage 269 The following code shows an example of opening a file for output (writing). ofstream outputFile; outputFile.open("Employees.txt"); The first statement defines an ofstream object named outputFile. The second statement calls the object’s open member function, passing the string "Employees.txt" as an argument. In this statement, the open member function creates the Employees.txt file and links it with the outputFile object. After this code executes, you will be able to use the outputFile object to write data to the Employees.txt file. It’s important to remember that when you call an ofstream object’s open member function, the specified file will be created. If the specified file already exists, it will be erased, and a new file with the same name will be created. Often, when opening a file, you will need to specify its path as well as its name. For example, on a Windows system the following statement opens the file C:\data\inventory.txt: inputFile.open("C:\\data\\inventory.txt") In this statement, the file C:\data\inventory.txt is opened and linked with inputFile. N OTE: Notice the use of two backslashes in the file’s path. Two backslashes are needed to represent one backslash in a string literal. It is possible to define a file stream object and open a file in one statement. Here is an example: ifstream inputFile("Customers.txt"); This statement defines an ifstream object named inputFile and opens the Customer.txt file. Here is an example that defines an ofstream object named outputFile and opens the Employees.txt file: ofstream outputFile("Employees.txt"); Closing a File The opposite of opening a file is closing it. Although a program’s files are automatically closed when the program shuts down, it is a good programming practice to write statements that close them. Here are two reasons a program should close files when it is finished using them: • Most operating systems temporarily store data in a file buffer before it is written to a file. A file buffer is a small “holding section” of memory that file-bound data is first written to. When the buffer is filled, all the data stored there is written to the file. This technique improves the system’s performance. Closing a file causes any unsaved data that may still be held in a buffer to be saved to its file. This means the data will be in the file if you need to read it later in the same program. • Some operating systems limit the number of files that may be open at one time. When a program closes files that are no longer being used, it will not deplete more of the operating system’s resources than necessary. Calling the file stream object’s close member function closes a file. Here is an example: inputFile.close(); 270 Chapter 5 Loops and Files Writing Data to a File You already know how to use the stream insertion operator (<<) with the cout object to write data to the screen. It can also be used with ofstream objects to write data to a file. Assuming outputFile is an ofstream object, the following statement demonstrates using the << operator to write a string literal to a file: outputFile << "I love C++ programming\n"; This statement writes the string literal "I love C++ programming\n" to the file associated with outputFile. As you can see, the statement looks like a cout statement, except the name of the ofstream object name replaces cout. Here is a statement that writes both a string literal and the contents of a variable to a file: outputFile << "Price: " << price << endl; The statement above writes the stream of data to outputFile exactly as cout would write it to the screen: It writes the string "Price: ", followed by the value of the price variable, followed by a newline character. Program 5-15 demonstrates opening a file, writing data to the file, and closing the file. After this code has executed, we can open the demofile.txt file using a text editor and look at its contents. Figure 5-15 shows how the file’s contents will appear in Notepad. Program 5-15 1 // This program writes data to a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ofstream outputFile; 9 outputFile.open("demofile.txt"); 10 11 cout << "Now writing data to the file.\n"; 12 13 // Write four names to the file. 14 outputFile << "Bach\n"; 15 outputFile << "Beethoven\n"; 16 outputFile << "Mozart\n"; 17 outputFile << "Schubert\n"; 18 19 // Close the file 20 outputFile.close(); 21 cout << "Done.\n"; 22 return 0; 23 } Program Screen Output Now writing data to the file. Done. Figure 5-15 5.11 Using Files for Data Storage 271 Notice that in lines 14 through 17 of Program 5-15, each string that was written to the file ends with a newline escape sequence (\n). The newline specifies the end of a line of text. Because a newline is written at the end of each string, the strings appear on separate lines when viewed in a text editor, as shown in Figure 5-15. Program 5-16 shows what happens if we write the same four names without the \n escape sequence. Figure 5-16 shows the contents of the file that Program 5-16 creates. As you can see, all of the names appear on the same line in the file. Program 5-16 1 // This program writes data to a single line in a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ofstream outputFile; 9 outputFile.open("demofile.txt"); 10 11 cout << "Now writing data to the file.\n"; 12 13 // Write four names to the file. 14 outputFile << "Bach"; 15 outputFile << "Beethoven"; 16 outputFile << "Mozart"; 17 outputFile << "Schubert"; 18 19 // Close the file 20 outputFile.close(); 21 cout << "Done.\n"; 22 return 0; 23 } Program Screen Output Now writing data to the file. Done. 272 Chapter 5 Loops and Files Figure 5-16 Program 5-17 shows another example. This program reads three numbers from the keyboard as input and then saves those numbers in a file named Numbers.txt. Program 5-17 1 // This program writes user input to a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ofstream outputFile; 9 int number1, number2, number3; 10 11 // Open an output file. 12 outputFile.open("Numbers.txt"); 13 14 // Get three numbers from the user. 15 cout << "Enter a number: "; 16 cin >> number1; 17 cout << "Enter another number: "; 18 cin >> number2; 19 cout << "One more time. Enter a number: "; 20 cin >> number3; 21 22 // Write the numbers to the file. 23 outputFile << number1 << endl; 24 outputFile << number2 << endl; 25 outputFile << number3 << endl; 26 cout << "The numbers were saved to a file.\n"; 27 28 // Close the file 29 outputFile.close(); 30 cout << "Done.\n"; 31 return 0; 32 } 5.11 Using Files for Data Storage 273 Program Screen Output with Example Input Shown in Bold Enter a number: 100 [Enter] Enter another number: 200 [Enter] One more time. Enter a number: 300 [Enter] The numbers were saved to a file. Done. In Program 5-17, lines 23 through 25 write the contents of the number1, number2, and number3 variables to the file. Notice that the endl manipulator is sent to the outputFile object immediately after each item. Sending the endl manipulator causes a newline to be written to the file. Figure 5-17 shows the file’s contents displayed in Notepad, using the example input values 100, 200, and 300. As you can see, each item appears on a separate line in the file because of the endl manipulators. Figure 5-17 Program 5-18 shows an example that reads strings as input from the keyboard and then writes those strings to a file. The program asks the user to enter the first names of three friends, and then it writes those names to a file named Friends.txt. Figure 5-18 shows an example of the Friends.txt file opened in Notepad. Program 5-18 1 // This program writes user input to a file. 2 #include 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 ofstream outputFile; 10 string name1, name2, name3; 11 12 // Open an output file. 13 outputFile.open("Friends.txt"); 14 (program continues) 274 Chapter 5 Loops and Files Program 5-18 (continued) 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 } // Get the names of three friends. cout << "Enter the names of three friends.\n"; cout << "Friend #1: "; cin >> name1; cout << "Friend #2: "; cin >> name2; cout << "Friend #3: "; cin >> name3; // Write the names to the file. outputFile << name1 << endl; outputFile << name2 << endl; outputFile << name3 << endl; cout << "The names were saved to a file.\n"; // Close the file outputFile.close(); return 0; Program Screen Output with Example Input Shown in Bold Enter the names of three friends. Friend #1: Joe [Enter] Friend #2: Chris [Enter] Friend #3: Geri [Enter] The names were saved to a file. Figure 5-18 VideoNote Reading Data from a File Reading Data from a File The >> operator not only reads user input from the cin object, but also data from a file. Assuming input File is an if stream object, the following statement shows the >> operator reading data from the file into the variable name: inputFile >> name; Let’s look at an example. Assume the file Friends.txt exists, and it contains the names shown in Figure 5-18. Program 5-19 opens the file, reads the names and displays them on the screen, and then closes the file. 5.11 Using Files for Data Storage 275 Program 5-19 1 // This program reads data from a file. 2 #include 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 ifstream inputFile; 10 string name; 11 12 inputFile.open("Friends.txt"); 13 cout << "Reading data from the file.\n"; 14 15 inputFile >> name; // Read name 1 from the file 16 cout << name << endl; // Display name 1 17 18 inputFile >> name; // Read name 2 from the file 19 cout << name << endl; // Display name 2 20 21 inputFile >> name; // Read name 3 from the file 22 cout << name << endl; // Display name 3 23 24 inputFile.close(); // Close the file 25 return 0; 26 } Program Output Reading data from the file. Joe Chris Geri The Read Position When a file has been opened for input, the file stream object internally maintains a special value known as a read position. A file’s read position marks the location of the next byte that will be read from the file. When an input file is opened, its read position is initially set to the first byte in the file. So, the first read operation extracts data starting at the first byte. As data is read from the file, the read position moves forward, toward the end of the file. Let’s see how this works with the example shown in Program 5-19. When the Friends.txt file is opened by the statement in line 12, the read position for the file will be positioned as shown in Figure 5-19. Figure 5-19 J o e \n C h r i s \n G e r i \n Read position 276 Chapter 5 Loops and Files Keep in mind that when the >> operator extracts data from a file, it expects to read pieces of data that are separated by whitespace characters (spaces, tabs, or newlines). When the statement in line 15 executes, the >> operator reads data from the file’s current read position, up to the \n character. The data that is read from the file is assigned to the name object. The \n character is also read from the file, but is not included as part of the data. So, the name object will hold the value "Joe" after this statement executes. The file’s read position will then be at the location shown in Figure 5-20. Figure 5-20 J o e \n C h r i s \n G e r i \n Read position When the statement in line 18 executes, it reads the next item from the file, which is "Chris", and assigns that value to the name object. After this statement executes, the file’s read position will be advanced to the next item, as shown in Figure 5-21. Figure 5-21 J o e \n C h r i s \n G e r i \n Read position When the statement in line 21 executes, it reads the next item from the file, which is "Geri", and assigns that value to the name object. After this statement executes, the file’s read position will be advanced to the end of the file, as shown in Figure 5-22. Figure 5-22 J o e \n C h r i s \n G e r i \n Read position Reading Numeric Data From a Text File Remember that when data is stored in a text file, it is encoded as text, using a scheme such as ASCII or Unicode. Even if the file contains numbers, those numbers are stored in the file as a series of characters. For example, suppose a text file contains numeric data, such as that shown in Figure 5-17. The numbers that you see displayed in the figure are stored in the file as the strings "10", "20", and "30". Fortunately, you can use the >> operator to read data such as this from a text file, into a numeric variable, and the >> operator will automatically convert the data to a numeric data type. Program 5-20 shows an example. It opens the file shown in Figure 5-23, reads the three numbers from the file into int variables, and calculates their sum. Figure 5-23 5.11 Using Files for Data Storage 277 Program 5-20 1 // This program reads numbers from a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ifstream inFile; 9 int value1, value2, value3, sum; 10 11 // Open the file. 12 inFile.open("NumericData.txt"); 13 14 // Read the three numbers from the file. 15 inFile >> value1; 16 inFile >> value2; 17 inFile >> value3; 18 19 // Close the file. 20 inFile.close(); 21 22 // Calculate the sum of the numbers. 23 sum = value1 + value2 + value3; 24 25 // Display the three numbers. 26 cout << "Here are the numbers:\n" 27 << value1 << " " << value2 28 << " " << value3 << endl; 29 30 // Display the sum of the numbers. 31 cout << "Their sum is: " << sum << endl; 32 return 0; 33 } Program Output Here are the numbers: 10 20 30 Their sum is: 60 278 Chapter 5 Loops and Files Using Loops to Process Files Although some programs use files to store only small amounts of data, files are typically used to hold large collections of data. When a program uses a file to write or read a large amount of data, a loop is typically involved. For example, look at the code in Program 5-21. This program gets sales amounts for a series of days from the user and writes those amounts to a file named Sales.txt. The user specifies the number of days of sales data he or she needs to enter. In the sample run of the program, the user enters sales amounts for five days. Figure 5-24 shows the contents of the Sales.txt file containing the data entered by the user in the sample run. Program 5-21 1 // This program reads data from a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ofstream outputFile; // File stream object 9 int numberOfDays; // Number of days of sales 10 double sales; // Sales amount for a day 11 12 // Get the number of days. 13 cout << "For how many days do you have sales? "; 14 cin >> numberOfDays; 15 16 // Open a file named Sales.txt. 17 outputFile.open("Sales.txt"); 18 19 // Get the sales for each day and write it 20 // to the file. 21 for (int count = 1; count <= numberOfDays; count++) 22 { 23 // Get the sales for a day. 24 cout << "Enter the sales for day " 25 << count << ": "; 26 cin >> sales; 27 28 // Write the sales to the file. 29 outputFile << sales << endl; 30 } 31 32 // Close the file. 33 outputFile.close(); 34 cout << "Data written to Sales.txt\n"; 35 return 0; 36 } Program Output (with Input Shown in Bold) For how many days do you have sales? 5 [Enter] Enter the sales for day 1: 1000.00 [Enter] Enter the sales for day 2: 2000.00 [Enter] Enter the sales for day 3: 3000.00 [Enter] Enter the sales for day 4: 4000.00 [Enter] Enter the sales for day 5: 5000.00 [Enter] Data written to sales.txt. 5.11 Using Files for Data Storage 279 Figure 5-24 Detecting the End of the File Quite often a program must read the contents of a file without knowing the number of items that are stored in the file. For example, suppose you need to write a program that displays all of the items in a file, but you do not know how many items the file contains. You can open the file and then use a loop to repeatedly read an item from the file and display it. However, an error will occur if the program attempts to read beyond the end of the file. The program needs some way of knowing when the end of the file has been reached so it will not try to read beyond it. Fortunately, the >> operator not only reads data from a file, but also returns a true or false value indicating whether the data was successfully read or not. If the operator returns true, then a value was successfully read. If the operator returns false, it means that no value was read from the file. Let’s look at an example. A file named ListOfNumbers.txt, which is shown in Figure 5-25, contains a list of numbers. Without knowing how many numbers the file contains, Program 5-22 opens the file, reads all of the values it contains, and displays them. 280 Chapter 5 Loops and Files Figure 5-25 Program 5-22 1 // This program reads data from a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ifstream inputFile; 9 int number; 10 11 // Open the file. 12 inputFile.open("ListOfNumbers.txt"); 13 14 // Read the numbers from the file and 15 // display them. 16 while (inputFile >> number) 17 { 18 cout << number << endl; 19 } 20 21 // Close the file. 22 inputFile.close(); 23 return 0; 24 } Program Output 100 200 300 400 500 600 700 5.11 Using Files for Data Storage 281 Take a closer look at line 16: while (inputFile >> number) Notice that the statement that extracts data from the file is used as the Boolean expression in the while loop. It works like this: • The expression inputFile >> number executes. • If an item is successfully read from the file, the item is stored in the number variable, and the expression returns true to indicate that it succeeded. In that case, the statement in line 18 executes and the loop repeats. • If there are no more items to read from the file, the expression inputFile >> number returns false, indicating that it did not read a value. In that case, the loop terminates. Because the value returned from the >> operator controls the loop, it will read items from the file until the end of the file has been reached. Testing for File Open Errors Under certain circumstances, the open member function will not work. For example, the following code will fail if the file info.txt does not exist: ifstream inputFile; inputFile.open("info.txt"); There is a way to determine whether the open member function successfully opened the file. After you call the open member function, you can test the file stream object as if it were a Boolean expression. Program 5-23 shows an example. Program 5-23 1 // This program tests for file open errors. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 ifstream inputFile; 9 int number; 10 11 // Open the file. 12 inputFile.open("BadListOfNumbers.txt"); 13 14 // If the file successfully opened, process it. 15 if (inputFile) 16 { 17 // Read the numbers from the file and 18 // display them. 19 while (inputFile >> number) 20 { 21 cout << number << endl; 22 } 23 (program continues) 282 Chapter 5 Loops and Files Program 5-23 (continued) 24 25 26 27 28 29 30 31 32 33 } // Close the file. inputFile.close(); } else { // Display an error message. cout << "Error opening the file.\n"; } return 0; Program Output (Assume BadListOfNumbers.txt does not exist) Error opening the file. Let’s take a closer look at certain parts of the code. Line 12 calls the inputFile object’s open member function to open the file ListOfNumbers.txt. Then the if statement in line 15 tests the value of the inputFile object as if it were a Boolean expression. When tested this way, the inputFile object will give a true value if the file was successfully opened. Otherwise it will give a false value. The example output shows this program will display an error message if it could not open the file. Another way to detect a failed attempt to open a file is with the fail member function, as shown in the following code: ifstream inputFile; inputFile.open("customers.txt"); if (inputFile.fail()) { cout << "Error opening file.\n"; } else { // Process the file. } The fail member function returns true when an attempted file operation is unsuccessful. When using file I/O, you should always test the file stream object to make sure the file was opened successfully. If the file could not be opened, the user should be informed and appropriate action taken by the program. Letting the User Specify a Filename 11 In each of the previous examples, the name of the file that is opened is hard-coded as a string literal into the program. In many cases, you will want the user to specify the name of a file for the program to open. In C++ 11, you can pass a string object as an argument to a file stream object’s open member function. Program 5-24 shows an example. This is a modified version of Program 5-23. This version prompts the user to enter the name of the file. In line 15, the name that the user enters is stored in a string object named filename. In line 18, the filename object is passed as an argument to the open function. 5.11 Using Files for Data Storage 283 Program 5-24 1 // This program lets the user enter a filename. 2 #include 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 ifstream inputFile; 10 string filename; 11 int number; 12 13 // Get the filename from the user. 14 cout << "Enter the filename: "; 15 cin >> filename; 16 17 // Open the file. 18 inputFile.open(filename); 19 20 // If the file successfully opened, process it. 21 if (inputFile) 22 { 23 // Read the numbers from the file and 24 // display them. 25 while (inputFile >> number) 26 { 27 cout << number << endl; 28 } 29 30 // Close the file. 31 inputFile.close(); 32 } 33 else 34 { 35 // Display an error message. 36 cout << "Error opening the file.\n"; 37 } 38 return 0; 39 } Program Output with Example Input Shown in Bold Enter the filename: ListOfNumbers.txt [Enter] 100 200 300 400 500 600 700 284 Chapter 5 Loops and Files Using the c_str Member Function in Older Versions of C++ In older versions of the C++ language (prior to C++ 11), a file stream object’s open member function will not accept a string object as an argument. The open member function requires that you pass the name of the file as a null-terminated string, which is also known as a C-string. String literals are stored in memory as null-terminated C-strings, but string objects are not. Fortunately, string objects have a member function named c_str that returns the contents of the object formatted as a null-terminated C-string. Here is the general format of how you call the function: stringObject.c_str() In the general format, stringObject is the name of a string object. The c_str function returns the string that is stored in stringObject as a null-terminated C-string. For example, line 18 in Program 5-24 could be rewritten in the following manner to make the program compatible with an older version of C++: inputFile.open(filename.c_str()); In this version of the statement, the value that is returned from filename.c_str() is passed as an argument to the open function. Checkpoint 5.16 What is an output file? What is an input file? 5.17 What three steps must be taken when a file is used by a program? 5.18 What is the difference between a text file and a binary file? 5.19 What is the difference between sequential access and random access? 5.20 What type of file stream object do you create if you want to write data to a file? 5.21 What type of file stream object do you create if you want to read data from a file? 5.22 Write a short program that uses a for loop to write the numbers 1 through 10 to a file. 5.23 Write a short program that opens the file created by the program you wrote for Checkpoint 5.22, reads all of the numbers from the file, and displays them. 5.12 Optional Topics: Breaking and Continuing a Loop CONCEPT: The break statement causes a loop to terminate early. The continue statement causes a loop to stop its current iteration and begin the next one. W A R N I N G ! Use the break and continue statements with great caution. Because they bypass the normal condition that controls the loop’s iterations, these statements make code difficult to understand and debug. For this reason, you should avoid using break and continue whenever possible. However, because they are part of the C++ language, we discuss them briefly in this section. 5.12 Optional Topics: Breaking and Continuing a Loop 285 Sometimes it’s necessary to stop a loop before it goes through all its iterations. The break statement, which was used with switch in Chapter 4, can also be placed inside a loop. When it is encountered, the loop stops, and the program jumps to the statement immediately following the loop. The while loop in the following program segment appears to execute 10 times, but the break statement causes it to stop after the fifth iteration. int count = 0; while (count++ < 10) { cout << count << endl; if (count == 5) break; } Program 5-25 uses the break statement to interrupt a for loop. The program asks the user for a number and then displays the value of that number raised to the powers of 0 through 10. The user can stop the loop at any time by entering Q. Program 5-25 1 // This program raises the user's number to the powers 2 // of 0 through 10. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 double value; 10 char choice; 11 12 cout << "Enter a number: "; 13 cin >> value; 14 cout << "This program will raise " << value; 15 cout << " to the powers of 0 through 10.\n"; 16 for (int count = 0; count <= 10; count++) 17 { 18 cout << value << " raised to the power of "; 19 cout << count << " is " << pow(value, count); 20 cout << "\nEnter Q to quit or any other key "; 21 cout << "to continue. "; 22 cin >> choice; 23 if (choice == 'Q' || choice == 'q') 24 break; 25 } 26 return 0; 27 } (program output continues) 286 Chapter 5 Loops and Files Program 5-25 (continued) Program Output with Example Input Shown in Bold Enter a number: 2 [Enter] This program will raise 2 to the powers of 0 through 10. 2 raised to the power of 0 is 1 Enter Q to quit or any other key to continue. C [Enter] 2 raised to the power of 1 is 2 Enter Q to quit or any other key to continue. C [Enter] 2 raised to the power of 2 is 4 Enter Q to quit or any other key to continue. Q [Enter] Using break in a Nested Loop In a nested loop, the break statement only interrupts the loop it is placed in. The following program segment displays five rows of asterisks on the screen. The outer loop controls the number of rows, and the inner loop controls the number of asterisks in each row. The inner loop is designed to display 20 asterisks, but the break statement stops it during the eleventh iteration. for (int row = 0; row < 5; row++) { for (int star = 0; star < 20; star++) { cout << '*'; if (star == 10) break; } cout << endl; } The output of the program segment above is: *********** *********** *********** *********** *********** The continue Statement The continue statement causes the current iteration of a loop to end immediately. When continue is encountered, all the statements in the body of the loop that appear after it are ignored, and the loop prepares for the next iteration. In a while loop, this means the program jumps to the test expression at the top of the loop. As usual, if the expression is still true, the next iteration begins. In a do-while loop, the program jumps to the test expression at the bottom of the loop, which determines whether the next iteration will begin. In a for loop, continue causes the update expression to be executed and then the test expression to be evaluated. The following program segment demonstrates the use of continue in a while loop: 5.12 Optional Topics: Breaking and Continuing a Loop 287 int testVal = 0; while (testVal++ < 10) { if (testVal == 4) continue; cout << testVal << " "; } This loop looks like it displays the integers 1 through 10. When testVal is equal to 4, however, the continue statement causes the loop to skip the cout statement and begin the next iteration. The output of the loop is 1 2 3 5 6 7 8 9 10 Program 5-26 demonstrates the continue statement. The program calculates the charges for DVD rentals, where current releases cost $3.50 and all others cost $2.50. If a customer rents several DVDs, every third one is free. The continue statement is used to skip the part of the loop that calculates the charges for every third DVD. Program 5-26 1 // This program calculates the charges for DVD rentals. 2 // Every third DVD is free. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int dvdCount = 1; // DVD counter 10 int numDVDs; // Number of DVDs rented 11 double total = 0.0; // Accumulator 12 char current; // Current release, Y or N 13 14 // Get the number of DVDs. 15 cout << "How many DVDs are being rented? "; 16 cin >> numDVDs; 17 18 // Determine the charges. 19 do 20 { 21 if ((dvdCount % 3) == 0) 22 { 23 cout << "DVD #" << dvdCount << " is free!\n"; 24 continue; // Immediately start the next iteration 25 } 26 cout << "Is DVD #" << dvdCount; 27 cout << " a current release? (Y/N) "; 28 cin >> current; 29 if (current == 'Y' || current == 'y') 30 total += 3.50; 31 else 32 total += 2.50; 33 } while (dvdCount++ < numDVDs); 34 (program continues) 288 Chapter 5 Loops and Files Program 5-26 (continued) 35 36 37 38 39 } // Display the total. cout << fixed << showpoint << setprecision(2); cout << "The total is $" << total << endl; return 0; Program Output with Example Input Shown in Bold How many DVDs are being rented? 6 [Enter] Is DVD #1 a current release? (Y/N) y [Enter] Is DVD #2 a current release? (Y/N) n [Enter] DVD #3 is free! Is DVD #4 a current release? (Y/N) n [Enter] Is DVD #5 a current release? (Y/N) y [Enter] DVD #6 is free! The total is $12.00 Case Study: See the Loan Amortization Case Study on this book’s companion Web site at www.pearsonhighered.com/gaddis. Review Questions and Exercises Short Answer 1. Why should you indent the statements in the body of a loop? 2. Describe the difference between pretest loops and posttest loops. 3. Why are the statements in the body of a loop called conditionally executed statements? 4. What is the difference between the while loop and the do-while loop? 5. Which loop should you use in situations where you wish the loop to repeat until the test expression is false, and the loop should not execute if the test expression is false to begin with? 6. Which loop should you use in situations where you wish the loop to repeat until the test expression is false, but the loop should execute at least one time? 7. Which loop should you use when you know the number of required iterations? 8. Why is it critical that counter variables be properly initialized? 9. Why is it critical that accumulator variables be properly initialized? 10. Why should you be careful not to place a statement in the body of a for loop that changes the value of the loop’s counter variable? 11. What header file do you need to include in a program that performs file operations? 12. What data type do you use when you want to create a file stream object that can write data to a file? 13. What data type do you use when you want to create a file stream object that can read data from a file? 14. Why should a program close a file when it’s finished using it? Review Questions and Exercises 289 15. What is a file’s read position? Where is the read position when a file is first opened for reading? Fill-in-the-Blank 16. To __________ a value means to increase it by one, and to __________ a value means to decrease it by one. 17. When the increment or decrement operator is placed before the operand (or to the operand’s left), the operator is being used in __________ mode. 18. When the increment or decrement operator is placed after the operand (or to the operand’s right), the operator is being used in __________ mode. 19. The statement or block that is repeated is known as the __________ of the loop. 20. Each repetition of a loop is known as a(n) __________. 21. A loop that evaluates its test expression before each repetition is a(n) __________ loop. 22. A loop that evaluates its test expression after each repetition is a(n) __________ loop. 23. A loop that does not have a way of stopping is a(n) __________ loop. 24. A(n) __________ is a variable that “counts” the number of times a loop repeats. 25. A(n) __________ is a sum of numbers that accumulates with each iteration of a loop. 26. A(n) __________ is a variable that is initialized to some starting value, usually zero, and then has numbers added to it in each iteration of a loop. 27. A(n) __________ is a special value that marks the end of a series of values. 28. The __________ loop always iterates at least once. 29. The __________ and __________ loops will not iterate at all if their test expressions are false to start with. 30. The __________ loop is ideal for situations that require a counter. 31. Inside the for loop’s parentheses, the first expression is the __________ , the second expression is the __________ , and the third expression is the __________. 32. A loop that is inside another is called a(n) __________ loop. 33. The __________ statement causes a loop to terminate immediately. 34. The __________ statement causes a loop to skip the remaining statements in the cur- rent iteration. Algorithm Workbench 35. Write a while loop that lets the user enter a number. The number should be multiplied by 10, and the result stored in the variable product. The loop should iterate as long as product contains a value less than 100. 36. Write a do-while loop that asks the user to enter two numbers. The numbers should be added and the sum displayed. The user should be asked if he or she wishes to perform the operation again. If so, the loop should repeat; otherwise it should terminate. 37. Write a for loop that displays the following set of numbers: 0, 10, 20, 30, 40, 50 . . . 1000 38. Write a loop that asks the user to enter a number. The loop should iterate 10 times and keep a running total of the numbers entered. 39. Write a nested loop that displays 10 rows of ‘#’ characters. There should be 15 ‘#’ characters in each row. 290 Chapter 5 Loops and Files 40. Convert the following while loop to a do-while loop: int x = 1; while (x > 0) { cout << "enter a number: "; cin >> x; } 41. Convert the following do-while loop to a while loop: char sure; do { cout << "Are you sure you want to quit? "; cin >> sure; } while (sure != 'Y' && sure != 'N'); 42. Convert the following while loop to a for loop: int count = 0; while (count < 50) { cout << "count is " << count << endl; count++; } 43. Convert the following for loop to a while loop: for (int x = 50; x > 0; x−−) { cout << x << " seconds to go.\n"; } 44. Write code that does the following: Opens an output file with the filename Numbers.txt, uses a loop to write the numbers 1 through 100 to the file, and then closes the file. 45. Write code that does the following: Opens the Numbers.txt file that was created by the code you wrote in question 44, reads all of the numbers from the file and displays them, and then closes the file. 46. Modify the code that you wrote in question 45 so it adds all of the numbers read from the file and displays their total. True or False 47. T 48. T F The operand of the increment and decrement operators can be any valid mathematical expression. F The cout statement in the following program segment will display 5: 49. T int x = 5; cout << x++; F The cout statement in the following program segment will display 5: 50. T 51. T 52. T 53. T int x = 5; cout << ++x; F The while loop is a pretest loop. F The do-while loop is a pretest loop. F The for loop is a posttest loop. F It is not necessary to initialize counter variables. Review Questions and Exercises 291 54. T 55. T 56. T 57. T 58. T 59. T 60. T 61. T 62. T 63. T 64. T F All three of the for loop’s expressions may be omitted. F One limitation of the for loop is that only one variable may be initialized in the initialization expression. F Variables may be defined inside the body of a loop. F A variable may be defined in the initialization expression of the for loop. F In a nested loop, the outer loop executes faster than the inner loop. F In a nested loop, the inner loop goes through all of its iterations for every single iteration of the outer loop. F To calculate the total number of iterations of a nested loop, add the number of iterations of all the loops. F The break statement causes a loop to stop the current iteration and begin the next one. F The continue statement causes a terminated loop to resume. F In a nested loop, the break statement only interrupts the loop it is placed in. F When you call an ofstream object’s open member function, the specified file will be erased if it already exists. Find the Errors Each of the following programs has errors. Find as many as you can. 65. // Find the error in this program. #include using namespace std; int main() { int num1 = 0, num2 = 10, result; num1++; result = ++(num1 + num2); cout << num1 << " " << num2 << " " << result; return 0; } 66. // This program adds two numbers entered by the user. #include using namespace std; int main() { int num1, num2; char again; while (again == 'y' || again == 'Y') cout << "Enter a number: "; cin >> num1; cout << "Enter another number: "; cin >> num2; cout << "Their sum is << (num1 + num2) << endl; cout << "Do you want to do this again? "; cin >> again; return 0; } 292 Chapter 5 Loops and Files 67. // This program uses a loop to raise a number to a power. #include using namespace std; int main() { int num, bigNum, power, count; cout << "Enter an integer: "; cin >> num; cout << "What power do you want it raised to? "; cin >> power; bigNum = num; while (count++ < power); bigNum *= num; cout << "The result is << bigNum << endl; return 0; } 68. // This program averages a set of numbers. #include using namespace std; int main() { int numCount, total; double average; cout << "How many numbers do you want to average? "; cin >> numCount; for (int count = 0; count < numCount; count++) { int num; cout << "Enter a number: "; cin >> num; total += num; count++; } average = total / numCount; cout << "The average is << average << endl; return 0; } 69. // This program displays the sum of two numbers. #include using namespace std; int main() { int choice, num1, num2; Programming Challenges 293 do { cout << "Enter a number: "; cin >> num1; cout << "Enter another number: "; cin >> num2; cout << "Their sum is " << (num1 + num2) << endl; cout << "Do you want to do this again?\n"; cout << "1 = yes, 0 = no\n"; cin >> choice; } while (choice = 1) return 0; } 70. // This program displays the sum of the numbers 1-100. #include using namespace std; int main() { int count = 1, total; while (count <= 100) total += count; cout << "The sum of the numbers 1-100 is "; cout << total << endl; return 0; } VideoNote Solving the Calories Burned Problem Programming Challenges 1. Sum of Numbers Write a program that asks the user for a positive integer value. The program should use a loop to get the sum of all the integers from 1 up to the number entered. For example, if the user enters 50, the loop will find the sum of 1, 2, 3, 4, … 50. Input Validation: Do not accept a negative starting number. 2. Characters for the ASCII Codes Write a program that uses a loop to display the characters for the ASCII codes 0 through 127. Display 16 characters on each line. 3. Ocean Levels Assuming the ocean’s level is currently rising at about 1.5 millimeters per year, write a program that displays a table showing the number of millimeters that the ocean will have risen each year for the next 25 years. 4. Calories Burned Running on a particular treadmill you burn 3.6 calories per minute. Write a program that uses a loop to display the number of calories burned after 5, 10, 15, 20, 25, and 30 minutes. 5. Membership Fees Increase A country club, which currently charges $2,500 per year for membership, has announced it will increase its membership fee by 4% each year for the next six years. Write a program that uses a loop to display the projected rates for the next six years. 294 Chapter 5 Loops and Files 6. Distance Traveled The distance a vehicle travels can be calculated as follows: distance = speed * time For example, if a train travels 40 miles per hour for 3 hours, the distance traveled is 120 miles. Write a program that asks the user for the speed of a vehicle (in miles per hour) and how many hours it has traveled. The program should then use a loop to display the distance the vehicle has traveled for each hour of that time period. Here is an example of the output: What is the speed of the vehicle in mph? 40 How many hours has it traveled? 3 Hour Distance Traveled -------------------------------- 1 40 2 80 3 120 Input Validation: Do not accept a negative number for speed and do not accept any value less than 1 for time traveled. 7. Pennies for Pay Write a program that calculates how much a person would earn over a period of time if his or her salary is one penny the first day and two pennies the second day, and continues to double each day. The program should ask the user for the number of days. Display a table showing how much the salary was for each day, and then show the total pay at the end of the period. The output should be displayed in a dollar amount, not the number of pennies. Input Validation: Do not accept a number less than 1 for the number of days worked. 8. Math Tutor This program started in Programming Challenge 15 of Chapter 3, and was modified in Programming Challenge 9 of Chapter 4. Modify the program again so it displays a menu allowing the user to select an addition, subtraction, multiplication, or division problem. The final selection on the menu should let the user quit the program. After the user has finished the math problem, the program should display the menu again. This process is repeated until the user chooses to quit the program. Input Validation: If the user selects an item not on the menu, display an error message and display the menu again. 9. Hotel Occupancy Write a program that calculates the occupancy rate for a hotel. The program should start by asking the user how many floors the hotel has. A loop should then iterate once for each floor. In each iteration, the loop should ask the user for the number of rooms on the floor and how many of them are occupied. After all the iterations, the program should display how many rooms the hotel has, how many of them are occupied, how many are unoccupied, and the percentage of rooms that are occupied. The percentage may be calculated by dividing the number of rooms occupied by the number of rooms. N OTE: It is traditional that most hotels do not have a thirteenth floor. The loop in this program should skip the entire thirteenth iteration. Programming Challenges 295 Input Validation: Do not accept a value less than 1 for the number of floors. Do not accept a number less than 10 for the number of rooms on a floor. 10. Average Rainfall Write a program that uses nested loops to collect data and calculate the average rainfall over a period of years. The program should first ask for the number of years. The outer loop will iterate once for each year. The inner loop will iterate twelve times, once for each month. Each iteration of the inner loop will ask the user for the inches of rainfall for that month. After all iterations, the program should display the number of months, the total inches of rainfall, and the average rainfall per month for the entire period. Input Validation: Do not accept a number less than 1 for the number of years. Do not accept negative numbers for the monthly rainfall. 11. Population Write a program that will predict the size of a population of organisms. The program should ask the user for the starting number of organisms, their average daily population increase (as a percentage), and the number of days they will multiply. A loop should display the size of the population for each day. Input Validation: Do not accept a number less than 2 for the starting size of the population. Do not accept a negative number for average daily population increase. Do not accept a number less than 1 for the number of days they will multiply. 12. Celsius to Fahrenheit Table In Programming Challenge 10 of Chapter 3 you were asked to write a program that converts a Celsius temperature to Fahrenheit. Modify that program so it uses a loop to display a table of the Celsius temperatures 0–20, and their Fahrenheit equivalents. 13. The Greatest and Least of These Write a program with a loop that lets the user enter a series of integers. The user should enter −99 to signal the end of the series. After all the numbers have been entered, the program should display the largest and smallest numbers entered. 14. Student Line Up A teacher has asked all her students to line up single file according to their first name. For example, in one class Amy will be at the front of the line and Yolanda will be at the end. Write a program that prompts the user to enter the number of students in the class, then loops to read that many names. Once all the names have been read it reports which student would be at the front of the line and which one would be at the end of the line. You may assume that no two students have the same name. Input Validation: Do not accept a number less than 1 or greater than 25 for the number of students. 15. Payroll Report Write a program that displays a weekly payroll report. A loop in the program should ask the user for the employee number, gross pay, state tax, federal tax, and FICA withholdings. The loop will terminate when 0 is entered for the employee number. After the data is entered, the program should display totals for gross pay, state tax, federal tax, FICA withholdings, and net pay. 296 Chapter 5 Loops and Files Input Validation: Do not accept negative numbers for any of the items entered. Do not accept values for state, federal, or FICA withholdings that are greater than the gross pay. If the sum state tax + federal tax + FICA withholdings for any employee is greater than gross pay, print an error message and ask the user to reenter the data for that employee. 16. Savings Account Balance Write a program that calculates the balance of a savings account at the end of a period of time. It should ask the user for the annual interest rate, the starting balance, and the number of months that have passed since the account was established. A loop should then iterate once for every month, performing the following: A) Ask the user for the amount deposited into the account during the month. (Do not accept negative numbers.) This amount should be added to the balance. B) Ask the user for the amount withdrawn from the account during the month. (Do not accept negative numbers.) This amount should be subtracted from the balance. C) Calculate the monthly interest. The monthly interest rate is the annual interest rate divided by twelve. Multiply the monthly interest rate by the balance, and add the result to the balance. After the last iteration, the program should display the ending balance, the total amount of deposits, the total amount of withdrawals, and the total interest earned. N OTE: If a negative balance is calculated at any point, a message should be displayed indicating the account has been closed and the loop should terminate. 17. Sales Bar Chart Write a program that asks the user to enter today’s sales for five stores. The program should then display a bar graph comparing each store’s sales. Create each bar in the bar graph by displaying a row of asterisks. Each asterisk should represent $100 of sales. Here is an example of the program’s output. Enter today's sales for store 1: 1000 [Enter] Enter today's sales for store 2: 1200 [Enter] Enter today's sales for store 3: 1800 [Enter] Enter today's sales for store 4: 800 [Enter] Enter today's sales for store 5: 1900 [Enter] SALES BAR CHART (Each * = $100) Store 1: ********** Store 2: ************ Store 3: ****************** Store 4: ******** Store 5: ******************* 18. Population Bar Chart Write a program that produces a bar chart showing the population growth of Prairieville, a small town in the Midwest, at 20-year intervals during the past 100 years. The program should read in the population figures (rounded to the nearest 1,000 people) for 1900, 1920, 1940, 1960, 1980, and 2000 from a file. For each year it should Programming Challenges 297 display the date and a bar consisting of one asterisk for each 1,000 people. The data can be found in the People.txt file. Here is an example of how the chart might begin: PRAIRIEVILLE POPULATION GROWTH (each * represents 1,000 people) 1900 ** 1920 **** 1940 ***** 19. Budget Analysis Write a program that asks the user to enter the amount that he or she has budgeted for a month. A loop should then prompt the user to enter each of his or her expenses for the month and keep a running total. When the loop finishes, the program should display the amount that the user is over or under budget. 20. Random Number Guessing Game Write a program that generates a random number and asks the user to guess what the number is. If the user’s guess is higher than the random number, the program should display “Too high, try again.” If the user’s guess is lower than the random number, the program should display “Too low, try again.” The program should use a loop that repeats until the user correctly guesses the random number. 21. Random Number Guessing Game Enhancement Enhance the program that you wrote for Programming Challenge 20 so it keeps a count of the number of guesses that the user makes. When the user correctly guesses the random number, the program should display the number of guesses. 22. Square Display Write a program that asks the user for a positive integer no greater than 15. The program should then display a square on the screen using the character ‘X’. The number entered by the user will be the length of each side of the square. For example, if the user enters 5, the program should display the following: XXXXX XXXXX XXXXX XXXXX XXXXX If the user enters 8, the program should display the following: XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX 298 Chapter 5 Loops and Files 23. Pattern Displays Write a program that uses a loop to display Pattern A below, followed by another loop that displays Pattern B. Pattern A + ++ +++ ++++ +++++ ++++++ +++++++ ++++++++ +++++++++ ++++++++++ Pattern B ++++++++++ +++++++++ ++++++++ +++++++ ++++++ +++++ ++++ +++ ++ + 24. Using Files—Numeric Processing If you have downloaded this book’s source code from the companion Web site, you will find a file named Random.txt in the Chapter 05 folder. (The companion Web site is at www.pearsonhighered.com/gaddis.) This file contains a long list of random numbers. Copy the file to your hard drive and then write a program that opens the file, reads all the numbers from the file, and calculates the following: A) The number of numbers in the file B) The sum of all the numbers in the file (a running total) C) The average of all the numbers in the file The program should display the number of numbers found in the file, the sum of the numbers, and the average of the numbers. 25. Using Files—Student Line Up Modify the Student Line Up program described in Programming Challenge 14 so that it gets the names from a file. Names should be read in until there is no more data to read. If you have downloaded this book’s source code from the companion Web site, you will find a file named LineUp.txt in the Chapter 05 folder. You can use this file to test the program. (The companion Web site is at www.pearsonhighered.com/gaddis.) 26. Using Files—Savings Account Balance Modification Modify the Savings Account Balance program described in Programming Challenge 16 so that it writes the final report to a file. CHAPTER 6 Functions TOPICS 6.1 Focus on Software Engineering: Modular Programming 6.2 Defining and Calling Functions 6.3 Function Prototypes 6.4 Sending Data into a Function 6.5 Passing Data by Value 6.6 Focus on Software Engineering: Using Functions in a Menu-Driven Program 6.7 The return Statement 6.8 Returning a Value from a Function 6.9 Returning a Boolean Value 6.10 Local and Global Variables 6.11 Static Local Variables 6.12 Default Arguments 6.13 Using Reference Variables as Parameters 6.14 Overloading Functions 6.15 The exit() Function 6.16 Stubs and Drivers 6.1 Focus on Software Engineering: Modular Programming CONCEPT: A program may be broken up into manageable functions. A function is a collection of statements that performs a specific task. So far you have experienced functions in two ways: (1) you have created a function named main in every program you’ve written, and (2) you have used library functions such as pow and strcmp. In this chapter you will learn how to create your own functions that can be used like library functions. Functions are commonly used to break a problem down into small manageable pieces. Instead of writing one long function that contains all of the statements necessary to solve a problem, several small functions that each solve a specific part of the problem can be written. These small functions can then be executed in the desired order to solve the problem. This approach is sometimes called divide and conquer because a large problem is 299 300 Chapter 6 Functions divided into several smaller problems that are easily solved. Figure 6-1 illustrates this idea by comparing two programs: one that uses a long complex function containing all of the statements necessary to solve a problem, and another that divides a problem into smaller problems, each of which are handled by a separate function. Figure 6-1 This program has one long, complex function containing all of the statements necessary to solve a problem. In this program the problem has been divided into smaller problems, each of which is handled by a separate function. int main() { statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; statement; } int main() { statement; statement; statement; } void function2() { statement; statement; statement; } void function3() { statement; statement; statement; } void function4() { statement; statement; statement; } main function function 2 function 3 function 4 Another reason to write functions is that they simplify programs. If a specific task is performed in several places in a program, a function can be written once to perform that task, and then be executed anytime it is needed. This benefit of using functions is known as code reuse because you are writing the code to perform a task once and then reusing it each time you need to perform the task. 6.2 Defining and Calling Functions CONCEPT: A function call is a statement that causes a function to execute. A function definition contains the statements that make up the function. 6.2 Defining and Calling Functions 301 When creating a function, you must write its definition. All function definitions have the following parts: Return type: Name: Parameter list: Body: A function can send a value to the part of the program that executed it. The return type is the data type of the value that is sent from the function. You should give each function a descriptive name. In general, the same rules that apply to variable names also apply to function names. The program can send data into a function. The parameter list is a list of variables that hold the values being passed to the function. The body of a function is the set of statements that perform the function’s operation. They are enclosed in a set of braces. Figure 6-2 shows the definition of a simple function with the various parts labeled. Figure 6-2 Return type Parameter list (This one is empty) Function name Function body int main () { cout << "Hello World\n"; return 0; } The line in the definition that reads int main() is called the function header. void Functions You already know that a function can return a value. The main function in all of the programs you have seen in this book is declared to return an int value to the operating system. The return 0; statement causes the value 0 to be returned when the main function finishes executing. It isn’t necessary for all functions to return a value, however. Some functions simply perform one or more statements, which follows terminate. These are called void functions. The displayMessage function, which follows, is an example. void displayMessage() { cout << "Hello from the function displayMessage.\n"; } The function’s name is displayMessage. This name gives an indication of what the function does: It displays a message. You should always give functions names that reflect their purpose. Notice that the function’s return type is void. This means the function does not return a value to the part of the program that executed it. Also notice the function has no return statement. It simply displays a message on the screen and exits. 302 Chapter 6 Functions Calling a Function A function is executed when it is called. Function main is called automatically when a program starts, but all other functions must be executed by function call statements. When a function is called, the program branches to that function and executes the statements in its body. Let’s look at Program 6-1, which contains two functions: main and displayMessage. Program 6-1 1 // This program has two functions: main and displayMessage 2 #include 3 using namespace std; 4 5 //***************************************** 6 // Definition of function displayMessage * 7 // This function displays a greeting. * 8 //***************************************** 9 10 void displayMessage() 11 { 12 cout << "Hello from the function displayMessage.\n"; 13 } 14 15 //***************************************** 16 // Function main * 17 //***************************************** 18 19 int main() 20 { 21 cout << "Hello from main.\n"; 22 displayMessage(); 23 cout << "Back in function main again.\n"; 24 return 0; 25 } Program Output Hello from main. Hello from the function displayMessage. Back in function main again. The function displayMessage is called by the following statement in line 22: displayMessage(); This statement is the function call. It is simply the name of the function followed by a set of parentheses and a semicolon. Let’s compare this with the function header: Function Header Function Call void displayMessage() displayMessage(); 6.2 Defining and Calling Functions 303 The function header is part of the function definition. It declares the function’s return type, name, and parameter list. It is not terminated with a semicolon because the definition of the function’s body follows it. The function call is a statement that executes the function, so it is terminated with a semicolon like all other C++ statements. The return type is not listed in the function call, and, if the program is not passing data into the function, the parentheses are left empty. N OTE: Later in this chapter you will see how data can be passed into a function by being listed inside the parentheses. Even though the program starts executing at main, the function displayMessage is defined first. This is because the compiler must know the function’s return type, the number of parameters, and the type of each parameter before the function is called. One way to ensure the compiler will know this information is to place the function definition before all calls to that function. (Later you will see an alternative, preferred method of accomplishing this.) N O T E : You should always document your functions by writing comments that describe what they do. These comments should appear just before the function definition. Notice how Program 6-1 flows. It starts, of course, in function main. When the call to displayMessage is encountered, the program branches to that function and performs its statements. Once displayMessage has finished executing, the program branches back to function main and resumes with the line that follows the function call. This is illustrated in Figure 6-3. Figure 6-3 void displayMessage() { cout << "Hello from the function displayMessage.\n"; } int main() { cout << "Hello from main.\n" displayMessage(); cout << "Back in function main again.\n"; return 0; } Function call statements may be used in control structures like loops, if statements, and switch statements. Program 6-2 places the displayMessage function call inside a loop. 304 Chapter 6 Functions Program 6-2 1 // The function displayMessage is repeatedly called from a loop. 2 #include 3 using namespace std; 4 5 //***************************************** 6 // Definition of function displayMessage * 7 // This function displays a greeting. * 8 //***************************************** 9 10 void displayMessage() 11 { 12 cout << "Hello from the function displayMessage.\n"; 13 } 14 15 //***************************************** 16 // Function main * 17 //***************************************** 18 19 int main() 20 { 21 cout << "Hello from main.\n"; 22 for (int count = 0; count < 5; count++) 23 displayMessage(); // Call displayMessage 24 cout << "Back in function main again.\n"; 25 return 0; 26 } Program Output Hello from main. Hello from the function displayMessage. Hello from the function displayMessage. Hello from the function displayMessage. Hello from the function displayMessage. Hello from the function displayMessage. Back in function main again. It is possible to have many functions and function calls in a program. Program 6-3 has three functions: main, first, and second. Program 6-3 1 // This program has three functions: main, first, and second. 2 #include 3 using namespace std; 4 6.2 Defining and Calling Functions 305 5 //***************************************** 6 // Definition of function first * 7 // This function displays a message. * 8 //***************************************** 9 10 void first() 11 { 12 cout << "I am now inside the function first.\n"; 13 } 14 15 //***************************************** 16 // Definition of function second * 17 // This function displays a message. * 18 //***************************************** 19 20 void second() 21 { 22 cout << "I am now inside the function second.\n"; 23 } 24 25 //***************************************** 26 // Function main * 27 //***************************************** 28 29 int main() 30 { 31 cout << "I am starting in function main.\n"; 32 first(); // Call function first 33 second(); // Call function second 34 cout << "Back in function main again. \n"; 35 return 0; 36 } Program Output I am starting in function main. I am now inside the function first. I am now inside the function second. Back in function main again. In lines 32 and 33 of Program 6-3, function main contains a call to first and a call to second: first(); second(); Each call statement causes the program to branch to a function and then back to main when the function is finished. Figure 6-4 illustrates the paths taken by the program. 306 Chapter 6 Functions Figure 6-4 void first() { cout << "I am now inside the function first.\n"; } void second() { cout << "I am now inside the function second.\n"; } int main() { cout << "I am starting in function main.\n" first(); second(); cout << "Back in function main again.\n"; return 0; } Functions may also be called in a hierarchical, or layered, fashion. This is demonstrated by Program 6-4, which has three functions: main, deep, and deeper. Program 6-4 1 // This program has three functions: main, deep, and deeper 2 #include 3 using namespace std; 4 5 //***************************************** 6 // Definition of function deeper * 7 // This function displays a message. * 8 //***************************************** 9 10 void deeper() 11 { 12 cout << "I am now inside the function deeper.\n"; 13 } 14 15 //***************************************** 16 // Definition of function deep * 17 // This function displays a message. * 18 //***************************************** 19 6.2 Defining and Calling Functions 307 20 void deep() 21 { 22 cout << "I am now inside the function deep.\n"; 23 deeper(); // Call function deeper 24 cout << "Now I am back in deep.\n"; 25 } 26 27 //***************************************** 28 // Function main * 29 //***************************************** 30 31 int main() 32 { 33 cout << "I am starting in function main.\n"; 34 deep(); // Call function deep 35 cout << "Back in function main again.\n"; 36 return 0; 37 } Program Output I am starting in function main. I am now inside the function deep. I am now inside the function deeper. Now I am back in deep. Back in function main again. In Program 6-4, function main only calls the function deep. In turn, deep calls deeper. The paths taken by the program are shown in Figure 6-5. Figure 6-5 void deep() { cout << "I am now inside the function deep.\n"; deeper(); cout << "Now I am back in deep.\n"; } void deeper() { cout << "I am now in the function deeper.\n"; } int main() { cout << "I am starting in function main.\n"; deep(); cout << "Back in function main again.\n"; return 0; } 308 Chapter 6 Functions Checkpoint 6.1 Is the following a function header or a function call? calcTotal(); 6.2 Is the following a function header or a function call? void showResults() 6.3 What will the output of the following program be if the user enters 10? #include using namespace std; void func1() { cout << "Able was I\n"; } void func2() { cout << "I saw Elba\n"; } int main() { int input; cout << "Enter a number: "; cin >> input; if (input < 10) { func1(); func2(); } else { func2(); func1(); } return 0; } 6.4 The following program skeleton determines whether a person qualifies for a credit card. To qualify, the person must have worked on his or her current job for at least two years and make at least $17,000 per year. Finish the program by writing the definitions of the functions qualify and noQualify. The function qualify should explain that the applicant qualifies for the card and that the annual interest rate is 12%. The function noQualify should explain that the applicant does not qualify for the card and give a general explanation why. #include using namespace std; // You must write definitions for the two functions qualify // and noQualify. 6.3 Function Prototypes 309 int main() { double salary; int years; cout << "This program will determine if you qualify\n"; cout << "for our credit card.\n"; cout << "What is your annual salary? "; cin >> salary; cout << "How many years have you worked at your "; cout << "current job? "; cin >> years; if (salary >= 17000.0 && years >= 2) qualify(); else noQualify(); return 0; } 6.3 Function Prototypes CONCEPT: A function prototype eliminates the need to place a function definition before all calls to the function. Before the compiler encounters a call to a particular function, it must already know the function’s return type, the number of parameters it uses, and the type of each parameter. (You will learn how to use parameters in the next section.) One way of ensuring that the compiler has this information is to place the function definition before all calls to that function. This was the approach taken in Programs 6-1, 6-2, 6-3, and 6-4. Another method is to declare the function with a function prototype. Here is a prototype for the displayMessage function in Program 6-1: void displayMessage(); The prototype looks similar to the function header, except there is a semicolon at the end. The statement above tells the compiler that the function displayMessage has a void return type (it doesn’t return a value) and uses no parameters. N OTE: Function prototypes are also known as function declarations. W A R N I N G ! You must place either the function definition or either/the function prototype ahead of all calls to the function. Otherwise the program will not compile. Function prototypes are usually placed near the top of a program so the compiler will encounter them before any function calls. Program 6-5 is a modification of Program 6-3. The definitions of the functions first and second have been placed after main, and a function prototype has been placed after the using namespace std statement. 310 Chapter 6 Functions Program 6-5 1 // This program has three functions: main, first, and second. 2 #include 3 using namespace std; 4 5 // Function Prototypes 6 void first(); 7 void second(); 8 9 int main() 10 { 11 cout << "I am starting in function main.\n"; 12 first(); // Call function first 13 second(); // Call function second 14 cout << "Back in function main again.\n"; 15 return 0; 16 } 17 18 //************************************* 19 // Definition of function first. * 20 // This function displays a message. * 21 //************************************* 22 23 void first() 24 { 25 cout << "I am now inside the function first.\n"; 26 } 27 28 //************************************* 29 // Definition of function second. * 30 // This function displays a message. * 31 //************************************* 32 33 void second() 34 { 35 cout << "I am now inside the function second.\n"; 36 } Program Output (The program’s output is the same as the output of Program 6-3.) When the compiler is reading Program 6-5, it encounters the calls to the functions first and second in lines 12 and 13 before it has read the definition of those functions. Because of the function prototypes, however, the compiler already knows the return type and parameter information of first and second. N O T E : Although some programmers make main the last function in the program, many prefer it to be first because it is the program’s starting point. 6.4 Sending Data into a Function 311 6.4 Sending Data into a Function VideoNote Functions and Arguments CONCEPT: When a function is called, the program may send values into the function. Values that are sent into a function are called arguments. You’re already familiar with how to use arguments in a function call. In the following statement the function pow is being called and two arguments, 2.0 and 4.0, are passed to it: result = pow(2.0, 4.0); By using parameters, you can design your own functions that accept data this way. A parameter is a special variable that holds a value being passed into a function. Here is the definition of a function that uses a parameter: void displayValue(int num) { cout << "The value is " << num << endl; } Notice the integer variable definition inside the parentheses (int num). The variable num is a parameter. This enables the function displayValue to accept an integer value as an argument. Program 6-6 is a complete program using this function. N OTE: In this text, the values that are passed into a function are called arguments, and the variables that receive those values are called parameters. There are several variations of these terms in use. Some call the arguments actual parameters and call the parameters formal parameters. Others use the terms actual argument and formal argument. Regardless of which set of terms you use, it is important to be consistent. Program 6-6 1 // This program demonstrates a function with a parameter. 2 #include 3 using namespace std; 4 5 // Function Prototype 6 void displayValue(int); 7 8 int main() 9{ 10 cout << "I am passing 5 to displayValue.\n"; 11 displayValue(5); // Call displayValue with argument 5 12 cout << "Now I am back in main.\n"; 13 return 0; 14 } 15 (program continues) 312 Chapter 6 Functions Program 6-6 (continued) 16 //********************************************************* 17 // Definition of function displayValue. * 18 // It uses an integer parameter whose value is displayed. * 19 //********************************************************* 20 21 void displayValue(int num) 22 { 23 cout << "The value is " << num << endl; 24 } Program Output I am passing 5 to displayValue. The value is 5 Now I am back in main. First, notice the function prototype for displayValue in line 6: void displayValue(int); It is not necessary to list the name of the parameter variable inside the parentheses. Only its data type is required. The function prototype shown above could optionally have been written as: void displayValue(int num); However, the compiler ignores the name of the parameter variable in the function prototype. In main, the displayValue function is called with the argument 5 inside the parentheses. The number 5 is passed into num, which is displayValue’s parameter. This is illustrated in Figure 6-6. Figure 6-6 displayValue(5); void displayValue(int num) { cout << "The value is " << num << endl; } Any argument listed inside the parentheses of a function call is copied into the function’s parameter variable. In essence, parameter variables are initialized to the value of their corresponding arguments. Program 6-7 shows the function displayValue being called several times with a different argument being passed each time. 6.4 Sending Data into a Function 313 Program 6-7 1 // This program demonstrates a function with a parameter. 2 #include 3 using namespace std; 4 5 // Function Prototype 6 void displayValue(int); 7 8 int main() 9{ 10 cout << "I am passing several values to displayValue.\n"; 11 displayValue(5); // Call displayValue with argument 5 12 displayValue(10); // Call displayValue with argument 10 13 displayValue(2); // Call displayValue with argument 2 14 displayValue(16); // Call displayValue with argument 16 15 cout << "Now I am back in main.\n"; 16 return 0; 17 } 18 19 //********************************************************* 20 // Definition of function displayValue. * 21 // It uses an integer parameter whose value is displayed. * 22 //********************************************************* 23 24 void displayValue(int num) 25 { 26 cout << "The value is " << num << endl; 27 } Program Output I am passing several values to displayValue. The value is 5 The value is 10 The value is 2 The value is 16 Now I am back in main. WARNING! When passing a variable as an argument, simply write the variable name inside the parentheses of the function call. Do not write the data type of the argument variable in the function call. For example, the following function call will cause an error: displayValue(int x); // Error! The function call should appear as displayValue(x); // Correct Each time the function is called in Program 6-7, num takes on a different value. Any expression whose value could normally be assigned to num may be used as an argument. For example, the following function call would pass the value 8 into num: displayValue(3 + 5); 314 Chapter 6 Functions If you pass an argument whose type is not the same as the parameter’s type, the argument will be promoted or demoted automatically. For instance, the argument in the following function call would be truncated, causing the value 4 to be passed to num: displayValue(4.7); Often, it’s useful to pass several arguments into a function. Program 6-8 shows the definition of a function with three parameters. Program 6-8 1 // This program demonstrates a function with three parameters. 2 #include 3 using namespace std; 4 5 // Function Prototype 6 void showSum(int, int, int); 7 8 int main() 9{ 10 int value1, value2, value3; 11 12 // Get three integers. 13 cout << "Enter three integers and I will display "; 14 cout << "their sum: "; 15 cin >> value1 >> value2 >> value3; 16 17 // Call showSum passing three arguments. 18 showSum(value1, value2, value3); 19 return 0; 20 } 21 22 //************************************************************ 23 // Definition of function showSum. * 24 // It uses three integer parameters. Their sum is displayed. * 25 //************************************************************ 26 27 void showSum(int num1, int num2, int num3) 28 { 29 cout << (num1 + num2 + num3) << endl; 30 } Program Output with Example Input Shown in Bold Enter three integers and I will display their sum: 4 8 7 [Enter] 19 In the function header for showSum, the parameter list contains three variable definitions separated by commas: void showSum(int num1, int num2, int num3) 6.4 Sending Data into a Function 315 WARN IN G ! Each parameter variable in a parameter list must have a data type listed before its name. For example, a compiler error would occur if the parameter list for the showSum function were defined as shown in the following header: void showSum(int num1, num2, num3) // Error! A data type for all three of the parameter variables must be listed, as shown here: void showSum(int num1, int num2, int num3) // Correct In the function call in line 18, the variables value1, value2, and value3 are passed as arguments: showSum(value1, value2, value3); When a function with multiple parameters is called, the arguments are passed to the parameters in order. This is illustrated in Figure 6-7. Figure 6-7 Function Call showSum(value1, value2, value3) void showSum(int num1, int num2, int num3) { cout << (num1 + num2 + num3) << endl; } The following function call will cause 5 to be passed into the num1 parameter, 10 to be passed into num2, and 15 to be passed into num3: showSum(5, 10, 15); However, the following function call will cause 15 to be passed into the num1 parameter, 5 to be passed into num2, and 10 to be passed into num3: showSum(15, 5, 10); N OTE: The function prototype must list the data type of each parameter. N O T E : Like all variables, parameters have a scope. The scope of a parameter is limited to the body of the function that uses it. 316 Chapter 6 Functions 6.5 Passing Data by Value CONCEPT: When an argument is passed into a parameter, only a copy of the argument’s value is passed. Changes to the parameter do not affect the original argument. As you’ve seen in this chapter, parameters are special-purpose variables that are defined inside the parentheses of a function definition. They are separate and distinct from the arguments that are listed inside the parentheses of a function call. The values that are stored in the parameter variables are copies of the arguments. Normally, when a parameter’s value is changed inside a function, it has no effect on the original argument. Program 6-9 demonstrates this concept. Program 6-9 1 // This program demonstrates that changes to a function parameter 2 // have no effect on the original argument. 3 #include 4 using namespace std; 5 6 // Function Prototype 7 void changeMe(int); 8 9 int main() 10 { 11 int number = 12; 12 13 // Display the value in number. 14 cout << "number is " << number << endl; 15 16 // Call changeMe, passing the value in number 17 // as an argument. 18 changeMe(number); 19 20 // Display the value in number again. 21 cout << "Now back in main again, the value of "; 22 cout << "number is " << number << endl; 23 return 0; 24 } 25 26 //************************************************************** 27 // Definition of function changeMe. * 28 // This function changes the value of the parameter myValue. * 29 //************************************************************** 6.5 Passing Data by Value 317 30 31 void changeMe(int myValue) 32 { 33 // Change the value of myValue to 0. 34 myValue = 0; 35 36 // Display the value in myValue. 37 cout << "Now the value is " << myValue << endl; 38 } Program Output number is 12 Now the value is 0 Now back in main again, the value of number is 12 Even though the parameter variable myValue is changed in the changeMe function, the argument number is not modified. The myValue variable contains only a copy of the number variable. The changeMe function does not have access to the original argument. When only a copy of an argument is passed to a function, it is said to be passed by value. This is because the function receives a copy of the argument’s value and does not have access to the original argument. Figure 6-8 illustrates that a parameter variable’s storage location in memory is separate from that of the original argument. Figure 6-8 Original Argument (in its memory location) 12 Function Parameter (in its memory location) 12 NOTE: Later in this chapter you will learn ways to give a function access to its original arguments. 318 Chapter 6 Functions 6.6 Focus on Software Engineering: Using Functions in a Menu-Driven Program CONCEPT: Functions are ideal for use in menu-driven programs. When the user selects an item from a menu, the program can call the appropriate function. In Chapters 4 and 5 you saw a menu-driven program that calculates the charges for a health club membership. Program 6-10 shows the program redesigned as a modular program. A modular program is broken up into functions that perform specific tasks. Program 6-10 1 // This is a menu-driven program that makes a function call 2 // for each selection the user makes. 3 #include 4 #include 5 using namespace std; 6 7 // Function prototypes 8 void showMenu(); 9 void showFees(double, int); 10 11 int main() 12 { 13 int choice; // To hold a menu choice 14 int months; // To hold a number of months 15 16 // Constants for the menu choices 17 const int ADULT_CHOICE = 1, 18 CHILD_CHOICE = 2, 19 SENIOR_CHOICE = 3, 20 QUIT_CHOICE = 4; 21 22 // Constants for membership rates 23 const double ADULT = 40.0, 24 CHILD = 20.0; 25 SENIOR = 30.0, 26 27 // Set up numeric output formatting. 28 cout << fixed << showpoint << setprecision(2); 29 30 do 31 { 32 // Display the menu and get the user's choice. 33 showMenu(); 34 cin >> choice; 35 36 // Validate the menu selection. 37 while (choice < ADULT_CHOICE || choice > QUIT_CHOICE) 38 { 6.6 Focus on Software Engineering: Using Functions in a Menu-Driven Program 319 39 cout << "Please enter a valid menu choice: "; 40 cin >> choice; 41 } 42 43 // If the user does not want to quit, proceed. 44 if (choice != QUIT_CHOICE) 45 { 46 // Get the number of months. 47 cout << "For how many months? "; 48 cin >> months; 49 50 // Display the membership fees. 51 switch (choice) 52 { 53 case ADULT_CHOICE: 54 showFees(ADULT, months); 55 break; 56 case CHILD_CHOICE: 57 showFees(CHILD, months); 58 break; 59 case SENIOR_CHOICE: 60 showFees(SENIOR, months); 61 } 62 } 63 } while (choice != QUIT_CHOICE); 64 return 0; 65 } 66 67 //***************************************************************** 68 // Definition of function showMenu which displays the menu. * 69 //***************************************************************** 70 71 void showMenu() 72 { 73 cout << "\n\t\tHealth Club Membership Menu\n\n" 74 << "1. Standard Adult Membership\n" 75 << "2. Child Membership\n" 76 << "3. Senior Citizen Membership\n" 77 << "4. Quit the Program\n\n" 78 << "Enter your choice: "; 79 } 80 81 //****************************************************************** 82 // Definition of function showFees. The memberRate parameter holds * 83 // the monthly membership rate and the months parameter holds the * 84 // number of months. The function displays the total charges. * 85 //****************************************************************** 86 87 void showFees(double memberRate, int months) 88 { 89 cout << "The total charges are $" 90 << (memberRate * months) << endl; 91 } (program output continues) 320 Chapter 6 Functions Program 6-10 (continued) Program Output with Example Input Shown in Bold Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 1 [Enter] For how many months? 12 [Enter] The total charges are $480.00 Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 4 [Enter] Let’s take a closer look at this program. First notice the showMenu function in lines 71 through 79. This function displays the menu and is called from the main function in line 33. The showFees function appears in lines 87 through 91. Its purpose is to display the total fees for a membership lasting a specified number of months. The function accepts two arguments: the monthly membership fee (a double) and the number of months of membership (an int). The function uses these values to calculate and display the total charges. For example, if we wanted the function to display the fees for an adult membership lasting six months, we would pass the ADULT constant as the first argument and 6 as the second argument. The showFees function is called from three different locations in the switch statement, which is in the main function. The first location is line 54. This statement is executed when the user has selected item 1, standard adult membership, from the menu. The showFees function is called with the ADULT constant and the months variable passed as arguments. The second location is line 57. This statement is executed when the user has selected item 2, child membership, from the menu. The showFees function is called in this line with the CHILD constant and the months variable passed as arguments. The third location is line 60. This statement is executed when the user has selected item 3, senior citizen membership, from the menu. The showFees function is called with the SENIOR constant and the months variable passed as arguments. Each time the showFees function is called, it displays the total membership fees for the specified type of membership, for the specified number of months. 6.6 Focus on Software Engineering: Using Functions in a Menu-Driven Program 321 Checkpoint 6.5 Indicate which of the following is the function prototype, the function header, and the function call: void showNum(double num) void showNum(double); showNum(45.67); 6.6 Write a function named timesTen. The function should have an integer parameter named number. When timesTen is called, it should display the product of number times ten. (Note: just write the function. Do not write a complete program.) 6.7 Write a function prototype for the timesTen function you wrote in Question 6.6. 6.8 What is the output of the following program? #include using namespace std; void showDouble(int); // Function prototype int main() { int num; for (num = 0; num < 10; num++) showDouble(num); return 0; } // Definition of function showDouble. void showDouble(int value) { cout << value << "\t" << (value * 2) << endl; } 6.9 What is the output of the following program? #include using namespace std; void func1(double, int); // Function prototype int main() { int x = 0; double y = 1.5; cout << x << " " << y << endl; func1(y, x); cout << x << " " << y << endl; return 0; } 322 Chapter 6 Functions void func1(double a, int b) { cout << a << " " << b << endl; a = 0.0; b = 10; cout << a << " " << b << endl; } 6.10 The following program skeleton asks for the number of hours you’ve worked and your hourly pay rate. It then calculates and displays your wages. The function showDollars, which you are to write, formats the output of the wages. #include using namespace std; void showDollars(double); // Function prototype int main() { double payRate, hoursWorked, wages; cout << "How many hours have you worked? " cin >> hoursWorked; cout << "What is your hourly pay rate? "; cin >> payRate; wages = hoursWorked * payRate; showDollars(wages); return 0; } // You must write the definition of the function showDollars // here. It should take one parameter of the type double. // The function should display the message "Your wages are $" // followed by the value of the parameter. It should be displayed // with 2 places of precision after the decimal point, in fixed // notation, and the decimal point should always display. 6.7 The return Statement CONCEPT: The return statement causes a function to end immediately. When the last statement in a void function has finished executing, the function terminates and the program returns to the statement following the function call. It’s possible, however, to force a function to return before the last statement has been executed. When the return statement is encountered, the function immediately terminates and control of the program returns to the statement that called the function. This is demonstrated in Program 6-11. The function divide shows the quotient of arg1 divided by arg2. If arg2 is set to zero, the function returns. 6.7 The return Statement 323 Program 6-11 1 // This program uses a function to perform division. If division 2 // by zero is detected, the function returns. 3 #include 4 using namespace std; 5 6 // Function prototype. 7 void divide(double, double); 8 9 int main() 10 { 11 double num1, num2; 12 13 cout << "Enter two numbers and I will divide the first\n"; 14 cout << "number by the second number: "; 15 cin >> num1 >> num2; 16 divide(num1, num2); 17 return 0; 18 } 19 20 //*************************************************************** 21 // Definition of function divide. * 22 // Uses two parameters: arg1 and arg2. The function divides arg1 * 23 // by arg2 and shows the result. If arg2 is zero, however, the * 24 // function returns. * 25 //*************************************************************** 26 27 void divide(double arg1, double arg2) 28 { 29 if (arg2 == 0.0) 30 { 31 cout << "Sorry, I cannot divide by zero.\n"; 32 return; 33 } 34 cout << "The quotient is " << (arg1 / arg2) << endl; 35 } Program Output with Example Input Shown in Bold Enter two numbers and I will divide the first number by the second number: 12 0 [Enter] Sorry, I cannot divide by zero. In the example running of the program, the user entered 12 and 0 as input. In line 16 the divide function was called, passing 12 into the arg1 parameter and 0 into the arg2 parameter. Inside the divide function, the if statement in line 29 executes. Because arg2 is equal to 0.0, the code in lines 31 and 32 executes. When the return statement in line 32 executes, the divide function immediately ends. This means the cout statement in line 34 does not execute. The program resumes at line 17 in the main function. 324 Chapter 6 Functions 6.8 Returning a Value from a Function CONCEPT: A function may send a value back to the part of the program that called the function. VideoNote Value-Returnlng Functions You’ve seen that data may be passed into a function by way of parameter variables. Data may also be returned from a function, back to the statement that called it. Functions that return a value are appropriately known as value-returning functions. The pow function, which you have already seen, is an example of a value-returning function. Here is an example: double x; x = pow(4.0, 2.0); The second line in this code calls the pow function, passing 4.0 and 2.0 as arguments. The function calculates the value of 4.0 raised to the power of 2.0 and returns that value. The value, which is 16.0, is assigned to the x variable by the = operator. Although several arguments may be passed into a function, only one value may be returned from it. Think of a function as having multiple communication channels for receiving data (parameters), but only one channel for sending data (the return value). This is illustrated in Figure 6-9. Figure 6-9 argument argument argument argument Function Return value N OTE: It is possible to return multiple values from a function, but they must be “packaged” in such a way that they are treated as a single value. This is a topic of Chapter 11. Defining a Value-Returning Function When you are writing a value-returning function, you must decide what type of value the function will return. This is because you must specify the data type of the return value in the function header, and in the function prototype. Recall that a void function, which does not return a value, uses the key word void as its return type in the function header. A 6.8 Returning a Value from a Function 325 value-returning function will use int, double, bool, or any other valid data type in its header. Here is an example of a function that returns an int value: int sum(int num1, int num2) { int result; result = num1 + num2; return result; } The name of this function is sum. Notice in the function header that the return type is int, as illustrated in Figure 6-10. Figure 6-10 Return Type int sum(int num1, int num2) This code defines a function named sum that accepts two int arguments. The arguments are passed into the parameter variables num1 and num2. Inside the function, a variable, result, is defined. Variables that are defined inside a function are called local variables. After the variable definition, the parameter variables num1 and num2 are added, and their sum is assigned to the result variable. The last statement in the function is return result; This statement causes the function to end, and it sends the value of the result variable back to the statement that called the function. A value-returning function must have a return statement written in the following general format: return expression; In the general format, expression is the value to be returned. It can be any expression that has a value, such as a variable, literal, or mathematical expression. The value of the expression is converted to the data type that the function returns and is sent back to the statement that called the function. In this case, the sum function returns the value in the result variable. However, we could have eliminated the result variable and returned the expression num1 + num2, as shown in the following code: int sum(int num1, int num2) { return num1 + num2; } When writing the prototype for a value-returning function, follow the same conventions that we have covered earlier. Here is the prototype for the sum function: int sum(int, int); 326 Chapter 6 Functions Calling a Value-Returning Function Program 6-12 shows an example of how to call the sum function. Program 6-12 1 // This program uses a function that returns a value. 2 #include 3 using namespace std; 4 5 // Function prototype 6 int sum(int, int); 7 8 int main() 9{ 10 int value1 = 20, // The first value 11 value2 = 40, // The second value 12 total; // To hold the total 13 14 // Call the sum function, passing the contents of 15 // value1 and value2 as arguments. Assign the return 16 // value to the total variable. 17 total = sum(value1, value2); 18 19 // Display the sum of the values. 20 cout << "The sum of " << value1 << " and " 21 << value2 << " is " << total << endl; 22 return 0; 23 } 24 25 //***************************************************** 26 // Definition of function sum. This function returns * 27 // the sum of its two parameters. * 28 //***************************************************** 29 30 int sum(int num1, int num2) 31 { 32 return num1 + num2; 33 } Program Output The sum of 20 and 40 is 60 Here is the statement in line 17 that calls the sum function, passing value1 and value2 as arguments. total = sum(value1, value2); This statement assigns the value returned by the sum function to the total variable. In this case, the function will return 60. Figure 6-11 shows how the arguments are passed into the function and how a value is passed back from the function. 6.8 Returning a Value from a Function 327 Figure 6-11 total = sum(value1, value2); 40 20 60 int sum(int num1, int num2) { return num + num; } When you call a value-returning function, you usually want to do something meaningful with the value it returns. Program 6-12 shows a function’s return value being assigned to a variable. This is commonly how return values are used, but you can do many other things with them. For example, the following code shows a mathematical expression that uses a call to the sum function: int x = 10, y = 15; double average; average = sum(x, y) / 2.0; In the last statement, the sum function is called with x and y as its arguments. The function’s return value, which is 25, is divided by 2.0. The result, 12.5, is assigned to average. Here is another example: int x = 10, y = 15; cout << "The sum is " << sum(x, y) << endl; This code sends the sum function’s return value to cout so it can be displayed on the screen. The message “The sum is 25” will be displayed. Remember, a value-returning function returns a value of a specific data type. You can use the function’s return value anywhere that you can use a regular value of the same data type. This means that anywhere an int value can be used, a call to an int value-returning function can be used. Likewise, anywhere a double value can be used, a call to a double value-returning function can be used. The same is true for all other data types. Let’s look at another example. Program 6-13, which calculates the area of a circle, has two functions in addition to main. One of the functions is named square, and it returns the square of any number passed to it as an argument. The square function is called in a mathematical statement. The program also has a function named getRadius, which prompts the user to enter the circle’s radius. The value entered by the user is returned from the function. Program 6-13 1 // This program demonstrates two value-returning functions. 2 // The square function is called in a mathematical statement. 3 #include 4 #include 5 using namespace std; (program continues) 328 Chapter 6 Functions Program 6-13 (continued) 6 7 //Function prototypes 8 double getRadius(); 9 double square(double); 10 11 int main() 12 { 13 const double PI = 3.14159; // Constant for pi 14 double radius; // To hold the circle's radius 15 double area; // To hold the circle's area 16 17 // Set the numeric output formatting. 18 cout << fixed << showpoint << setprecision(2); 19 20 // Get the radius of the circle. 21 cout << "This program calculates the area of "; 22 cout << "a circle.\n"; 23 radius = getRadius(); 24 25 // Calculate the area of the circle. 26 area = PI * square(radius); 27 28 // Display the area. 29 cout << "The area is " << area << endl; 30 return 0; 31 } 32 33 //******************************************************* 34 // Definition of function getRadius. * 35 // This function asks the user to enter the radius of * 36 // the circle and then returns that number as a double. * 37 //******************************************************* 38 39 double getRadius() 40 { 41 double rad; 42 43 cout << "Enter the radius of the circle: "; 44 cin >> rad; 45 return rad; 46 } 47 48 //******************************************************* 49 // Definition of function square. * 50 // This function accepts a double argument and returns * 51 // the square of the argument as a double. * 52 //******************************************************* 53 54 double square(double number) 6.8 Returning a Value from a Function 329 55 { 56 57 } return number * number; Program Output with Example Input Shown in Bold This program calculates the area of a circle. Enter the radius of the circle: 10 [Enter] The area is 314.16 First, look at the getRadius function defined in lines 39 through 46. The purpose of the function is to prompt the user to enter the radius of a circle. In line 41 the function defines a local variable, rad. Lines 43 and 44 prompt the user to enter the circle’s radius, which is stored in the rad variable. In line 45 the value of the rad value is returned. The getRadius function is called in the main function, in line 23. The value that is returned from the function is assigned to the radius variable. Next look at the square function, which is defined in lines 54 through 57. When the function is called, a double argument is passed to it. The function stores the argument in the number parameter. The return statement in line 56 returns the value of the expression number * number, which is the square of the number parameter. The square function is called in the main function, in line 26, with the value of radius passed as an argument. The function will return the square of the radius variable, and that value will be used in the mathematical expression. Assuming the user has entered 10 as the radius, and this value is passed as an argument to the square function, the square function will return the value 100. Figure 6-12 illustrates how the value 100 is passed back to the mathematical expression in line 26. The value 100 will then be used in the mathematical expression. Figure 6-12 area = PI * square(radius); 10 100 double square(double number) { return number * number; } Functions can return values of any type. Both the getRadius and square functions in Program 6-13 return a double. The sum function you saw in Program 6-12 returned an int. When a statement calls a value-returning function, it should properly handle the return value. For example, if you assign the return value of the square function to a variable, the variable should be a double. If the return value of the function has a fractional portion and you assign it to an int variable, the value will be truncated. 330 Chapter 6 Functions In the Spotlight: Using Functions Your friend Michael runs a catering company. Some of the ingredients that his recipes require are measured in cups. When he goes to the grocery store to buy those ingredients, however, they are sold only by the fluid ounce. He has asked you to write a simple program that converts cups to fluid ounces. You design the following algorithm: 1. Display an introductory screen that explains what the program does. 2. Get the number of cups. 3. Convert the number of cups to fluid ounces and display the result. This algorithm lists the top level of tasks that the program needs to perform and becomes the basis of the program’s main function. The hierarchy chart shown in Figure 6-13 shows how the program will broken down into functions. Figure 6-13 Hierarchy chart for the program main() showIntro( ) getCups( ) cupsToOunces (double cups) As shown in the hierarchy chart, the main function will call three other functions. Here are summaries of those functions: • showIntro—This function will display a message on the screen that explains what the program does. • getCups—This function will prompt the user to enter the number of cups and then will return that value as a double. • cupsToOunces—This function will accept the number of cups as an argument and then return an equivalent number of fluid ounces as a double. Program 6-14 shows the code for the program. Program 6-14 1 // This program converts cups to fluid ounces. 2 #include 3 #include 4 using namespace std; 6.8 Returning a Value from a Function 331 5 6 // Function prototypes 7 void showIntro(); 8 double getCups(); 9 double cupsToOunces(double); 10 11 int main() 12 { 13 // Variables for the cups and ounces. 14 double cups, ounces; 15 16 // Set up numeric output formatting. 17 cout << fixed << showpoint << setprecision(1); 18 19 // Display an intro screen. 20 showIntro(); 21 22 // Get the number of cups. 23 cups = getCups(); 24 25 // Convert cups to fluid ounces. 26 ounces = cupsToOunces(cups); 27 28 // Display the number of ounces. 29 cout << cups << " cups equals " 30 << ounces << " ounces.\n"; 31 32 return 0; 33 } 34 35 //****************************************** 36 // The showIntro function displays an * 37 // introductory screen. * 38 //****************************************** 39 40 void showIntro() 41 { 42 cout << "This program converts measurements\n" 43 << "in cups to fluid ounces. For your\n" 44 << "reference the formula is:\n" 45 << " 1 cup = 8 fluid ounces\n\n"; 46 } 47 48 //****************************************** 49 // The getCups function prompts the user * 50 // to enter the number of cups and then * 51 // returns that value as a double. * 52 //****************************************** 53 54 double getCups() (program continues) 332 Chapter 6 Functions Program 6-14 (continued) 55 { 56 double numCups; 57 58 cout << "Enter the number of cups: "; 59 cin >> numCups; 60 return numCups; 61 } 62 63 //****************************************** 64 // The cupsToOunces function accepts a * 65 // number of cups as an argument and * 66 // returns the equivalent number of fluid * 67 // ounces as a double. * 68 //****************************************** 69 70 double cupsToOunces(double numCups) 71 { 72 return numCups * 8.0; 73 } Program Output with Example Input Shown in Bold This program converts measurements in cups to fluid ounces. For your reference the formula is: 1 cup = 8 fluid ounces Enter the number of cups: 2 [Enter] 2.0 cups equals 16.0 ounces. 6.9 Returning a Boolean Value CONCEPT: Functions may return true or false values. Frequently there is a need for a function that tests an argument and returns a true or false value indicating whether or not a condition exists. Such a function would return a bool value. For example, the following function accepts an int argument and returns true if the argument is within the range of 1 through 100, or false otherwise. bool isValid(int number) { bool status; if (number >= 1 && number <= 100) status = true; else status = false; return status; } 6.9 Returning a Boolean Value 333 The following code shows an if/else statement that uses a call to the function: int value = 20; if (isValid(value)) cout << "The value is within range.\n"; else cout << "The value is out of range.\n"; When this code executes, the message “The value is within range.” will be displayed. Program 6-15 shows another example. This program has a function named isEven which returns true if its argument is an even number. Otherwise, the function returns false. Program 6-15 1 // This program uses a function that returns true or false. 2 #include 3 using namespace std; 4 5 // Function prototype 6 bool isEven(int); 7 8 int main() 9{ 10 int val; 11 12 // Get a number from the user. 13 cout << "Enter an integer and I will tell you "; 14 cout << "if it is even or odd: "; 15 cin >> val; 16 17 // Indicate whether it is even or odd. 18 if (isEven(val)) 19 cout << val << " is even.\n"; 20 else 21 cout << val << " is odd.\n"; 22 return 0; 23 } 24 25 //***************************************************************** 26 // Definition of function isEven. This function accepts an * 27 // integer argument and tests it to be even or odd. The function * 28 // returns true if the argument is even or false if the argument * 29 // is odd. The return value is a bool. * 30 //***************************************************************** 31 32 bool isEven(int number) 33 { 34 bool status; 35 36 if (number % 2 == 0) 37 status = true; // The number is even if there is no remainder. 38 else 39 status = false; // Otherwise, the number is odd. 40 41 } return status; (program output continues) 334 Chapter 6 Functions Program 6-15 (continued) Program Output with Example Input Shown in Bold Enter an integer and I will tell you if it is even or odd: 5 [Enter] 5 is odd. The isEven function is called in line 18, in the following statement: if (isEven(val)) When the if statement executes, isEven is called with val as its argument. If val is even, isEven returns true , otherwise it returns false. Checkpoint 6.11 How many return values may a function have? 6.12 Write a header for a function named distance. The function should return a double and have two double parameters: rate and time. 6.13 Write a header for a function named days. The function should return an int and have three int parameters: years, months, and weeks. 6.14 Write a header for a function named getKey. The function should return a char and use no parameters. 6.15 Write a header for a function named lightYears. The function should return a long and have one long parameter: miles. 6.10 Local and Global Variables CONCEPT: A local variable is defined inside a function and is not accessible outside the function. A global variable is defined outside all functions and is accessible to all functions in its scope. Local Variables Variables defined inside a function are local to that function. They are hidden from the statements in other functions, which normally cannot access them. Program 6-16 shows that because the variables defined in a function are hidden, other functions may have separate, distinct variables with the same name. Program 6-16 1 // This program shows that variables defined in a function 2 // are hidden from other functions. 3 #include 4 using namespace std; 5 6 void anotherFunction(); // Function prototype 6.10 Local and Global Variables 335 7 8 int main() 9{ 10 int num = 1; // Local variable 11 12 cout << "In main, num is " << num << endl; 13 anotherFunction(); 14 cout << "Back in main, num is " << num << endl; 15 return 0; 16 } 17 18 //***************************************************** 19 // Definition of anotherFunction * 20 // It has a local variable, num, whose initial value * 21 // is displayed. * 22 //***************************************************** 23 24 void anotherFunction() 25 { 26 int num = 20; // Local variable 27 28 cout << "In anotherFunction, num is " << num << endl; 29 } Program Output In main, num is 1 In anotherFunction, num is 20 Back in main, num is 1 Even though there are two variables named num, the program can only “see” one of them at a time because they are in different functions. When the program is executing in main, the num variable defined in main is visible. When anotherFunction is called, however, only variables defined inside it are visible, so the num variable in main is hidden. Figure 6-14 illustrates the closed nature of the two functions. The boxes represent the scope of the variables. Figure 6-14 Function main int num = 1; Function anotherFunction int num = 20; This num variable is visible only in main. This num variable is visible only in anotherFunction. Local Variable Lifetime A function’s local variables exist only while the function is executing. This is known as the lifetime of a local variable. When the function begins, its local variables and its parameter 336 Chapter 6 Functions variables are created in memory, and when the function ends, the local variables and parameter variables are destroyed. This means that any value stored in a local variable is lost between calls to the function in which the variable is declared. Initializing Local Variables with Parameter Values It is possible to use a parameter variable to initialize a local variable. Sometimes this simplifies the code in a function. For example, recall the first version of the sum function we discussed earlier: int sum(int num1, int num2) { int result; result = num1 + num2; return result; } In the body of the function, the result variable is defined and then a separate assignment statement assigns num1 + num2 to result. We can combine these statements into one, as shown in the following modified version of the function. int sum(int num1, int num2) { int result = num1 + num2; return result; } Because the scope of a parameter variable is the entire function in which it is declared, we can use parameter variables to initialize local variables. Global Variables A global variable is any variable defined outside all the functions in a program. The scope of a global variable is the portion of the program from the variable definition to the end. This means that a global variable can be accessed by all functions that are defined after the global variable is defined. Program 6-17 shows two functions, main and anotherFunction, that access the same global variable, num. Program 6-17 1 // This program shows that a global variable is visible 2 // to all the functions that appear in a program after 3 // the variable's declaration. 4 #include 5 using namespace std; 6 7 void anotherFunction(); // Function prototype 8 int num = 2; // Global variable 9 10 int main() 11 { 12 cout << "In main, num is " << num << endl; 6.10 Local and Global Variables 337 13 anotherFunction(); 14 cout << "Back in main, num is " << num << endl; 15 return 0; 16 } 17 18 //***************************************************** 19 // Definition of anotherFunction * 20 // This function changes the value of the * 21 // global variable num. * 22 //***************************************************** 23 24 void anotherFunction() 25 { 26 cout << "In anotherFunction, num is " << num << endl; 27 num = 50; 28 cout << "But, it is now changed to " << num << endl; 29 } Program Output In main, num is 2 In anotherFunction, num is 2 But, it is now changed to 50 Back in main, num is 50 In Program 6-17, num is defined outside of all the functions. Because its definition appears before the definitions of main and anotherFunction, both functions have access to it. Unless you explicitly initialize numeric global variables, they are automatically initialized to zero. Global character variables are initialized to NULL.* The variable globalNum in Program 6-18 is never set to any value by a statement, but because it is global, it is automatically set to zero. Program 6-18 1 // This program has an uninitialized global variable. 2 #include 3 using namespace std; 4 5 int globalNum; // Global variable, automatically set to zero 6 7 int main() 8{ 9 cout << "globalNum is " << globalNum << endl; 10 return 0; 11 } Program Output globalNum is 0 *The NULL character is stored as ASCII code 0. 338 Chapter 6 Functions Now that you’ve had a basic introduction to global variables, I must warn you to restrict your use of them. When beginning students first learn to write programs with multiple functions, they are sometimes tempted to make all their variables global. This is usually because global variables can be accessed by any function in the program without being passed as arguments. Although this approach might make a program easier to create, it usually causes problems later. The reasons are as follows: • Global variables make debugging difficult. Any statement in a program can change the value of a global variable. If you find that the wrong value is being stored in a global variable, you have to track down every statement that accesses it to determine where the bad value is coming from. In a program with thousands of lines of code, this can be difficult. • Functions that use global variables are usually dependent on those variables. If you want to use such a function in a different program, most likely you will have to redesign it so it does not rely on the global variable. • Global variables make a program hard to understand. A global variable can be modified by any statement in the program. If you are to understand any part of the program that uses a global variable, you have to be aware of all the other parts of the program that access the global variable. Because of this, you should not use global variables for the conventional purposes of storing, manipulating, and retrieving data. In most cases, you should declare variables locally and pass them as arguments to the functions that need to access them. Global Constants Although you should try to avoid the use of global variables, it is generally permissible to use global constants in a program. A global constant is a named constant that is available to every function in a program. Because a global constant’s value cannot be changed during the program’s execution, you do not have to worry about the potential hazards that are associated with the use of global variables. Global constants are typically used to represent unchanging values that are needed throughout a program. For example, suppose a banking program uses a named constant to represent an interest rate. If the interest rate is used in several functions, it is easier to create a global constant, rather than a local named constant in each function. This also simplifies maintenance. If the interest rate changes, only the declaration of the global constant has to be changed, instead of several local declarations. Program 6-19 shows an example of how global constants might be used. The program calculates an employee’s gross pay, including overtime. In addition to main, this program has two functions: getBasePay and getOvertimePay. The getBasePay function accepts the number of hours worked and returns the amount of pay for the non-overtime hours. The getOvertimePay function accepts the number of hours worked and returns the amount of pay for the overtime hours, if any. Program 6-19 1 // This program calculates gross pay. 2 #include 3 #include 6.10 Local and Global Variables 339 4 using namespace std; 5 6 // Global constants 7 const double PAY_RATE = 22.55; // Hourly pay rate 8 const double BASE_HOURS = 40.0; // Max non-overtime hours 9 const double OT_MULTIPLIER = 1.5; // Overtime multiplier 10 11 // Function prototypes 12 double getBasePay(double); 13 double getOvertimePay(double); 14 15 int main() 16 { 17 double hours, // Hours worked 18 basePay, // Base pay 19 overtime = 0.0, // Overtime pay 20 totalPay; // Total pay 21 22 // Get the number of hours worked. 23 cout << "How many hours did you work? "; 24 cin >> hours; 25 26 // Get the amount of base pay. 27 basePay = getBasePay(hours); 28 29 // Get overtime pay, if any. 30 if (hours > BASE_HOURS) 31 overtime = getOvertimePay(hours); 32 33 // Calculate the total pay. 34 totalPay = basePay + overtime; 35 36 // Set up numeric output formatting. 37 cout << setprecision(2) << fixed << showpoint; 38 39 // Display the pay. 40 cout << "Base pay: $" << basePay << endl 41 << "Overtime pay $" << overtime << endl 42 << "Total pay $" << totalPay << endl; 43 44 } return 0; 45 46 //************************************************* 47 // The getBasePay function accepts the number of * 48 // hours worked as an argument and returns the * 49 // employee's pay for non-overtime hours. * 50 //************************************************* 51 52 double getBasePay(double hoursWorked) 53 { 54 double basePay; // To hold base pay 55 (program continues) 340 Chapter 6 Functions Program 6-19 (continued) 56 // Determine base pay. 57 if (hoursWorked > BASE_HOURS) 58 basePay = BASE_HOURS * PAY_RATE; 59 else 60 basePay = hoursWorked * PAY_RATE; 61 62 return basePay; 63 } 64 65 //************************************************* 66 // The getOvertimePay function accepts the number * 67 // of hours worked as an argument and returns the * 68 // employee's overtime pay. * 69 //************************************************* 70 71 double getOvertimePay(double hoursWorked) 72 { 73 double overtimePay; // To hold overtime pay 74 75 // Determine overtime pay. 76 if (hoursWorked > BASE_HOURS) 77 { 78 overtimePay = (hoursWorked - BASE_HOURS) * 79 PAY_RATE * OT_MULTIPLIER; 80 } 81 else 82 overtimePay = 0.0; 83 84 return overtimePay; 85 } Program Output with Example Input Shown in Bold How many hours did you work? 48 [Enter] Base pay: $902.00 Overtime pay: $270.60 Total pay: $1172.60 Let’s take a closer look at the program. Three global constants are defined in lines 7, 8, and 9. The PAY_RATE constant is set to the employee’s hourly pay rate, which is 22.55. The BASE_HOURS constant is set to 40, which is the number of hours an employee can work in a week without getting paid overtime. The OT_MULTIPLIER constant is set to 1.5, which is the pay rate multiplier for overtime hours. This means that the employee’s hourly pay rate is multiplied by 1.5 for all overtime hours. Because these constants are global and are defined before all of the functions in the program, all the functions may access them. For example, the getBasePay function accesses the BASE_HOURS constant in lines 57 and 58 and accesses the PAY_RATE constant in lines 58 and 60. The getOvertimePay function accesses the BASE_HOURS constant in lines 76 and 78, the PAY_RATE constant in line 79, and the OT_MULTIPLIER constant in line 79. 6.10 Local and Global Variables 341 Local and Global Variables with the Same Name You cannot have two local variables with the same name in the same function. This applies to parameter variables as well. A parameter variable is, in essence, a local variable. So, you cannot give a parameter variable and a local variable in the same function the same name. However, you can have a local variable or a parameter variable with the same name as a global variable, or a global constant. When you do, the name of the local or parameter variable shadows the name of the global variable or global constant. This means that the global variable or constant’s name is hidden by the name of the local or parameter variable. For example, look at Program 6-20. This program has a global constant named BIRDS, set to 500. The california function has a local constant named BIRDS, set to 10000. Program 6-20 1 // This program demonstrates how a local variable 2 // can shadow the name of a global constant. 3 #include 4 using namespace std; 5 6 // Global constant. 7 const int BIRDS = 500; 8 9 // Function prototype 10 void california(); 11 12 int main() 13 { 14 cout << "In main there are " << BIRDS 15 << " birds.\n"; 16 california(); 17 return 0; 18 } 19 20 //******************************************** 21 // california function * 22 //******************************************** 23 24 void california() 25 { 26 const int BIRDS = 10000; 27 cout << "In california there are " << BIRDS 28 << " birds.\n"; 29 } Program Output In main there are 500 birds. In california there are 10000 birds. When the program is executing in the main function, the global constant BIRDS, which is set to 500, is visible. The cout statement in lines 14 and 15 displays “In main there are 500 birds.” (My apologies to folks living in Maine for the difference in spelling.) When the program is executing in the california function, however, the local constant BIRDS shadows the global constant BIRDS. When the california function accesses BIRDS, it accesses the local constant. That is why the cout statement in lines 27 and 28 displays “In california there are 10000 birds.” 342 Chapter 6 Functions 6.11 Static Local Variables If a function is called more than once in a program, the values stored in the function’s local variables do not persist between function calls. This is because the local variables are destroyed when the function terminates and are then re-created when the function starts again. This is shown in Program 6-21. Program 6-21 1 // This program shows that local variables do not retain 2 // their values between function calls. 3 #include 4 using namespace std; 5 6 // Function prototype 7 void showLocal(); 8 9 int main() 10 { 11 showLocal(); 12 showLocal(); 13 return 0; 14 } 15 16 //*********************************************************** 17 // Definition of function showLocal. * 18 // The initial value of localNum, which is 5, is displayed. * 19 // The value of localNum is then changed to 99 before the * 20 // function returns. * 21 //*********************************************************** 22 23 void showLocal() 24 { 25 int localNum = 5; // Local variable 26 27 cout << "localNum is " << localNum << endl; 28 localNum = 99; 29 } Program Output localNum is 5 localNum is 5 Even though in line 28 the last statement in the showLocal function stores 99 in localNum, the variable is destroyed when the function returns. The next time the function is called, localNum is re-created and initialized to 5 again. Sometimes it’s desirable for a program to “remember” what value is stored in a local variable between function calls. This can be accomplished by making the variable static. 6.11 Static Local Variables 343 Static local variables are not destroyed when a function returns. They exist for the lifetime of the program, even though their scope is only the function in which they are defined. Program 6-22 demonstrates some characteristics of static local variables: Program 6-22 1 // This program uses a static local variable. 2 #include 3 using namespace std; 4 5 void showStatic(); // Function prototype 6 7 int main() 8{ 9 // Call the showStatic function five times. 10 for (int count = 0; count < 5; count++) 11 showStatic(); 12 return 0; 13 } 14 15 //************************************************************** 16 // Definition of function showStatic. * 17 // statNum is a static local variable. Its value is displayed * 18 // and then incremented just before the function returns. * 19 //************************************************************** 20 21 void showStatic() 22 { 23 static int statNum; 24 25 cout << "statNum is " << statNum << endl; 26 statNum++; 27 } Program Output statNum is 0 statNum is 1 statNum is 2 statNum is 3 statNum is 4 In line 26 of Program 6-22, statNum is incremented in the showStatic function, and it retains its value between each function call. Notice that even though statNum is not explicitly initialized, it starts at zero. Like global variables, all static local variables are initialized to zero by default. (Of course, you can provide your own initialization value, if necessary.) If you do provide an initialization value for a static local variable, the initialization only occurs once. This is because initialization normally happens when the variable is created, and static local variables are only created once during the running of a program. Program 6-23, which is a slight modification of Program 6-22, illustrates this point. 344 Chapter 6 Functions Program 6-23 1 // This program shows that a static local variable is only 2 // initialized once. 3 #include 4 using namespace std; 5 6 void showStatic(); // Function prototype 7 8 int main() 9{ 10 // Call the showStatic function five times. 11 for (int count = 0; count < 5; count++) 12 showStatic(); 13 return 0; 14 } 15 16 //************************************************************* 17 // Definition of function showStatic. * 18 // statNum is a static local variable. Its value is displayed * 19 // and then incremented just before the function returns. * 20 //************************************************************* 21 22 void showStatic() 23 { 24 static int statNum = 5; 25 26 cout << "statNum is " << statNum << endl; 27 statNum++; 28 } Program Output statNum is 5 statNum is 6 statNum is 7 statNum is 8 statNum is 9 Even though the statement that defines statNum in line 24 initializes it to 5, the initialization does not happen each time the function is called. If it did, the variable would not be able to retain its value between function calls. Checkpoint 6.16 What is the difference between a static local variable and a global variable? 6.17 What is the output of the following program? #include using namespace std; void myFunc(); // Function prototype int main() { 6.12 Default Arguments 345 6.18 int var = 100; cout << var << endl; myFunc(); cout << var << endl; return 0; } // Definition of function myFunc void myFunc() { int var = 50; cout << var << endl; } What is the output of the following program? #include using namespace std; void showVar(); // Function prototype int main() { for (int count = 0; count < 10; count++) showVar(); return 0; } // Definition of function showVar void showVar() { static int var = 10; cout << var << endl; var++; } 6.12 Default Arguments CONCEPT: Default arguments are passed to parameters automatically if no argument is provided in the function call. It’s possible to assign default arguments to function parameters. A default argument is passed to the parameter when the actual argument is left out of the function call. The default arguments are usually listed in the function prototype. Here is an example: void showArea(double = 20.0, double = 10.0); Default arguments are literal values or constants with an = operator in front of them, appearing after the data types listed in a function prototype. Since parameter names are optional in function prototypes, the example prototype could also be declared as void showArea(double length = 20.0, double width = 10.0); 346 Chapter 6 Functions In both example prototypes, the function showArea has two double parameters. The first is assigned the default argument 20.0 and the second is assigned the default argument 10.0. Here is the definition of the function: void showArea(double length, double width) { double area = length * width; cout << "The area is " << area << endl; } The default argument for length is 20.0 and the default argument for width is 10.0. Because both parameters have default arguments, they may optionally be omitted in the function call, as shown here: showArea(); In this function call, both default arguments will be passed to the parameters. The parameter length will take the value 20.0 and width will take the value 10.0. The output of the function will be The area is 200 The default arguments are only used when the actual arguments are omitted from the function call. In the call below, the first argument is specified, but the second is omitted: showArea(12.0); The value 12.0 will be passed to length, while the default value 10.0 will be passed to width. The output of the function will be The area is 120 Of course, all the default arguments may be overridden. In the function call below, arguments are supplied for both parameters: showArea(12.0, 5.5); The output of the function call above will be The area is 66 N OTE: If a function does not have a prototype, default arguments may be specified in the function header. The showArea function could be defined as follows: void showArea(double length = 20.0, double width = 10.0) { double area = length * width; cout << "The area is " << area << endl; } W ARN IN G ! A function’s default arguments should be assigned in the earliest occurrence of the function name. This will usually be the function prototype. Program 6-24 uses a function that displays asterisks on the screen. Arguments are passed to the function specifying how many columns and rows of asterisks to display. Default arguments are provided to display one row of 10 asterisks. 6.12 Default Arguments 347 Program 6-24 1 // This program demonstrates default function arguments. 2 #include 3 using namespace std; 4 5 // Function prototype with default arguments 6 void displayStars(int = 10, int = 1); 7 8 int main() 9{ 10 displayStars(); // Use default values for cols and rows. 11 cout << endl; 12 displayStars(5); // Use default value for rows. 13 cout << endl; 14 displayStars(7, 3); // Use 7 for cols and 3 for rows. 15 return 0; 16 } 17 18 //********************************************************* 19 // Definition of function displayStars. * 20 // The default argument for cols is 10 and for rows is 1.* 21 // This function displays a square made of asterisks. * 22 //********************************************************* 23 24 void displayStars(int cols, int rows) 25 { 26 // Nested loop. The outer loop controls the rows 27 // and the inner loop controls the columns. 28 for (int down = 0; down < rows; down++) 29 { 30 for (int across = 0; across < cols; across++) 31 cout << "*"; 32 cout << endl; 33 } 34 } Program Output ********** ***** ******* ******* ******* Although C++’s default arguments are very convenient, they are not totally flexible in their use. When an argument is left out of a function call, all arguments that come after it must be left out as well. In the displayStars function in Program 6-24, it is not possible to omit the argument for cols without also omitting the argument for rows. For example, the following function call would be illegal: displayStars(, 3); // Illegal function call. 348 Chapter 6 Functions It’s possible for a function to have some parameters with default arguments and some without. For example, in the following function (which displays an employee’s gross pay), only the last parameter has a default argument: // Function prototype void calcPay(int empNum, double payRate, double hours = 40.0); // Definition of function calcPay void calcPay(int empNum, double payRate, double hours) { double wages; wages = payRate * hours; cout << fixed << showpoint << setprecision(2); cout << "Gross pay for employee number "; cout << empNum << " is " << wages << endl; } When calling this function, arguments must always be specified for the first two parameters (empNum and payRate) since they have no default arguments. Here are examples of valid calls: calcPay(769, 15.75); // Use default arg for 40 hours calcPay(142, 12.00, 20); // Specify number of hours When a function uses a mixture of parameters with and without default arguments, the parameters with default arguments must be defined last. In the calcPay function, hours could not have been defined before either of the other parameters. The following prototypes are illegal: // Illegal prototype void calcPay(int empNum, double hours = 40.0, double payRate); // Illegal prototype void calcPay(double hours = 40.0, int empNum, double payRate); Here is a summary of the important points about default arguments: • The value of a default argument must be a literal value or a named constant. • When an argument is left out of a function call (because it has a default value), all the arguments that come after it must be left out too. • When a function has a mixture of parameters both with and without default argu- ments, the parameters with default arguments must be declared last. 6.13 Using Reference Variables as Parameters CONCEPT: When used as parameters, reference variables allow a function to access the parameter’s original argument. Changes to the parameter are also made to the argument. Earlier you saw that arguments are normally passed to a function by value, and that the function cannot change the source of the argument. C++ provides a special type of variable 6.13 Using Reference Variables as Parameters 349 called a reference variable that, when used as a function parameter, allows access to the original argument. A reference variable is an alias for another variable. Any changes made to the reference variable are actually performed on the variable for which it is an alias. By using a reference variable as a parameter, a function may change a variable that is defined in another function. Reference variables are defined like regular variables, except you place an ampersand (&) in front of the name. For example, the following function definition makes the parameter refVar a reference variable: void doubleNum(int &refVar) { refVar *= 2; } N O T E : The variable refVar is called “a reference to an int.” This function doubles refVar by multiplying it by 2. Since refVar is a reference variable, this action is actually performed on the variable that was passed to the function as an argument. When prototyping a function with a reference variable, be sure to include the ampersand after the data type. Here is the prototype for the doubleNum function: void doubleNum(int &); N OTE: Some programmers prefer not to put a space between the data type and the ampersand. The following prototype is equivalent to the one above: void doubleNum(int &); N OTE: The ampersand must appear in both the prototype and the header of any function that uses a reference variable as a parameter. It does not appear in the function call. Program 6-25 demonstrates how the doubleNum function works. Program 6-25 1 // This program uses a reference variable as a function 2 // parameter. 3 #include 4 using namespace std; 5 6 // Function prototype. The parameter is a reference variable. 7 void doubleNum(int &); 8 (program continues) 350 Chapter 6 Functions Program 6-25 (continued) 9 int main() 10 { 11 int value = 4; 12 13 cout << "In main, value is " << value << endl; 14 cout << "Now calling doubleNum..." << endl; 15 doubleNum(value); 16 cout << "Now back in main. value is " << value << endl; 17 return 0; 18 } 19 20 //********************************************************** 21 // Definition of doubleNum. * 22 // The parameter refVar is a reference variable. The value * 23 // in refVar is doubled. * 24 //********************************************************** 25 26 void doubleNum (int &refVar) 27 { 28 refVar *= 2; 29 } Program Output In main, value is 4 Now calling doubleNum... Now back in main. value is 8 The parameter refVar in Program 6-25 “points” to the value variable in function main. When a program works with a reference variable, it is actually working with the variable it references, or points to. This is illustrated in Figure 6-15. Figure 6-15 Reference Variable Original Argument 4 Recall that function arguments are normally passed by value, which means a copy of the argument’s value is passed into the parameter variable. When a reference parameter is used, it is said that the argument is passed by reference. Program 6-26 is a modification of Program 6-25. The function getNum has been added. The function asks the user to enter a number, which is stored in userNum. userNum is a reference to main’s variable value. 6.13 Using Reference Variables as Parameters 351 Program 6-26 1 // This program uses reference variables as function parameters. 2 #include 3 using namespace std; 4 5 // Function prototypes. Both functions use reference variables 6 // as parameters. 7 void doubleNum(int &); 8 void getNum(int &); 9 10 int main() 11 { 12 int value; 13 14 // Get a number and store it in value. 15 getNum(value); 16 17 // Double the number stored in value. 18 doubleNum(value); 19 20 // Display the resulting number. 21 cout << "That value doubled is " << value << endl; 22 return 0; 23 } 24 25 //************************************************************** 26 // Definition of getNum. * 27 // The parameter userNum is a reference variable. The user is * 28 // asked to enter a number, which is stored in userNum. * 29 //************************************************************** 30 31 void getNum(int &userNum) 32 { 33 cout << "Enter a number: "; 34 cin >> userNum; 35 } 36 37 //*********************************************************** 38 // Definition of doubleNum. * 39 // The parameter refVar is a reference variable. The value * 40 // in refVar is doubled. * 41 //*********************************************************** 42 43 void doubleNum (int &refVar) 44 { 45 refVar *= 2; 46 } Program Output with Example Input Shown in Bold Enter a number: 12 [Enter] That value doubled is 24 352 Chapter 6 Functions N OTE: Only variables may be passed by reference. If you attempt to pass a nonvariable argument, such as a literal, a constant, or an expression, into a reference parameter, an error will result. Using the doubleNum function as an example, the following statements will generate an error. doubleNum(5); // Error doubleNum(userNum + 10); // Error If a function uses more than one reference variable as a parameter, be sure to place the ampersand before each reference variable name. Here is the prototype and definition for a function that uses four reference variable parameters: // Function prototype with four reference variables // as parameters. void addThree(int &, int &, int &, int &); // Definition of addThree. // All four parameters are reference variables. void addThree(int &sum, int &num1, int &num2, int &num3) { cout << "Enter three integer values: "; cin >> num1 >> num2 >> num3; sum = num1 + num2 + num3; } WARNING! Don’t get carried away with using reference variables as function parameters. Any time you allow a function to alter a variable that’s outside the function, you are creating potential debugging problems. Reference variables should only be used as parameters when the situation requires them. Checkpoint 6.19 What kinds of values may be specified as default arguments? 6.20 Write the prototype and header for a function called compute. The function should have three parameters: an int, a double, and a long (not necessarily in that order). The int parameter should have a default argument of 5, and the long parameter should have a default argument of 65536. The double parameter should not have a default argument. 6.21 Write the prototype and header for a function called calculate. The function should have three parameters: an int, a reference to a double, and a long (not necessarily in that order.) Only the int parameter should have a default argument, which is 47. 6.22 What is the output of the following program? #include using namespace std; void test(int = 2, int = 4, int = 6); 6.13 Using Reference Variables as Parameters 353 6.23 int main() { test(); test(6); test(3, 9); test(1, 5, 7); return 0; } void test (int first, int second, int third) { first += 3; second += 6; third += 9; cout << first << " " << second << " " << third << endl; } The following program asks the user to enter two numbers. What is the output of the program if the user enters 12 and 14? #include using namespace std; void func1(int &, int &); void func2(int &, int &, int &); void func3(int, int, int); int main() { int x = 0, y = 0, z = 0; cout << x << " " << y << " " << z << endl; func1(x, y); cout << x << " " << y << " " << z << endl; func2(x, y, z); cout << x << " " << y << " " << z << endl; func3(x, y, z); cout << x << " " << y << " " << z << endl; return 0; } void func1(int &a, int &b) { cout << "Enter two numbers: "; cin >> a >> b; } void func2(int &a, int &b, int &c) { b++; c−−; a = b + c; } void func3(int a, int b, int c) { a = b − c; } 354 Chapter 6 Functions 6.14 Overloading Functions CONCEPT: Two or more functions may have the same name, as long as their parameter lists are different. Sometimes you will create two or more functions that perform the same operation, but use a different set of parameters or parameters of different data types. For instance, in Program 6-13 there is a square function that uses a double parameter. But, suppose you also wanted a square function that works exclusively with integers, accepting an int as its argument. Both functions would do the same thing: return the square of their argument. The only difference is the data type involved in the operation. If you were to use both these functions in the same program, you could assign a unique name to each function. For example, the function that squares an int might be named squareInt, and the one that squares a double might be named squareDouble. C++, however, allows you to overload function names. That means you may assign the same name to multiple functions, as long as their parameter lists are different. Program 6-27 uses two overloaded square functions. Program 6-27 1 // This program uses overloaded functions. 2 #include 3 #include 4 using namespace std; 5 6 // Function prototypes 7 int square(int); 8 double square(double); 9 10 int main() 11 { 12 int userInt; 13 double userFloat; 14 15 // Get an int and a double. 16 cout << fixed << showpoint << setprecision(2); 17 cout << "Enter an integer and a floating-point value: "; 18 cin >> userInt >> userFloat; 19 20 // Display their squares. 21 cout << "Here are their squares: "; 22 cout << square(userInt) << " and " << square(userFloat); 23 return 0; 24 } 25 26 //*************************************************************** 27 // Definition of overloaded function square. * 28 // This function uses an int parameter, number. It returns the * 29 // square of number as an int. * 30 //*************************************************************** 6.14 Overloading Functions 355 31 32 int square(int number) 33 { 34 return number * number; 35 } 36 37 //*************************************************************** 38 // Definition of overloaded function square. * 39 // This function uses a double parameter, number. It returns * 40 // the square of number as a double. * 41 //*************************************************************** 42 43 double square(double number) 44 { 45 return number * number; 46 } Program Output with Example Input Shown in Bold Enter an integer and a floating-point value: 12 4.2 [Enter] Here are their squares: 144 and 17.64 Here are the headers for the square functions used in Program 6-27: int square(int number) double square(double number) In C++, each function has a signature. The function signature is the name of the function and the data types of the function’s parameters in the proper order. The square functions in Program 6-27 would have the following signatures: square(int) square(double) When an overloaded function is called, C++ uses the function signature to distinguish it from other functions with the same name. In Program 6-27, when an int argument is passed to square, the version of the function that has an int parameter is called. Likewise, when a double argument is passed to square, the version with a double parameter is called. Note that the function’s return value is not part of the signature. The following functions could not be used in the same program because their parameter lists aren’t different. int square(int number) { return number * number } double square(int number) // Wrong! Parameter lists must differ { return number * number } 356 Chapter 6 Functions Overloading is also convenient when there are similar functions that use a different number of parameters. For example, consider a program with functions that return the sum of integers. One returns the sum of two integers, another returns the sum of three integers, and yet another returns the sum of four integers. Here are their function headers: int sum(int num1, int num2) int sum(int num1, int num2, int num3) int sum(int num1, int num2, int num3, int num4) Because the number of parameters is different in each, they all may be used in the same program. Program 6-28 is an example that uses two functions, each named calcWeeklyPay, to determine an employee’s gross weekly pay. One version of the function uses an int and a double parameter, while the other version only uses a double parameter. Program 6-28 1 // This program demonstrates overloaded functions to calculate 2 // the gross weekly pay of hourly paid or salaried employees. 3 #include 4 #include 5 using namespace std; 6 7 // Function prototypes 8 void getChoice(char &); 9 double calcWeeklyPay(int, double); 10 double calcWeeklyPay(double); 11 12 int main() 13 { 14 char selection; // Menu selection 15 int worked; // Hours worked 16 double rate; // Hourly pay rate 17 double yearly; // Yearly salary 18 19 // Set numeric output formatting. 20 cout << fixed << showpoint << setprecision(2); 21 22 // Display the menu and get a selection. 23 cout << "Do you want to calculate the weekly pay of\n"; 24 cout << "(H) an hourly paid employee, or \n"; 25 cout << "(S) a salaried employee?\n"; 26 getChoice(selection); 27 28 // Process the menu selection. 29 switch (selection) 30 { 31 // Hourly paid employee 32 case 'H' : 33 case 'h' : cout << "How many hours were worked? "; 6.14 Overloading Functions 357 34 cin >> worked; 35 cout << "What is the hourly pay rate? "; 36 cin >> rate; 37 cout << "The gross weekly pay is $"; 38 cout << calcWeeklyPay(worked, rate) << endl; 39 break; 40 41 // Salaried employee 42 case 'S' : 43 case 's' : cout << "What is the annual salary? "; 44 cin >> yearly; 45 cout << "The gross weekly pay is $"; 46 cout << calcWeeklyPay(yearly) << endl; 47 break; 48 } 49 return 0; 50 } 51 52 //************************************************************* 53 // Definition of function getChoice. * 54 // The parameter letter is a reference to a char. * 55 // This function asks the user for an H or an S and returns * 56 // the validated input. * 57 //************************************************************* 58 59 void getChoice(char & letter) 60 { 61 // Get the user's selection. 62 cout << "Enter your choice (H or S): "; 63 cin >> letter; 64 65 // Validate the selection. 66 while (letter != 'H' && letter != 'h' && 67 letter != 'S' && letter != 's') 68 { 69 cout << "Please enter H or S: "; 70 cin >> letter; 71 } 72 } 73 74 //*********************************************************** 75 // Definition of overloaded function calcWeeklyPay. * 76 // This function calculates the gross weekly pay of * 77 // an hourly paid employee. The parameter hours holds the * 78 // number of hours worked. The parameter payRate holds the * 79 // hourly pay rate. The function returns the weekly salary.* 80 //*********************************************************** 81 82 double calcWeeklyPay(int hours, double payRate) 83 { (program continues) 358 Chapter 6 Functions Program 6-28 (continued) 84 return hours * payRate; 85 } 86 87 //*********************************************************** 88 // Definition of overloaded function calcWeeklyPay. * 89 // This function calculates the gross weekly pay of * 90 // a salaried employee. The parameter holds the employee's * 91 // annual salary. The function returns the weekly salary. * 92 //*********************************************************** 93 94 double calcWeeklyPay(double annSalary) 95 { 96 return annSalary / 52; 97 } Program Output with Example Input Shown in Bold Do you want to calculate the weekly pay of (H) an hourly paid employee, or (S) a salaried employee? Enter your choice (H or S): H [Enter] How many hours were worked? 40 [Enter] What is the hourly pay rate? 18.50 [Enter] The gross weekly pay is $740.00 Program Output with Example Input Shown in Bold Do you want to calculate the weekly pay of (H) an hourly paid employee, or (S) a salaried employee? Enter your choice (H or S): S [Enter] What is the annual salary? 68000.00 [Enter] The gross weekly pay is $1307.69 6.15 The exit() Function CONCEPT: The exit() function causes a program to terminate, regardless of which function or control mechanism is executing. A C++ program stops executing when the return statement in function main is encountered. When other functions end, however, the program does not stop. Control of the program goes back to the place immediately following the function call. Sometimes, rare circumstances make it necessary to terminate a program in a function other than main. To accomplish this, the exit function is used. When the exit function is called, it causes the program to stop, regardless of which function contains the call. Program 6-29 demonstrates its use. 6.15 The exit() Function 359 Program 6-29 1 // This program shows how the exit function causes a program 2 // to stop executing. 3 #include 4 #include // Needed for the exit function 5 using namespace std; 6 7 void function(); // Function prototype 8 9 int main() 10 { 11 function(); 12 return 0; 13 } 14 15 //*********************************************************** 16 // This function simply demonstrates that exit can be used * 17 // to terminate a program from a function other than main. * 18 //*********************************************************** 19 20 void function() 21 { 22 cout << "This program terminates with the exit function.\n"; 23 cout << "Bye!\n"; 24 exit(0); 25 cout << "This message will never be displayed\n"; 26 cout << "because the program has already terminated.\n"; 27 } Program Output This program terminates with the exit function. Bye! To use the exit function, you must include the cstdlib header file. Notice the function takes an integer argument. This argument is the exit code you wish the program to pass back to the computer’s operating system. This code is sometimes used outside of the program to indicate whether the program ended successfully or as the result of a failure. In Program 6-29, the exit code zero is passed, which commonly indicates a successful exit. If you are unsure which code to use with the exit function, there are two named constants, EXIT_FAILURE and EXIT_SUCCESS, defined in cstdlib for you to use. The constant EXIT_FAILURE is defined as the termination code that commonly represents an unsuccessful exit under the current operating system. Here is an example of its use: exit(EXIT_FAILURE); The constant EXIT_SUCCESS is defined as the termination code that commonly represents a successful exit under the current operating system. Here is an example: exit(EXIT_SUCCESS); N OTE: Generally, the exit code is important only if you know it will be tested outside the program. If it is not used, just pass zero, or EXIT_SUCCESS. 360 Chapter 6 Functions W A R N I N G ! The exit() function unconditionally shuts down your program. Because it bypasses a program’s normal logical flow, you should use it with caution. Checkpoint 6.24 What is the output of the following program? #include #include using namespace std; void showVals(double, double); int main() { double x = 1.2, y = 4.5; showVals(x, y); return 0; } void showVals(double p1, double p2) { cout << p1 << endl; exit(0); cout << p2 << endl; } 6.25 What is the output of the following program? #include using namespace std; int manip(int); int manip(int, int); int manip(int, double); int main() { int x = 2, y= 4, z; double a = 3.1; z = manip(x) + manip(x, y) + manip(y, a); cout << z << endl; return 0; } int manip(int val) { return val + val * 2; } int manip(int val1, int val2) { return (val1 + val2) * 2; } int manip(int val1, double val2) { return val1 * static_cast(val2); } 6.16 Stubs and Drivers 361 6.16 Stubs and Drivers Stubs and drivers are very helpful tools for testing and debugging programs that use functions. They allow you to test the individual functions in a program, in isolation from the parts of the program that call the functions. A stub is a dummy function that is called instead of the actual function it represents. It usually displays a test message acknowledging that it was called, and nothing more. For example, if a stub were used for the showFees function in Program 6-10 (the modular health club membership program), it might look like this: void showFees(double memberRate, int months) { cout << "The showFees function was called with " << "the following arguments:\n" << "memberRate: " << memberRate << endl << "months: " << months << endl; } The following is an example output of the program if it were run with the stub instead of the actual showFees function. (A version of the health club program using this stub function is available from the book’s companion Web site at www.pearsonhighered.com/gaddis. The program is named HealthClubWithStub.cpp.) Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 1 [Enter] For how many months? 4 [Enter] The showFees function was called with the following arguments: memberRate: 40.00 months: 4 Health Club Membership Menu 1. Standard Adult Membership 2. Child Membership 3. Senior Citizen Membership 4. Quit the Program Enter your choice: 4 [Enter] As you can see, by replacing an actual function with a stub, you can concentrate your testing efforts on the parts of the program that call the function. Primarily, the stub allows you to determine whether your program is calling a function when you expect it to, and to confirm that valid values are being passed to the function. If the stub represents a function that returns a value, then the stub should return a test value. This helps you confirm that the return value is being handled properly. When the parts of the program that call a function are debugged to your satisfaction, you can move on to testing and debugging the actual functions themselves. This is where drivers become useful. 362 Chapter 6 Functions A driver is a program that tests a function by simply calling it. If the function accepts arguments, the driver passes test data. If the function returns a value, the driver displays the return value on the screen. This allows you to see how the function performs in isolation from the rest of the program it will eventually be part of. Program 6-30 shows a driver for testing the showFees function in the health club membership program. Program 6-30 1 // This program is a driver for testing the showFees function. 2 #include 3 using namespace std; 4 5 // Prototype 6 void showFees(double, int); 7 8 int main() 9{ 10 // Constants for membership rates 11 const double ADULT = 40.0; 12 const double SENIOR = 30.0; 13 const double CHILD = 20.0; 14 15 // Perform a test for adult membership. 16 cout << "Testing an adult membership...\n" 17 << "Calling the showFees function with arguments " 18 << ADULT << " and 10.\n"; 19 showFees(ADULT, 10); 20 21 // Perform a test for senior citizen membership. 22 cout << "\nTesting a senior citizen membership...\n" 23 << "Calling the showFees function with arguments " 24 << SENIOR << " and 10.\n"; 25 showFees(SENIOR, 10); 26 27 // Perform a test for child membership. 28 cout << "\nTesting a child membership...\n" 29 << "\nCalling the showFees function with arguments " 30 << CHILD << " and 10.\n"; 31 showFees(CHILD, 10); 32 return 0; 33 } 34 35 //****************************************************************** 36 // Definition of function showFees. The memberRate parameter holds * 37 // the monthly membership rate and the months parameter holds the * 38 // number of months. The function displays the total charges. * 39 //****************************************************************** 40 41 void showFees(double memberRate, int months) 42 { 43 cout << "The total charges are $" 44 << (memberRate * months) << endl; 45 } Review Questions and Exercises 363 Program Output Testing an adult membership... Calling the showFees function with arguments 40 and 10. The total charges are $400 Testing a senior citizen membership... Calling the showFees function with arguments 30 and 10. The total charges are $300 Testing a child membership... Calling the showFees function with arguments 20 and 10. The total charges are $200 As shown in Program 6-30, a driver can be used to thoroughly test a function. It can repeatedly call the function with different test values as arguments. When the function performs as desired, it can be placed into the actual program it will be part of. Case Study: See High Adventure Travel Agency Part 1 Case Study on the book’s companion Web site at www.pearsonhighered.com/gaddis. Review Questions and Exercises Short Answer 1. Why do local variables lose their values between calls to the function in which they are defined? 2. What is the difference between an argument and a parameter variable? 3. Where do you define parameter variables? 4. If you are writing a function that accepts an argument and you want to make sure the function cannot change the value of the argument, what do you do? 5. When a function accepts multiple arguments, does it matter in what order the arguments are passed? 6. How do you return a value from a function? 7. What is the advantage of breaking your application’s code into several small procedures? 8. How would a static local variable be useful? 9. Give an example where passing an argument by reference would be useful. Fill-in-the-Blank 10. The _________ is the part of a function definition that shows the function name, return type, and parameter list. 11. If a function doesn’t return a value, the word _________ will appear as its return type. 12. Either a function’s _________ or its _________ must precede all calls to the function. 13. Values that are sent into a function are called _________. 364 Chapter 6 Functions 14. Special variables that hold copies of function arguments are called _________. 15. When only a copy of an argument is passed to a function, it is said to be passed by _________. 16. A(n) _________ eliminates the need to place a function definition before all calls to the function. 17. A(n) _________ variable is defined inside a function and is not accessible outside the function. 18. _________ variables are defined outside all functions and are accessible to any function within their scope. 19. _________ variables provide an easy way to share large amounts of data among all the functions in a program. 20. Unless you explicitly initialize global variables, they are automatically initialized to _________. 21. If a function has a local variable with the same name as a global variable, only the _________ variable can be seen by the function. 22. _________ local variables retain their value between function calls. 23. The _________ statement causes a function to end immediately. 24. _________ arguments are passed to parameters automatically if no argument is pro- vided in the function call. 25. When a function uses a mixture of parameters with and without default arguments, the parameters with default arguments must be defined _________. 26. The value of a default argument must be a(n) _________. 27. When used as parameters, _________ variables allow a function to access the param- eter’s original argument. 28. Reference variables are defined like regular variables, except there is a(n) _________ in front of the name. 29. Reference variables allow arguments to be passed by ____________. 30. The _________ function causes a program to terminate. 31. Two or more functions may have the same name, as long as their _________ are different. Algorithm Workbench 32. Examine the following function header, then write an example call to the function. void showValue(int quantity) 33. The following statement calls a function named half. The half function returns a value that is half that of the argument. Write the function. result = half(number); 34. A program contains the following function. int cube(int num) { return num * num * num; } Review Questions and Exercises 365 Write a statement that passes the value 4 to this function and assigns its return value to the variable result. 35. Write a function named timesTen that accepts an argument. When the function is called, it should display the product of its argument multiplied times 10. 36. A program contains the following function. void display(int arg1, double arg2, char arg3) { cout << "Here are the values: " << arg1 << " " << arg2 << " " << arg3 << endl; } Write a statement that calls the procedure and passes the following variables to it: int age; double income; char initial; 37. Write a function named getNumber that uses a reference parameter variable to accept an integer argument. The function should prompt the user to enter a number in the range of 1 through 100. The input should be validated and stored in the parameter variable. True or False 38. T 39. T 40. T 41. T 42. T 43. T 44. T 45. T 46. T 47. T 48. T 49. T 50. T 51. T 52. T 53. T F Functions should be given names that reflect their purpose. F Function headers are terminated with a semicolon. F Function prototypes are terminated with a semicolon. F If other functions are defined before main, the program still starts executing at function main. F When a function terminates, it always branches back to main, regardless of where it was called from. F Arguments are passed to the function parameters in the order they appear in the function call. F The scope of a parameter is limited to the function which uses it. F Changes to a function parameter always affect the original argument as well. F In a function prototype, the names of the parameter variables may be left out. F Many functions may have local variables with the same name. F Overuse of global variables can lead to problems. F Static local variables are not destroyed when a function returns. F All static local variables are initialized to −1 by default. F Initialization of static local variables only happens once, regardless of how many times the function in which they are defined is called. F When a function with default arguments is called and an argument is left out, all arguments that come after it must be left out as well. F It is not possible for a function to have some parameters with default argu- ments and some without. 366 Chapter 6 Functions 54. T 55. T F The exit function can only be called from main. F A stub is a dummy function that is called instead of the actual function it represents. Find the Errors Each of the following functions has errors. Locate as many errors as you can. 56. void total(int value1, value2, value3) { return value1 + value2 + value3; } 57. double average(int value1, int value2, int value3) { double average; average = value1 + value2 + value3 / 3; } 58. void area(int length = 30, int width) { return length * width; } 59. void getValue(int value&) { cout << "Enter a value: "; cin >> value&; } 60. (Overloaded functions) int getValue() { int inputValue; cout << "Enter an integer: "; cin >> inputValue; return inputValue; } double getValue() { double inputValue; cout << "Enter a floating-point number: "; cin >> inputValue; return inputValue; } VideoNote Solving the Markup Problem Programming Challenges 1. Markup Write a program that asks the user to enter an item’s wholesale cost and its markup percentage. It should then display the item’s retail price. For example: • If an item’s wholesale cost is 5.00 and its markup percentage is 100%, then the item’s retail price is 10.00. Programming Challenges 367 • If an item’s wholesale cost is 5.00 and its markup percentage is 50%, then the item’s retail price is 7.50. The program should have a function named calculateRetail that receives the wholesale cost and the markup percentage as arguments and returns the retail price of the item. Input Validation: Do not accept negative values for either the wholesale cost of the item or the markup percentage. 2. Rectangle Area—Complete the Program If you have downloaded this book’s source code from the companion Web site, you will find a partially written program named AreaRectangle.cpp in the Chapter 06 folder. (The companion Web site is at www.pearsonhighered.com/gaddis.) Your job is to complete the program. When it is complete, the program will ask the user to enter the width and length of a rectangle and then display the rectangle’s area. The program calls the following functions, which have not been written: • getLength – This function should ask the user to enter the rectangle’s length and then return that value as a double. • getWidth – This function should ask the user to enter the rectangle’s width and then return that value as a double. • getArea – This function should accept the rectangle’s length and width as arguments and return the rectangle’s area. The area is calculated by multiplying the length by the width. • displayData – This function should accept the rectangle’s length, width, and area as arguments and display them in an appropriate message on the screen. 3. Winning Division Write a program that determines which of a company’s four divisions (Northeast, Southeast, Northwest, and Southwest) had the greatest sales for a quarter. It should include the following two functions, which are called by main. • double getSales() is passed the name of a division. It asks the user for a division’s quarterly sales figure, validates the input, then returns it. It should be called once for each division. • void findHighest() is passed the four sales totals. It determines which is the largest and prints the name of the high grossing division, along with its sales figure. Input Validation: Do not accept dollar amounts less than $0.00. 4. Safest Driving Area Write a program that determines which of five geographic regions within a major city (north, south, east, west, and central) had the fewest reported automobile accidents last year. It should have the following two functions, which are called by main. • int getNumAccidents() is passed the name of a region. It asks the user for the number of automobile accidents reported in that region during the last year, validates the input, then returns it. It should be called once for each city region. • void findLowest() is passed the five accident totals. It determines which is the smallest and prints the name of the region, along with its accident figure. Input Validation: Do not accept an accident number that is less than 0. 368 Chapter 6 Functions 5. Falling Distance When an object is falling because of gravity, the following formula can be used to determine the distance the object falls in a specific time period: d ϭ 1Ͳ2gt2 The variables in the formula are as follows: d is the distance in meters, g is 9.8, and t is the amount of time, in seconds, that the object has been falling. Write a function named fallingDistance that accepts an object’s falling time (in seconds) as an argument. The function should return the distance, in meters, that the object has fallen during that time interval. Write a program that demonstrates the function by calling it in a loop that passes the values 1 through 10 as arguments and displays the return value. 6. Kinetic Energy In physics, an object that is in motion is said to have kinetic energy. The following formula can be used to determine a moving object’s kinetic energy: KE ϭ 1Ͳ2 mv2 The variables in the formula are as follows: KE is the kinetic energy, m is the object’s mass in kilograms, and v is the object’s velocity, in meters per second. Write a function named kineticEnergy that accepts an object’s mass (in kilograms) and velocity (in meters per second) as arguments. The function should return the amount of kinetic energy that the object has. Demonstrate the function by calling it in a program that asks the user to enter values for mass and velocity. 7. Celsius Temperature Table The formula for converting a temperature from Fahrenheit to Celsius is 5 C ϭ 9 (F Ϫ 32) where F is the Fahrenheit temperature and C is the Celsius temperature. Write a function named celsius that accepts a Fahrenheit temperature as an argument. The function should return the temperature, converted to Celsius. Demonstrate the function by calling it in a loop that displays a table of the Fahrenheit temperatures 0 through 20 and their Celsius equivalents. 8. Coin Toss Write a function named coinToss that simulates the tossing of a coin. When you call the function, it should generate a random number in the range of 1 through 2. If the random number is 1, the function should display “heads.” If the random number is 2, the function should display “tails.” Demonstrate the function in a program that asks the user how many times the coin should be tossed and then simulates the tossing of the coin that number of times. 9. Present Value Suppose you want to deposit a certain amount of money into a savings account and then leave it alone to draw interest for the next 10 years. At the end of 10 years you would like to have $10,000 in the account. How much do you need to deposit today to Programming Challenges 369 make that happen? You can use the following formula, which is known as the present value formula, to find out: P ϭ (1 F ϩ r)n The terms in the formula are as follows: • P is the present value, or the amount that you need to deposit today. • F is the future value that you want in the account. (In this case, F is $10,000.) • r is the annual interest rate. • n is the number of years that you plan to let the money sit in the account. Write a program that has a function named presentValue that performs this calculation. The function should accept the future value, annual interest rate, and number of years as arguments. It should return the present value, which is the amount that you need to deposit today. Demonstrate the function in a program that lets the user experiment with different values for the formula’s terms. 10. Future Value Suppose you have a certain amount of money in a savings account that earns compound monthly interest, and you want to calculate the amount that you will have after a specific number of months. The formula, which is known as the future value formula, is: F ϭ P ϫ (1 ϩ i)t The terms in the formula are as follows: • F is the future value of the account after the specified time period. • P is the present value of the account. • i is the monthly interest rate. • t is the number of months. Write a program that prompts the user to enter the account’s present value, monthly interest rate, and the number of months that the money will be left in the account. The program should pass these values to a function named futureValue that returns the future value of the account, after the specified number of months. The program should display the account’s future value. 11. Lowest Score Drop Write a program that calculates the average of a group of test scores, where the lowest score in the group is dropped. It should use the following functions: • void getScore() should ask the user for a test score, store it in a reference parameter variable, and validate it. This function should be called by main once for each of the five scores to be entered. • void calcAverage() should calculate and display the average of the four highest scores. This function should be called just once by main and should be passed the five scores. • int findLowest() should find and return the lowest of the five scores passed to it. It should be called by calcAverage, which uses the function to determine which of the five scores to drop. Input Validation: Do not accept test scores lower than 0 or higher than 100. 370 Chapter 6 Functions 12. Star Search A particular talent competition has five judges, each of whom awards a score between 0 and 10 to each performer. Fractional scores, such as 8.3, are allowed. A performer’s final score is determined by dropping the highest and lowest score received, then averaging the three remaining scores. Write a program that uses this method to calculate a contestant’s score. It should include the following functions: • void getJudgeData() should ask the user for a judge’s score, store it in a reference parameter variable, and validate it. This function should be called by main once for each of the five judges. • void calcScore() should calculate and display the average of the three scores that remain after dropping the highest and lowest scores the performer received. This function should be called just once by main and should be passed the five scores. The last two functions, described below, should be called by calcScore, which uses the returned information to determine which of the scores to drop. • int findLowest() should find and return the lowest of the five scores passed to it. • int findHighest() should find and return the highest of the five scores passed to it. Input Validation: Do not accept judge scores lower than 0 or higher than 10. 13. Days Out Write a program that calculates the average number of days a company’s employees are absent. The program should have the following functions: • A function called by main that asks the user for the number of employees in the company. This value should be returned as an int. (The function accepts no arguments.) • A function called by main that accepts one argument: the number of employees in the company. The function should ask the user to enter the number of days each employee missed during the past year. The total of these days should be returned as an int. • A function called by main that takes two arguments: the number of employees in the company and the total number of days absent for all employees during the year. The function should return, as a double, the average number of days absent. (This function does not perform screen output and does not ask the user for input.) Input Validation: Do not accept a number less than 1 for the number of employees. Do not accept a negative number for the days any employee missed. 14. Order Status The Middletown Wholesale Copper Wire Company sells spools of copper wiring for $100 each. Write a program that displays the status of an order. The program should have a function that asks for the following data: • The number of spools ordered. • The number of spools in stock. • Whether there are special shipping and handling charges. (Shipping and handling is normally $10 per spool.) If there are special charges, the program should ask for the special charges per spool. Programming Challenges 371 The gathered data should be passed as arguments to another function that displays • The number of spools ready to ship from current stock. • The number of spools on backorder (if the number ordered is greater than what is in stock). • Subtotal of the portion ready to ship (the number of spools ready to ship times $100). • Total shipping and handling charges on the portion ready to ship. • Total of the order ready to ship. The shipping and handling parameter in the second function should have the default argument 10.00. Input Validation: Do not accept numbers less than 1 for spools ordered. Do not accept a number less than 0 for spools in stock or shipping and handling charges. 15. Overloaded Hospital Write a program that computes and displays the charges for a patient’s hospital stay. First, the program should ask if the patient was admitted as an in-patient or an outpatient. If the patient was an in-patient, the following data should be entered: • The number of days spent in the hospital • The daily rate • Hospital medication charges • Charges for hospital services (lab tests, etc.) The program should ask for the following data if the patient was an out-patient: • Charges for hospital services (lab tests, etc.) • Hospital medication charges The program should use two overloaded functions to calculate the total charges. One of the functions should accept arguments for the in-patient data, while the other function accepts arguments for out-patient information. Both functions should return the total charges. Input Validation: Do not accept negative numbers for any data. 16. Population In a population, the birth rate is the percentage increase of the population due to births, and the death rate is the percentage decrease of the population due to deaths. Write a program that displays the size of a population for any number of years. The program should ask for the following data: • The starting size of a population • The annual birth rate • The annual death rate • The number of years to display Write a function that calculates the size of the population for a year. The formula is N = P + BP − DP where N is the new population size, P is the previous population size, B is the birth rate, and D is the death rate. 372 Chapter 6 Functions Input Validation: Do not accept numbers less than 2 for the starting size. Do not accept negative numbers for birth rate or death rate. Do not accept numbers less than 1 for the number of years. 17. Transient Population Modify Programming Challenge 16 to also consider the effect on population caused by people moving into or out of a geographic area. Given as input a starting population size, the annual birth rate, the annual death rate, the number of individuals who typically move into the area each year, and the number of individuals who typically leave the area each year, the program should project what the population will be numYears from now. You can either prompt the user to input a value for numYears, or you can set it within the program. Input Validation: Do not accept numbers less than 2 for the starting size. Do not accept negative numbers for birth rate, death rate, arrivals, or departures. 18. Paint Job Estimator A painting company has determined that for every 110 square feet of wall space, one gallon of paint and eight hours of labor will be required. The company charges $25.00 per hour for labor. Write a modular program that allows the user to enter the number of rooms that are to be painted and the price of the paint per gallon. It should also ask for the square feet of wall space in each room. It should then display the following data: • The number of gallons of paint required • The hours of labor required • The cost of the paint • The labor charges • The total cost of the paint job Input validation: Do not accept a value less than 1 for the number of rooms. Do not accept a value less than $10.00 for the price of paint. Do not accept a negative value for square footage of wall space. 19. Using Files—Hospital Report Modify Programming Challenge 15, Overloaded Hospital, to write the report it creates to a file. 20. Stock Profit The profit from the sale of a stock can be calculated as follows: Profit ϭ ((NS ϫ SP) Ϫ SC) Ϫ ((NS ϫ PP) ϩ PC) where NS is the number of shares, SP is the sale price per share, SC is the sale commission paid, PP is the purchase price per share, and PC is the purchase commission paid. If the calculation yields a positive value, then the sale of the stock resulted in a profit. If the calculation yields a negative number, then the sale resulted in a loss. Write a function that accepts as arguments the number of shares, the purchase price per share, the purchase commission paid, the sale price per share, and the sale commission paid. The function should return the profit (or loss) from the sale of stock. Programming Challenges 373 Demonstrate the function in a program that asks the user to enter the necessary data and displays the amount of the profit or loss. 21. Multiple Stock Sales Use the function that you wrote for Programming Challenge 20 (Stock Profit) in a program that calculates the total profit or loss from the sale of multiple stocks. The program should ask the user for the number of stock sales and the necessary data for each stock sale. It should accumulate the profit or loss for each stock sale and then display the total 22. isPrime Function A prime number is a number that is only evenly divisible by itself and 1. For example, the number 5 is prime because it can only be evenly divided by 1 and 5. The number 6, however, is not prime because it can be divided evenly by 1, 2, 3, and 6. Write a function name isPrime, which takes an integer as an argument and returns true if the argument is a prime number, or false otherwise. Demonstrate the function in a complete program. TIP: Recall that the % operator divides one number by another, and returns the remainder of the division. In an expression such as num1 % num2, the % operator will return 0 if num1 is evenly divisible by num2. 23. Prime Number List Use the isPrime function that you wrote in Programming Challenge 22 in a program that stores a list of all the prime numbers from 1 through 100 in a file. 24. Rock, Paper, Scissors Game Write a program that lets the user play the game of Rock, Paper, Scissors against the computer. The program should work as follows. 1. When the program begins, a random number in the range of 1 through 3 is generated. If the number is 1, then the computer has chosen rock. If the number is 2, then the computer has chosen paper. If the number is 3, then the computer has chosen scissors. (Don’t display the computer’s choice yet.) 2. The user enters his or her choice of “rock”, “paper”, or “scissors” at the keyboard. (You can use a menu if you prefer.) 3. The computer’s choice is displayed. 4. A winner is selected according to the following rules: • If one player chooses rock and the other player chooses scissors, then rock wins. (The rock smashes the scissors.) • If one player chooses scissors and the other player chooses paper, then scissors wins. (Scissors cuts paper.) • If one player chooses paper and the other player chooses rock, then paper wins. (Paper wraps rock.) • If both players make the same choice, the game must be played again to determine the winner. Be sure to divide the program into functions that perform each major task. 374 Chapter 6 Functions Group Project 25. Travel Expenses This program should be designed and written by a team of students. Here are some suggestions: • One student should design function main, which will call the other functions in the program. The remainder of the functions will be designed by other members of the team. • The requirements of the program should be analyzed so each student is given about the same workload. • The parameters and return types of each function should be decided in advance. • Stubs and drivers should be used to test and debug the program. • The program can be implemented as a multifile program, or all the functions can be cut and pasted into the main file. Here is the assignment: Write a program that calculates and displays the total travel expenses of a businessperson on a trip. The program should have functions that ask for and return the following: • The total number of days spent on the trip • The time of departure on the first day of the trip, and the time of arrival back home on the last day of the trip • The amount of any round-trip airfare • The amount of any car rentals • Miles driven, if a private vehicle was used. Calculate the vehicle expense as $0.27 per mile driven • Parking fees (The company allows up to $6 per day. Anything in excess of this must be paid by the employee.) • Taxi fees, if a taxi was used anytime during the trip (The company allows up to $10 per day, for each day a taxi was used. Anything in excess of this must be paid by the employee.) • Conference or seminar registration fees • Hotel expenses (The company allows up to $90 per night for lodging. Anything in excess of this must be paid by the employee.) • The amount of each meal eaten. On the first day of the trip, breakfast is allowed as an expense if the time of departure is before 7 a.m. Lunch is allowed if the time of departure is before 12 noon. Dinner is allowed on the first day if the time of departure is before 6 p.m. On the last day of the trip, breakfast is allowed if the time of arrival is after 8 a.m. Lunch is allowed if the time of arrival is after 1 p.m. Dinner is allowed on the last day if the time of arrival is after 7 p.m. The program should only ask for the amounts of allowable meals. (The company allows up to $9 for breakfast, $12 for lunch, and $16 for dinner. Anything in excess of this must be paid by the employee.) The program should calculate and display the total expenses incurred by the businessperson, the total allowable expenses for the trip, the excess that must be reimbursed by the businessperson, if any, and the amount saved by the businessperson if the expenses were under the total allowed. Input Validation: Do not accept negative numbers for any dollar amount or for miles driven in a private vehicle. Do not accept numbers less than 1 for the number of days. Only accept valid times for the time of departure and the time of arrival. CHAPTER 7 Arrays TOPICS 7.1 Arrays Hold Multiple Values 7.2 Accessing Array Elements 7.3 No Bounds Checking in C++ 7.4 Array Initialization 7.5 The Range-Based for Loop 7.6 Processing Array Contents 7.7 Focus on Software Engineering: Using Parallel Arrays 7.8 Arrays as Function Arguments 7.9 Two-Dimensional Arrays 7.10 Arrays with Three or More Dimensions 7.11 Focus on Problem Solving and Program Design: A Case Study 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 7.1 Arrays Hold Multiple Values CONCEPT: An array allows you to store and work with multiple values of the same data type. The variables you have worked with so far are designed to hold only one value at a time. Each of the variable definitions in Figure 7-1 causes only enough memory to be reserved to hold one value of the specified data type. Figure 7-1 int count; Enough memory for 1 int 12314 float price; Enough memory for 1 float 56.981 char letter; Enough memory for 1 char A 375 376 Chapter 7 Arrays An array works like a variable that can store a group of values, all of the same type. The values are stored together in consecutive memory locations. Here is a definition of an array of integers: int days[6]; The name of this array is days. The number inside the brackets is the array’s size declarator. It indicates the number of elements, or values, the array can hold. The days array can store six elements, each one an integer. This is depicted in Figure 7-2. Figure 7-2 days array: enough memory for six int values Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 An array’s size declarator must be a constant integer expression with a value greater than zero. It can be either a literal, as in the previous example, or a named constant, as shown in the following: const int NUM_DAYS = 6; int days[NUM_DAYS]; Arrays of any data type can be defined. The following are all valid array definitions: float temperatures[100]; string names[10]; long units[50]; double sizes[1200]; // Array of 100 floats // Array of 10 string objects // Array of 50 long integers // Array of 1200 doubles Memory Requirements of Arrays The amount of memory used by an array depends on the array’s data type and the number of elements. The hours array, defined here, is an array of six shorts. short hours[6]; On a typical PC, a short uses two bytes of memory, so the hours array would occupy 12 bytes. This is shown in Figure 7-3. Figure 7-3 hours array: Each element uses two bytes Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 7.2 Accessing Array Elements 377 The size of an array can be calculated by multiplying the size of an individual element by the number of elements in the array. Table 7-1 shows the typical sizes of various arrays. Table 7-1 Array Definition char letters[25]; short rings[100]; int miles[84]; float temp[12]; double distance[1000]; Number of Elements 25 100 84 12 1000 Size of Each Element 1 byte 2 bytes 4 bytes 4 bytes 8 bytes Size of the Array 25 bytes 200 bytes 336 bytes 48 bytes 8000 bytes 7.2 Accessing Array Elements CONCEPT: The individual elements of an array are assigned unique subscripts. These subscripts are used to access the elements. Even though an entire array has only one name, the elements may be accessed and used as individual variables. This is possible because each element is assigned a number known as a subscript. A subscript is used as an index to pinpoint a specific element within an array. The first element is assigned the subscript 0, the second element is assigned 1, and so forth. The six elements in the array hours would have the subscripts 0 through 5. This is shown in Figure 7-4. Figure 7-4 Subscripts 0 1 2 3 4 5 N OTE: Subscript numbering in C++ always starts at zero. The subscript of the last element in an array is one less than the total number of elements in the array. This means that in the array shown in Figure 7-4, the element hours[6] does not exist. hours[5] is the last element in the array. Each element in the hours array, when accessed by its subscript, can be used as a short variable. Here is an example of a statement that stores the number 20 in the first element of the array: hours[0] = 20; 378 Chapter 7 Arrays N O T E : The expression hours[0] is pronounced “hours sub zero.” You would read this assignment statement as “hours sub zero is assigned twenty.” Figure 7-5 shows the contents of the array hours after the statement assigns 20 to hours[0]. Figure 7-5 hours[0] hours[1] hours[2] hours[3] hours[4] hours[5] 20 ? ? ? ? ? N O T E : Because values have not been assigned to the other elements of the array, question marks will be used to indicate that the contents of those elements are unknown. If an array is defined globally, all of its elements are initialized to zero by default. Local arrays, however, have no default initialization value. The following statement stores the integer 30 in hours[3]. hours[3] = 30; Figure 7-6 shows the contents of the array after the previous statement executes: Figure 7-6 hours[0] hours[1] hours[2] hours[3] hours[4] hours[5] 20 ? ? 30 ? ? N OTE: Understand the difference between the array size declarator and a subscript. The number inside the brackets of an array definition is the size declarator. The number inside the brackets of an assignment statement or any statement that works with the contents of an array is a subscript. Inputting and Outputting Array Contents Array elements may be used with the cin and cout objects like any other variable. Program 7-1 shows the array hours being used to store and display values entered by the user. 7.2 Accessing Array Elements 379 Program 7-1 1 // This program asks for the number of hours worked 2 // by six employees. It stores the values in an array. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int NUM_EMPLOYEES = 6; 9 int hours[NUM_EMPLOYEES]; 10 11 // Get the hours worked by each employee. 12 cout << "Enter the hours worked by " 13 << NUM_EMPLOYEES << " employees: "; 14 cin >> hours[0]; 15 cin >> hours[1]; 16 cin >> hours[2]; 17 cin >> hours[3]; 18 cin >> hours[4]; 19 cin >> hours[5]; 20 21 // Display the values in the array. 22 cout << "The hours you entered are:”; 23 cout << " " << hours[0]; 24 cout << " " << hours[1]; 25 cout << " " << hours[2]; 26 cout << " " << hours[3]; 27 cout << " " << hours[4]; 28 cout << " " << hours[5] << endl; 29 return 0; 30 } Program Output with Example Input Shown in Bold Enter the hours worked by 6 employees: 20 12 40 30 30 15 [Enter] The hours you entered are: 20 12 40 30 30 15 Figure 7-7 shows the contents of the array hours with the values entered by the user in the example output above. Figure 7-7 hours[0] hours[1] hours[2] hours[3] hours[4] hours[5] 20 12 40 30 30 15 380 Chapter 7 Arrays VideoNote Accessing Array Elements With a Loop Even though the size declarator of an array definition must be a constant or a literal, subscript numbers can be stored in variables. This makes it possible to use a loop to “cycle through” an entire array, performing the same operation on each element. For example, look at the following code: const int ARRAY_SIZE = 5; int numbers[ARRAY_SIZE]; for (int count = 0; count < ARRAY_SIZE; count++) numbers[count] = 99; This code first defines a constant int named ARRAY_SIZE and initializes it with the value 5. Then it defines an int array named numbers, using ARRAY_SIZE as the size declarator. As a result, the numbers array will have five elements. The for loop uses a counter variable named count. This loop will iterate five times, and during the loop iterations the count variable will take on the values 0 through 4. Notice that the statement inside the loop uses the count variable as a subscript. It assigns 99 to numbers[count]. During the first iteration, 99 is assigned to numbers[0]. During the next iteration, 99 is assigned to numbers[1]. This continues until 99 has been assigned to all of the array’s elements. Figure 7-8 illustrates that the loop’s initialization, test, and update expressions have been written so the loop starts and ends the counter variable with valid subscript values (0 through 4). This ensures that only valid subscripts are used in the body of the loop. Figure 7-8 The variable count starts at 0, which is the first valid subscript value. The loop ends when the variable count reaches 5, which is the first invalid subscript value. for (count = 0; count < ARRAY_SIZE; count++) numbers[count] = 99; The variable count is incremented after each iteration. Program 7-1 could be simplified by using two for loops: one for inputting the values into the array and another for displaying the contents of the array. This is shown in Program 7-2. Program 7-2 1 // This program asks for the number of hours worked 2 // by six employees. It stores the values in an array. 3 #include 4 using namespace std; 5 7.2 Accessing Array Elements 381 6 int main() 7{ 8 const int NUM_EMPLOYEES = 6; // Number of employees 9 int hours[NUM_EMPLOYEES]; // Each employee's hours 10 int count; // Loop counter 11 12 // Input the hours worked. 13 for (count = 0; count < NUM_EMPLOYEES; count++) 14 { 15 cout << “Enter the hours worked by employee ” 16 << (count + 1) << ": "; 17 cin >> hours[count]; 18 } 19 20 // Display the contents of the array. 21 cout << "The hours you entered are:"; 22 for (count = 0; count < NUM_EMPLOYEES; count++) 23 cout << " " << hours[count]; 24 cout << endl; 25 return 0; 26 } Program Output with Example Input Shown in Bold Enter the hours worked by employee 1: 20 [Enter] Enter the hours worked by employee 2: 12 [Enter] Enter the hours worked by employee 3: 40 [Enter] Enter the hours worked by employee 4: 30 [Enter] Enter the hours worked by employee 5: 30 [Enter] Enter the hours worked by employee 6: 15 [Enter] The hours you entered are: 20 12 40 30 30 15 The first for loop, in lines 13 through 18, prompts the user for each employee’s hours. Take a closer look at lines 15 through 17: cout << "Enter the hours worked by employee " << (count + 1) << ": "; cin >> hours[count]; Notice that the cout statement uses the expression count + 1 to display the employee number, but the cin statement uses count as the array subscript. This is because the hours for employee number 1 are stored in hours[0], the hours for employee number 2 are stored in hours[1], and so forth. The loop in lines 22 through 23 also uses the count variable to step through the array, displaying each element. 382 Chapter 7 Arrays N OTE: You can use any integer expression as an array subscript. For example, the first loop in Program 7-2 could have been written like this: for (count = 1; count <= NUM_EMPLOYEES; count++) { cout << "Enter the hours worked by employee " << count << ": "; cin >> hours[count − 1]; } In this code the cin statement uses the expression count − 1 as a subscript. Inputting data into an array must normally be done one element at a time. For example, the following cin statement will not input data into the hours array: cin >> hours; // Wrong! This will NOT work. Instead, you must use multiple cin statements to read data into each array element, or use a loop to step through the array, reading data into its elements. Also, outputting an array’s contents must normally be done one element at a time. For example, the following cout statement will not display the contents of the hours array: cout << hours; // Wrong! This will NOT work. Instead, you must output each element of the array separately. Reading Data from a File into an Array Reading the contents of a file into an array is straightforward: Open the file and use a loop to read each item from the file, storing each item in an array element. The loop should iterate until either the array is filled or the end of the file is reached. Program 7-3 demonstrates by opening a file that has 10 numbers stored in it and then reading the file’s contents into an array. Program 7-3 1 // This program reads data from a file into an array. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int ARRAY_SIZE = 10; // Array size 9 int numbers[ARRAY_SIZE]; // Array with 10 elements 10 int count = 0; // Loop counter variable 11 ifstream inputFile; // Input file stream object 12 13 // Open the file. 14 inputFile.open("TenNumbers.txt"); 15 7.2 Accessing Array Elements 383 16 17 18 19 20 21 22 23 24 25 26 27 28 29 } // Read the numbers from the file into the array. while (count < ARRAY_SIZE && inputFile >> numbers[count]) count++; // Close the file. inputFile.close(); // Display the numbers read: cout << "The numbers are: "; for (count = 0; count < ARRAY_SIZE; count++) cout << numbers[count] << " "; cout << endl; return 0; Program Output The numbers are: 101 102 103 104 105 106 107 108 109 110 The while loop in lines 17 and 18 reads items from the file and assigns them to elements of the numbers array. Notice that the loop tests two Boolean expressions, connected by the && operator: • The first expression is count < ARRAY_SIZE. The purpose of this expression is to prevent the loop from writing beyond the end of the array. If the expression is true, the second Boolean expression is tested. If this expression is false, however, the loop stops. • The second expression is inputFile >> numbers[count]. This expression reads a value from the file and stores it in the numbers[count] array element. If a value is successfully read from the file, the expression is true and the loop continues. If no value can be read from the file, however, the expression is false and the loop stops. Each time the loop iterates, it increments count in line 18. Writing the Contents of an Array to a File Writing the contents of an array to a file is also a straightforward matter. Use a loop to step through each element of the array, writing its contents to a file. Program 7-4 demonstrates. Program 7-4 1 // This program writes the contents of an array to a file. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int ARRAY_SIZE = 10; // Array size 9 int numbers[ARRAY_SIZE]; // Array with 10 elements 10 int count; // Loop counter variable 11 ofstream outputFile; // Output file stream object 12 (program continues) 384 Chapter 7 Arrays Program 7-4 (continued) 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 } // Store values in the array. for (count = 0; count < ARRAY_SIZE; count++) numbers[count] = count; // Open a file for output. outputFile.open("SavedNumbers.txt"); // Write the array contents to the file. for (count = 0; count < ARRAY_SIZE; count++) outputFile << numbers[count] << endl; // Close the file. outputFile.close(); // That's it! cout << "The numbers were saved to the file.\n "; return 0; Program Output The numbers were saved to the file. Contents of the File SavedNumbers.txt 0 1 2 3 4 5 6 7 8 9 7.3 No Bounds Checking in C++ CONCEPT: C++ does not prevent you from overwriting an array’s bounds. C++ is a popular language for software developers who have to write fast, efficient code. To increase runtime efficiency, C++ does not provide many of the common safeguards to prevent unsafe memory access found in other languages. For example, C++ does not perform array bounds checking. This means you can write programs with subscripts that go beyond the boundaries of a particular array. Program 7-5 demonstrates this capability. 7.3 No Bounds Checking in C++ 385 WARN IN G ! Think twice before you compile and run Program 7-5. The program will attempt to write to an area of memory outside the array. This is an invalid operation and will most likely cause the program to crash. Program 7-5 1 // This program unsafely accesses an area of memory by writing 2 // values beyond an array's boundary. 3 // WARNING: If you compile and run this program, it could crash. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 const int SIZE = 3; // Constant for the array size 10 int values[SIZE]; // An array of 3 integers 11 int count; // Loop counter variable 12 13 // Attempt to store five numbers in the three-element array. 14 cout << "I will store 5 numbers in a 3-element array!\n"; 15 for (count = 0; count < 5; count++) 16 values[count] = 100; 17 18 // If the program is still running, display the numbers. 19 cout << "If you see this message, it means the program\n"; 20 cout << "has not crashed! Here are the numbers:\n"; 21 for (count = 0; count < 5; count++) 22 cout << values[count] << endl; 23 return 0; 24 } The values array has three integer elements, with the subscripts 0, 1, and 2. The loop, however, stores the number 100 in elements 0, 1, 2, 3, and 4. The elements with subscripts 3 and 4 do not exist, but C++ allows the program to write beyond the boundary of the array, as if those elements were there. Figure 7-9 depicts the way the array is set up in memory when the program first starts to execute, and what happens when the loop writes data beyond the boundary of the array. 386 Chapter 7 Arrays Figure 7-9 Memory outside the array (Each block = 4 bytes) The way the values array is set up in memory. The outlined area represents the array. Memory outside the array (Each block = 4 bytes) values[0] values[1] values[2] How the numbers assigned to the array overflow the array's boundaries. The shaded area is the section of memory illegally written to. Anything previously stored here is overwritten. 100 100 100 100 100 values[0] values[1] values[2] values[3] values[4] (Does not exist) (Does not exist) Although C++ programs are fast and efficient, the absence of safeguards such as array bounds checking usually proves to be a bad thing. It’s easy for C++ programmers to make careless mistakes that allow programs to access areas of memory that are supposed to be off-limits. You must always make sure that any time you assign values to array elements, the values are written within the array’s boundaries. Watch for Off-by-One Errors In working with arrays, a common type of mistake is the off-by-one error. This is an easy mistake to make because array subscripts start at 0 rather than 1. For example, look at the following code: // This code has an off-by-one error. const int SIZE = 100; int numbers[SIZE]; for (int count = 1; count <= SIZE; count++) numbers[count] = 0; The intent of this code is to create an array of integers with 100 elements, and store the value 0 in each element. However, this code has an off-by-one error. The loop uses its counter variable, count, as a subscript with the numbers array. During the loop’s execution, the variable count takes on the values 1 through 100, when it should take on the values 0 through 99. As a result, the first element, which is at subscript 0, is skipped. In addition, the loop attempts to use 100 as a subscript during the last iteration. Because 100 is an invalid subscript, the program will write data beyond the array’s boundaries. 7.4 Array Initialization 387 Checkpoint 7.1 Define the following arrays: A) empNums, a 100-element array of ints B) payRates, a 25-element array of floats C) miles, a 14-element array of longs D) cityName, a 26-element array of string objects E) lightYears, a 1,000-element array of doubles 7.2 What’s wrong with the following array definitions? int readings[-1]; float measurements[4.5]; int size; string names[size]; 7.3 What would the valid subscript values be in a four-element array of doubles? 7.4 What is the difference between an array’s size declarator and a subscript? 7.5 What is “array bounds checking”? Does C++ perform it? 7.6 What is the output of the following code? int values[5], count; for (count = 0; count < 5; count++) values[count] = count + 1; for (count = 0; count < 5; count++) cout << values[count] << endl; 7.7 The following program skeleton contains a 20-element array of ints called fish. When completed, the program should ask how many fish were caught by fishermen 1 through 20, and store this data in the array. Complete the program. #include using namespace std; int main() { const int NUM_FISH = 20; int fish[NUM_FISH]; // You must finish this program. It should ask how // many fish were caught by fishermen 1-20, and // store this data in the array fish. return 0; } 7.4 Array Initialization CONCEPT: Arrays may be initialized when they are defined. Like regular variables, C++ allows you to initialize an array’s elements when you create the array. Here is an example: const int MONTHS = 12; int days[MONTHS] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; 388 Chapter 7 Arrays The series of values inside the braces and separated with commas is called an initialization list. These values are stored in the array elements in the order they appear in the list. (The first value, 31, is stored in days[0], the second value, 28, is stored in days[1], and so forth.) Figure 7-10 shows the contents of the array after the initialization. Figure 7-10 Subscripts 0 1 2 3 4 5 6 7 8 9 10 11 31 28 31 30 31 30 31 31 30 31 30 31 Program 7-6 demonstrates how an array may be initialized. Program 7-6 1 // This program displays the number of days in each month. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 const int MONTHS = 12; 8 int days[MONTHS] = { 31, 28, 31, 30, 9 31, 30, 31, 31, 10 30, 31, 30, 31}; 11 12 for (int count = 0; count < MONTHS; count++) 13 { 14 cout << "Month " << (count + 1) << " has "; 15 cout << days[count] << “ days.\n"; 16 } 17 return 0; 18 } Program Output Month 1 has 31 days. Month 2 has 28 days. Month 3 has 31 days. Month 4 has 30 days. Month 5 has 31 days. Month 6 has 30 days. Month 7 has 31 days. Month 8 has 31 days. Month 9 has 30 days. Month 10 has 31 days. Month 11 has 30 days. Month 12 has 31 days. 7.4 Array Initialization 389 N OTE: Notice that C++ allows you to spread the initialization list across multiple lines. Both of the following array definitions are equivalent: double coins[5] = {0.05, 0.1, 0.25, 0.5, 1.0}; double coins[5] = {0.05, 0.1, 0.25, 0.5, 1.0}; Program 7-7 shows an example with a string array that is initialized with strings. Program 7-7 1 // This program initializes a string array. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int SIZE = 9; 9 string planets[SIZE] = { "Mercury", "Venus", "Earth", "Mars", 10 "Jupiter", "Saturn", "Uranus", 11 "Neptune", "Pluto (a dwarf planet)" }; 12 13 cout << "Here are the planets:\n"; 14 15 for (int count = 0; count < SIZE; count++) 16 cout << planets[count] << endl; 17 return 0; 18 } Program Output Here are the planets: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto (a dwarf planet) Program 7-8 shows a character array being initialized with the first ten letters of the alphabet. The array is then used to display those characters’ ASCII codes. 390 Chapter 7 Arrays Program 7-8 1 // This program uses an array of ten characters to store the 2 // first ten letters of the alphabet. The ASCII codes of the 3 // characters are displayed. 4 #include 5 using namespace std; 6 7 int main() 8{ 9 const int NUM_LETTERS = 10; 10 char letters[NUM_LETTERS] = {'A', 'B', 'C', 'D', 'E', 11 'F', 'G', 'H', 'I', 'J'}; 12 13 cout << "Character" << "\t" << "ASCII Code\n"; 14 cout << "---------" << "\t" << "----------\n"; 15 for (int count = 0; count < NUM_LETTERS; count++) 16 { 17 cout << letters[count] << "\t\t"; 18 cout << static_cast(letters[count]) << endl; 19 } 20 return 0; 21 } Program Output Character --------A B C D E F G H I J ASCII Code ---------65 66 67 68 69 70 71 72 73 74 N OTE: An array’s initialization list cannot have more values than the array has elements. Partial Array Initialization When an array is being initialized, C++ does not require a value for every element. It’s possible to only initialize part of an array, such as: int numbers[7] = {1, 2, 4, 8}; This definition initializes only the first four elements of a seven-element array, as illustrated in Figure 7-11. 7.4 Array Initialization 391 Figure 7-11 int numbers[7] = {1, 2, 4, 8}; Uninitialized Elements 1 2 4 8 0 0 0 numbers numbers numbers numbers numbers numbers numbers [0] [1] [2] [3] [4] [5] [6] It’s important to note that if an array is partially initialized, the uninitialized elements will be set to zero. The uninitialized elements of a string array will contain empty strings. This is true even if the array is defined locally. (If a local array is completely uninitialized, its elements will contain “garbage,” like all other local variables.) Program 7-9 shows the contents of the array numbers after it is partially initialized. Program 7-9 1 // This program has a partially initialized array. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 const int SIZE = 7; 8 int numbers[SIZE] = {1, 2, 4, 8}; // Initialize first 4 elements 9 10 cout << "Here are the contents of the array:\n"; 11 for (int index = 0; index < SIZE; index++) 12 cout << numbers[index] << " "; 13 14 cout << endl; 15 return 0; 16 } Program Output Here are the contents of the array: 1248000 If you leave an element uninitialized, you must leave all the elements that follow it uninitialized as well. C++ does not provide a way to skip elements in the initialization list. For example, the following is not legal: int numbers[6] = {2, 4, , 8, , 12}; // NOT Legal! Implicit Array Sizing It’s possible to define an array without specifying its size, as long as you provide an initialization list. C++ automatically makes the array large enough to hold all the initialization values. For example, the following definition creates an array with five elements: double ratings[] = {1.0, 1.5, 2.0, 2.5, 3.0}; 392 Chapter 7 Arrays Because the size declarator is omitted, C++ counts the number of items in the initialization list and gives the array that many elements. N OTE: You must provide an initialization list if you leave out an array’s size declarator. Otherwise, C++ doesn’t know how large to make the array. 11 7.5 The Range-Based for Loop CONCEPT: The range-based for loop is a loop that iterates once for each element in an array. Each time the loop iterates, it copies an element from the array to a variable. The range-based for loop was introduced in C++ 11. C++ 11 provides a specialized version of the for loop that, in many circumstances, simplifies array processing. It is known as the range-based for loop. When you use the rangebased for loop with an array, the loop automatically iterates once for each element in the array. For example, if you use the range-based for loop with an eight-element array, the loop will iterate eight times. Because the range-based for loop automatically knows the number of elements in an array, you do not have to use a counter variable to control its iterations, as with a regular for loop. Additionally, you do not have to worry about stepping outside the bounds of an array when you use the range-based for loop. The range-based for loop is designed to work with a built-in variable known as the range variable. Each time the range-based for loop iterates, it copies an array element to the range variable. For example, the first time the loop iterates, the range variable will contain the value of element 0, the second time the loop iterates, the range variable will contain the value of element 1, and so forth. Here is the general format of the range-based for loop: for (dataType rangeVariable : array) statement; Let’s look at the syntax more closely as follows: • dataType is the data type of the range variable. It must be the same as the data type of the array elements, or a type that the elements can automatically be converted to. • rangeVariable is the name of the range variable. This variable will receive the value of a different array element during each loop iteration. During the first loop iteration, it receives the value of the first element; during the second iteration, it receives the value of the second element, and so forth. • array is the name of an array on which you wish the loop to operate. The loop will iterate once for every element in the array. • statement is a statement that executes during a loop iteration. If you need to execute more than one statement in the loop, enclose the statements in a set of braces. For example, assume that you have the following array definition: int numbers[] = { 3, 6, 9 }; 7.5 The Range-Based for Loop 393 You can use the following range-based for loop to display the contents of the numbers array: for (int val : numbers) cout << val << endl; Because the numbers array has three elements, this loop will iterate three times. The first time it iterates, the val variable will receive the value in numbers[0]. During the second iteration, val will receive the value in numbers[1]. During the third iteration, val will receive the value in numbers[2]. The code’s output will be as follows: 3 6 9 Here is an example of a range-based for loop that executes more than one statement in the body of the loop: int[] numbers = { 3, 6, 9 }; for (int val : numbers) { cout << "The next value is "; cout << val << endl; } This code will produce the following output: The next value is 3 The next value is 6 The next value is 9 If you wish, you can use the auto key word to specify the range variable’s data type. Here is an example: int[] numbers = { 3, 6, 9 }; for (auto val : numbers) cout << val << endl; Program 7-10 demonstrates the range-based for loop by displaying the elements of an int array. Program 7-10 1 // This program demonstrates the range-based for loop. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 // Define an array of integers. 8 int numbers[] = { 10, 20, 30, 40, 50 }; 9 10 // Display the values in the array. 11 for (int val : numbers) 12 cout << val << endl; 13 14 return 0; 15 } (program output continues) 394 Chapter 7 Arrays Program 7-10 (continued) Program Output 10 20 30 40 50 Program 7-11 shows another example of the range-based for loop. This program displays the elements of a string array. Program 7-11 1 // This program demonstrates the range-based for loop. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 string planets[] = { "Mercury", "Venus", "Earth", "Mars", 9 "Jupiter", "Saturn", "Uranus", 10 "Neptune", "Pluto (a dwarf planet)" }; 11 12 cout << "Here are the planets:\n"; 13 14 // Display the values in the array. 15 for (string val : planets) 16 cout << val << endl; 17 18 return 0; 19 } Program Output Here are the planets: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto (a dwarf planet) Modifying an Array with a Range-Based for Loop As the range-based for loop executes, its range variable contains only a copy of an array element. As a consequence, you cannot use a range-based for loop to modify the contents 7.5 The Range-Based for Loop 395 of an array unless you declare the range variable as a reference. Recall from Chapter 6 that a reference variable is an alias for another value. Any changes made to the reference variable are actually made to the value for which it is an alias. To declare the range variable as a reference variable, simply write an ampersand (&) in front of its name in the loop header. Program 7-12 shows an example. Program 7-12 1 // This program uses a range-based for loop to 2 // modify the contents of an array. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int SIZE = 5; 9 int numbers[5]; 10 11 // Get values for the array. 12 for (int &val : numbers) 13 { 14 cout << "Enter an integer value: "; 15 cin >> val; 16 } 17 18 // Display the values in the array. 19 cout << "Here are the values you entered:\n"; 20 for (int val : numbers) 21 cout << val << endl; 22 23 return 0; 24 } Program Output with Example Input Shown in Bold Enter an integer value: 1 [Enter] Enter an integer value: 2 [Enter] Enter an integer value: 3 [Enter] Enter an integer value: 4 [Enter] Enter an integer value: 5 [Enter] Here are the values you entered: 1 2 3 4 5 Notice that in line 12 the range variable, val, has an ampersand (&) written in front of its name. This declares val as a reference variable. As the loop executes, the val variable will not merely contain a copy of an array element, but it will be an alias for the element. Any changes made to the val variable will actually be made to the array element it references. 396 Chapter 7 Arrays Also notice that in line 20 we did not declare val as a reference variable (there is no ampersand written in front of the variable’s name). Because the loop is simply displaying the array elements, and does not need to change the array’s contents, there is no need to make val a reference variable. The Range-Based for Loop versus the Regular for Loop The range-based for loop can be used in any situation where you need to step through the elements of an array, and you do not need to use the element subscripts. It will not work, however, in situations where you need the element subscript for some purpose. In those situations, you need to use the regular for loop. N O T E : You can use the auto key word with a reference range variable. For example, the code in lines 12 through 16 in Program 7-12 could have been written like this: for (auto &val : numbers) { cout << "Enter an integer value: "; cin >> val; } 7.6 Processing Array Contents CONCEPT: Individual array elements are processed like any other type of variable. Processing array elements is no different than processing other variables. For example, the following statement multiplies hours[3] by the variable rate: pay = hours[3] * rate; And the following are examples of pre-increment and post-increment operations on array elements: int score[5] = {7, 8, 9, 10, 11}; ++score[2]; // Pre-increment operation on the value in score[2] score[4]++; // Post-increment operation on the value in score[4] N OTE: When using increment and decrement operators, be careful not to confuse the subscript with the array element. For example, the following statement decrements the variable count, but does nothing to the value in amount[count]: amount[count--]; To decrement the value stored in amount[count], use the following statement: amount[count]--; 7.6 Processing Array Contents 397 Program 7-13 demonstrates the use of array elements in a simple mathematical statement. A loop steps through each element of the array, using the elements to calculate the gross pay of five employees. Program 7-13 1 // This program stores, in an array, the hours worked by 2 // employees who all make the same hourly wage. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 const int NUM_EMPLOYEES = 5; // Number of employees 10 int hours[NUM_EMPLOYEES]; // Array to hold hours 11 double payrate; // Hourly pay rate 12 double grossPay; // To hold the gross pay 13 14 // Input the hours worked. 15 cout << "Enter the hours worked by "; 16 cout << NUM_EMPLOYEES << " employees who all\n"; 17 cout << "earn the same hourly rate.\n"; 18 for (int index = 0; index < NUM_EMPLOYEES; index++) 19 { 20 cout << "Employee #" << (index + 1) << ": "; 21 cin >> hours[index]; 22 } 23 24 // Input the hourly rate for all employees. 25 cout << "Enter the hourly pay rate for all the employees: "; 26 cin >> payrate; 27 28 // Display each employee's gross pay. 29 cout << "Here is the gross pay for each employee:\n"; 30 cout << fixed << showpoint << setprecision(2); 31 for (int index = 0; index < NUM_EMPLOYEES; index++) 32 { 33 grossPay = hours[index] * payrate; 34 cout << "Employee #" << (index + 1); 35 cout << ": $" << grossPay << endl; 36 } 37 return 0; 38 } Program Output with Example Input Shown in Bold Enter the hours worked by 5 employees who all earn the same hourly rate. Employee #1: 5 [Enter] Employee #2: 10 [Enter] (program output continues) 398 Chapter 7 Arrays Program 7-13 (continued) Employee #3: 15 [Enter] Employee #4: 20 [Enter] Employee #5: 40 [Enter] Enter the hourly pay rate for all the employees: 12.75 [Enter] Here is the gross pay for each employee: Employee #1: $63.75 Employee #2: $127.50 Employee #3: $191.25 Employee #4: $255.00 Employee #5: $510.00 The following statement in line 33 assigns the value of hours[index] times payRate to the grossPay variable: grossPay = hours[index] * payRate; Array elements may also be used in relational expressions. For example, the following if statement tests cost[20] to determine whether it is less than cost[0]: if (cost[20] < cost[0]) And the following statement sets up a while loop to iterate as long as value[place] does not equal 0: while (value[place] != 0) Thou Shall Not Assign The following code defines two integer arrays: newValues and oldValues. newValues is uninitialized and oldValues is initialized with 10, 100, 200, and 300: const int SIZE = 4; int oldValues[SIZE] = {10, 100, 200, 300}; int newValues[SIZE]; At first glance, it might appear that the following statement assigns the contents of the array oldValues to newValues: newValues = oldValues; // Wrong! Unfortunately, this statement will not work. The only way to assign one array to another is to assign the individual elements in the arrays. Usually, this is best done with a loop, such as: for (int count = 0; count < SIZE; count++) newValues[count] = oldValues[count]; The reason the assignment operator will not work with an entire array at once is complex, but important to understand. Anytime the name of an array is used without brackets and a subscript, it is seen as the array’s beginning memory address. To illustrate this, consider the definition of the arrays newValues and oldValues above. Figure 7-12 depicts the two arrays in memory. Figure 7-12 Memory Address 8012 newValues Array ? ? ? ? 7.6 Processing Array Contents 399 Memory Address 8024 oldValues Array 10 100 200 300 In the figure, newValues is shown starting at memory address 8012 and oldValues is shown starting at 8024. (Of course, these are just arbitrary addresses, picked for illustration purposes. In reality the addresses would probably be different.) Table 7-2 shows various expressions that use the names of these arrays, and their values. Table 7-2 Expression oldValues[0] oldValues[1] oldValues[2] oldValues[3] newValues oldValues Value 10 (Contents of Element 0 of oldValues) 100 (Contents of Element 1 of oldValues) 200 (Contents of Element 2 of oldValues) 300 (Contents of Element 3 of oldValues) 8012 (Memory Address of newValues) 8024 (Memory Address of oldValues) Because the name of an array without the brackets and subscript stands for the array’s starting memory address, the following statement newValues = oldValues; is interpreted by C++ as 8012 = 8024; The statement will not work because you cannot change the starting memory address of an array. Printing the Contents of an Array Suppose we have the following array definition: const int SIZE = 5; int numbers [SIZE] = {10, 20, 30, 40, 50}; You now know that an array’s name is seen as the array’s beginning memory address. This explains why the following statement cannot be used to display the contents of array: cout << numbers << endl; //Wrong! 400 Chapter 7 Arrays When this statement executes, cout will display the array’s memory address, not the array’s contents. You must use a loop to display the contents of each of the array’s elements, as follows. for (int count = 0; count < SIZE; count++) cout << numbers[count] << endl; 11 In C++ 11, you can use the range-based for loop to display an array’s contents, as shown here: for (int val : numbers) cout << val << endl; Summing the Values in a Numeric Array To sum the values in an array, you must use a loop with an accumulator variable. The loop adds the value in each array element to the accumulator. For example, assume that the following statements appear in a program and that values have been stored in the units array. const int NUM_UNITS = 24; int units[NUM_UNITS]; The following code uses a regular for loop to add the values of each element in the array to the total variable. When the code is finished, total will contain the sum of the units array’s elements. int total = 0; // Initialize accumulator for (int count = 0; count < NUM_UNITS; count++) total += units[count]; 11 In C++ 11, you can use the range-based for loop, as shown here. When the code is finished, total will contain the sum of the units array’s elements. int total = 0; // Initialize accumulator for (int val : units) total += val; N O T E : The first statement in both of these code segments sets total to 0. Recall from Chapter 5 that an accumulator variable must be set to 0 before it is used to keep a running total or the sum will not be correct. Getting the Average of the Values in a Numeric Array The first step in calculating the average of all the values in an array is to sum the values. The second step is to divide the sum by the number of elements in the array. Assume that the following statements appear in a program and that values have been stored in the scores array. const int NUM_SCORES = 10; double scores[NUM_SCORES]; The following code calculates the average of the values in the scores array. When the code completes, the average will be stored in the average variable. double total = 0; // Initialize accumulator double average; // Will hold the average for (int count = 0; count < NUM_SCORES; count++) total += scores[count]; average = total / NUM_SCORES; 7.6 Processing Array Contents 401 Notice that the last statement, which divides total by numScores, is not inside the loop. This statement should only execute once, after the loop has finished its iterations. 11 In C++ 11, you can use the range-based for loop, as shown here. When the code completes, the average will be stored in the average variable. double total = 0; // Initialize accumulator double average; // Will hold the average for (int val : scores) total += val; average = total / NUM_SCORES; Finding the Highest and Lowest Values in a Numeric Array The algorithms for finding the highest and lowest values in an array are very similar. First, let’s look at code for finding the highest value in an array. Assume that the following code exists in a program, and that values have already been stored in the numbers array. const int SIZE = 50; int numbers[SIZE]; The code to find the highest value in the array is as follows. int count; int highest; highest = numbers[0]; for (count = 1; count < SIZE; count++) { if (numbers[count] > highest) highest = numbers[count]; } First we copy the value in the first array element to the variable highest. Then the loop compares all of the remaining array elements, beginning at subscript 1, to the value in highest. Each time it finds a value in the array that is greater than highest, it copies that value to highest. When the loop has finished, highest will contain the highest value in the array. The following code finds the lowest value in the array. As you can see, it is nearly identical to the code for finding the highest value. int count; int lowest; lowest = numbers[0]; for (count = 1; count < SIZE; count++) { if (numbers[count] < lowest) lowest = numbers[count]; } When the loop has finished, lowest will contain the lowest value in the array. Partially Filled Arrays Sometimes you need to store a series of items in an array, but you do not know the number of items that there are. As a result, you do not know the exact number of elements needed 402 Chapter 7 Arrays for the array. One solution is to make the array large enough to hold the largest possible number of items. This can lead to another problem, however. If the actual number of items stored in the array is less than the number of elements, the array will be only partially filled. When you process a partially filled array, you must only process the elements that contain valid data items. A partially filled array is normally used with an accompanying integer variable that holds the number of items stored in the array. For example, suppose a program uses the following code to create an array with 100 elements, and an int variable named count that will hold the number of items stored in the array: const int SIZE = 100; int numbers[SIZE]; int count = 0; Each time we add an item to the array, we must increment count. The following code demonstrates. int num; cout << "Enter a number or −1 to quit: "; cin >> num; while (num != −1 && count < SIZE) { count++; numbers[count − 1] = num; cout << "Enter a number or −1 to quit: "; cin >> num; } Each iteration of this sentinel-controlled loop allows the user to enter a number to be stored in the array, or −1 to quit. The count variable is incremented and then used to calculate the subscript of the next available element in the array. When the user enters −1, or count exceeds 99, the loop stops. The following code displays all of the valid items in the array. for (int index = 0; index < count; index++) { cout << numbers[index] << endl; } Notice that this code uses count to determine the maximum array subscript to use. Program 7-14 shows how this technique can be used to read an unknown number of items from a file into an array. The program reads values from the file numbers.txt. Program 7-14 1 //This program reads data from a file into an array. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int ARRAY_SIZE = 100; // Array size 9 int numbers[ARRAY_SIZE]; // Array with 100 elements 7.6 Processing Array Contents 403 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 } int count = 0; ifstream inputFile; // Loop counter variable // Input file stream object inputFile.open("numbers.txt"); // Open the file. // Read the numbers from the file into the array. // After this loop executes, the count variable will hold // the number of values that were stored in the array. while (count < ARRAY_SIZE && inputFile >> numbers[count]) count++; // Close the file. inputFile.close(); // Display the numbers read. cout << "The numbers are: "; for (int index = 0; index < count; index++) cout << numbers[index] << " "; cout << endl; return 0; Program Output The numbers are: 47 89 65 36 12 25 17 8 62 10 87 62 Look closer at the while loop that begins in line 18. It repeats as long as count is less than ARRAY_SIZE and the end of the file has not been encountered. The first part of the while loop’s test expression, count < ARRAY_SIZE, prevents the loop from writing outside the array boundaries. Recall from Chapter 4 that the && operator performs shortcircuit evaluation, so the second part of the while loop’s test expression, inputFile >> values[count], will be executed only if count is less than ARRAY_SIZE. Comparing Arrays We have already noted that you cannot simply assign one array to another array. You must assign each element of the first array to an element of the second array. In addition, you cannot use the == operator with the names of two arrays to determine whether the arrays are equal. For example, the following code appears to compare two arrays, but in reality does not. int firstArray[] = { 5, 10, 15, 20, 25 }; int secondArray[] = { 5, 10, 15, 20, 25 }; if (firstArray == secondArray) // This is a mistake. cout << "The arrays are the same.\n"; else cout << "The arrays are not the same.\n"; When you use the == operator with array names, the operator compares the beginning memory addresses of the arrays, not the contents of the arrays. The two array names in this code will obviously have different memory addresses. Therefore, the result of the expression firstArray == secondArray is false and the code reports that the arrays are not the same. 404 Chapter 7 Arrays To compare the contents of two arrays, you must compare the elements of the two arrays. For example, look at the following code. const int SIZE = 5; int firstArray[SIZE] = { 5, 10, 15, 20, 25 }; int secondArray[SIZE] = { 5, 10, 15, 20, 25 }; bool arraysEqual = true; // Flag variable int count = 0; // Loop counter variable // Determine whether the elements contain the same data. while (arraysEqual && count < SIZE) { if (firstArray[count] != secondArray[count]) arraysEqual = false; count++; } if (arraysEqual) cout << "The arrays are equal.\n"; else cout << "The arrays are not equal.\n"; This code determines whether firstArray and secondArray contain the same values. A bool variable, arraysEqual, which is initialized to true, is used to signal whether the arrays are equal. Another variable, count, which is initialized to 0, is used as a loop counter variable. Then a while loop begins. The loop executes as long as arraysEqual is true and the counter variable count is less than SIZE. During each iteration, it compares a different set of corresponding elements in the arrays. When it finds two corresponding elements that have different values, the arraysEqual variable is set to false. After the loop finishes, an if statement examines the arraysEqual variable. If the variable is true, then the arrays are equal and a message indicating so is displayed. Otherwise, they are not equal, so a different message is displayed. 7.7 Focus on Software Engineering: Using Parallel Arrays CONCEPT: By using the same subscript, you can build relationships between data stored in two or more arrays. Sometimes it’s useful to store related data in two or more arrays. It’s especially useful when the related data is of unlike types. For example, Program 7-15 is another variation of the payroll program. It uses two arrays: one to store the hours worked by each employee (as ints), and another to store each employee’s hourly pay rate (as doubles). Program 7-15 1 // This program uses two parallel arrays: one for hours 2 // worked and one for pay rate. 3 #include 7.7 Focus on Software Engineering: Using Parallel Arrays 405 4 #include 5 using namespace std; 6 7 int main() 8{ 9 const int NUM_EMPLOYEES = 5; // Number of employees 10 int hours[NUM_EMPLOYEES]; // Holds hours worked 11 double payRate[NUM_EMPLOYEES]; // Holds pay rates 12 13 // Input the hours worked and the hourly pay rate. 14 cout << "Enter the hours worked by " << NUM_EMPLOYEES 15 << " employees and their\n" 16 << "hourly pay rates.\n"; 17 for (int index = 0; index < NUM_EMPLOYEES; index++) 18 { 19 cout << "Hours worked by employee #" << (index+1) << ": "; 20 cin >> hours[index]; 21 cout << "Hourly pay rate for employee #" << (index+1) << ": "; 22 cin >> payRate[index]; 23 } 24 25 // Display each employee's gross pay. 26 cout << "Here is the gross pay for each employee:\n"; 27 cout << fixed << showpoint << setprecision(2); 28 for (int index = 0; index < NUM_EMPLOYEES; index++) 29 { 30 double grossPay = hours[index] * payRate[index]; 31 cout << "Employee #" << (index + 1); 32 cout << ": $" << grossPay << endl; 33 } 34 return 0; 35 } Program Output with Example Input Shown in Bold Enter the hours worked by 5 employees and their hourly pay rates. Hours worked by employee #1: 10 [Enter] Hourly pay rate for employee #1: 9.75 [Enter] Hours worked by employee #2: 15 [Enter] Hourly pay rate for employee #2: 8.62 [Enter] Hours worked by employee #3: 20 [Enter] Hourly pay rate for employee #3: 10.50 [Enter] Hours worked by employee #4: 40 [Enter] Hourly pay rate for employee #4: 18.75 [Enter] Hours worked by employee #5: 40 [Enter] Hourly pay rate for employee #5: 15.65 [Enter] Here is the gross pay for each employee: Employee #1: $97.50 Employee #2: $129.30 Employee #3: $210.00 Employee #4: $750.00 Employee #5: $626.00 406 Chapter 7 Arrays Notice in the loops that the same subscript is used to access both arrays. That’s because the data for one employee is stored in the same relative position in each array. For example, the hours worked by employee #1 are stored in hours[0], and the same employee’s pay rate is stored in payRate[0]. The subscript relates the data in both arrays. This concept is illustrated in Figure 7-13. Figure 7-13 10 15 20 40 40 hours[0] hours[1] hours[2] hours[3] hours[4] Employee Employee Employee Employee Employee #1 #2 #3 #4 #5 9.75 8.62 10.50 18.75 15.65 payRate[0] payRate[1] payRate[2] payRate[3] payRate[4] Checkpoint 7.8 Define the following arrays: A) ages, a 10-element array of ints initialized with the values 5, 7, 9, 14, 15, 17, 18, 19, 21, and 23. B) temps, a 7-element array of floats initialized with the values 14.7, 16.3, 18.43, 21.09, 17.9, 18.76, and 26.7. C) alpha, an 8-element array of chars initialized with the values ‘J’, ‘B’, ‘L’, ‘A’, ‘*’, ‘$’, ‘H’, and ‘M’. 7.9 Is each of the following a valid or invalid array definition? (If a definition is invalid, explain why.) 7.10 int numbers[10] = {0, 0, 1, 0, 0, 1, 0, 0, 1, 1}; int matrix[5] = {1, 2, 3, 4, 5, 6, 7}; double radii[10] = {3.2, 4.7}; int table[7] = {2, , , 27, , 45, 39}; char codes[] = {'A', 'X', '1', '2', 's'}; int blanks[]; Given the following array definition: int values[] = {2, 6, 10, 14}; What does each of the following display? A) cout << values[2]; B) cout << ++values[0]; C) cout << values[1]++; D) x = 2; cout << values[++x]; 7.8 Arrays as Function Arguments 407 7.11 7.12 7.13 Given the following array definition: int nums[5] = {1, 2, 3}; What will the following statement display? cout << nums[3]; What is the output of the following code? (You may need to use a calculator.) double balance[5] = {100.0, 250.0, 325.0, 500.0, 1100.0}; const double INTRATE = 0.1; cout << fixed << showpoint << setprecision(2); for (int count = 0; count < 5; count++) cout << (balance[count] * INTRATE) << endl; What is the output of the following code? (You may need to use a calculator.) const int SIZE = 5; int time[SIZE] = {1, 2, 3, 4, 5}, speed[SIZE] = {18, 4, 27, 52, 100}, dist[SIZE]; for (int count = 0; count < SIZE; count++) dist[count] = time[count] * speed[count]; for (int count = 0; count < SIZE; count++) { cout << time[count] << " "; cout << speed[count] << " "; cout << dist[count] << endl; } 7.8 Arrays as Function Arguments VideoNote Passing an Array to a Function CONCEPT: To pass an array as an argument to a function, pass the name of the array. Quite often you’ll want to write functions that process the data in arrays. For example, functions could be written to put values in an array, display an array’s contents on the screen, total all of an array’s elements, or calculate their average. Usually, such functions accept an array as an argument. When a single element of an array is passed to a function, it is handled like any other variable. For example, Program 7-16 shows a loop that passes one element of the array numbers to the function showValue each time the loop iterates. Program 7-16 1 // This program demonstrates that an array element is passed 2 // to a function like any other variable. 3 #include 4 using namespace std; 5 6 void showValue(int); // Function prototype 7 (program continues) 408 Chapter 7 Arrays Program 7-16 (continued) 8 int main() 9{ 10 const int SIZE = 8; 11 int numbers[SIZE] = {5, 10, 15, 20, 25, 30, 35, 40}; 12 13 for (int index = 0; index < SIZE; index++) 14 showValue(numbers[index]); 15 return 0; 16 } 17 18 //********************************************** 19 // Definition of function showValue. * 20 // This function accepts an integer argument. * 21 // The value of the argument is displayed. * 22 //********************************************** 23 24 void showValue(int num) 25 { 26 cout << num << " "; 27 } Program Output 5 10 15 20 25 30 35 40 Each time showValue is called in line 14, a copy of an array element is passed into the parameter variable num. The showValue function simply displays the contents of num and doesn’t work directly with the array element itself. (In other words, the array element is passed by value.) If the function were written to accept the entire array as an argument, however, the parameter would be set up differently. In the following function definition, the parameter nums is followed by an empty set of brackets. This indicates that the argument will be an array, not a single value. void showValues(int nums[], int size) { for (int index = 0; index < size; index++) cout << nums[index] << " "; cout << endl; } The reason there is no size declarator inside the brackets of nums is because nums is not actually an array. It’s a special variable that can accept the address of an array. When an entire array is passed to a function, it is not passed by value, but passed by reference. Imagine the CPU time and memory that would be necessary if a copy of a 10,000-element array were created each time it was passed to a function! Instead, only the starting memory address of the array is passed. Program 7-17 shows the function showValues in use. N OTE: Notice that in the function prototype, empty brackets appear after the data type of the array parameter. This indicates that showValues accepts the address of an array of integers. 7.8 Arrays as Function Arguments 409 Program 7-17 1 // This program demonstrates an array being passed to a function. 2 #include 3 using namespace std; 4 5 void showValues(int [], int); // Function prototype 6 7 int main() 8{ 9 const int ARRAY_SIZE = 8; 10 int numbers[ARRAY_SIZE] = {5, 10, 15, 20, 25, 30, 35, 40}; 11 12 showValues(numbers, ARRAY_SIZE); 13 return 0; 14 } 15 16 //*************************************************** 17 // Definition of function showValue. * 18 // This function accepts an array of integers and * 19 // the array's size as its arguments. The contents * 20 // of the array are displayed. * 21 //*************************************************** 22 23 void showValues(int nums[], int size) 24 { 25 for (int index = 0; index < size; index++) 26 cout << nums[index] << " "; 27 cout << endl; 28 } Program Output 5 10 15 20 25 30 35 40 In Program 7-17, the function showValues is called in the following statement which appears in line 12: showValues(numbers, ARRAY_SIZE); The first argument is the name of the array. Remember, in C++ the name of an array without brackets and a subscript is actually the beginning address of the array. In this function call, the address of the numbers array is being passed as the first argument to the function. The second argument is the size of the array. In the showValues function, the beginning address of the numbers array is copied into the nums parameter variable. The nums variable is then used to reference the numbers array. Figure 7-14 illustrates the relationship between the numbers array and the nums parameter variable. When the contents of nums[0] is displayed, it is actually the contents of numbers[0] that appears on the screen. 410 Chapter 7 Arrays Figure 7-14 numbers Array of eight integers 5 10 15 20 25 30 35 40 nums[0] references numbers[0] nums[1] references numbers[1] nums[2] references numbers[2] ... and so forth N O T E : Although nums is not a reference variable, it works like one. The nums parameter variable in the showValues function can accept the address of any integer array and can be used to reference that array. So, we can use the showValues function to display the contents of any integer array by passing the name of the array and its size as arguments. Program 7-18 uses the function to display the contents of two different arrays. Program 7-18 1 // This program demonstrates the showValues function being 2 // used to display the contents of two arrays. 3 #include 4 using namespace std; 5 6 void showValues(int [], int); // Function prototype 7 8 int main() 9{ 10 const int SIZE1 = 8; // Size of set1 array 11 const int SIZE2 = 5; // Size of set2 array 12 int set1[SIZE1] = {5, 10, 15, 20, 25, 30, 35, 40}; 13 int set2[SIZE2] = {2, 4, 6, 8, 10}; 14 15 // Pass set1 to showValues. 16 showValues(set1, SIZE1); 17 18 // Pass set2 to showValues. 19 showValues(set2, SIZE2); 20 return 0; 21 } 22 23 //*************************************************** 24 // Definition of function showValues. * 25 // This function accepts an array of integers and * 26 // the array's size as its arguments. The contents * 27 // of the array are displayed. * 28 //*************************************************** 29 7.8 Arrays as Function Arguments 411 30 void showValues(int nums[], int size) 31 { 32 for (int index = 0; index < size; index++) 33 cout << nums[index] << " "; 34 cout << endl; 35 } Program Output 5 10 15 20 25 30 35 40 2 4 6 8 10 Recall from Chapter 6 that when a reference variable is used as a parameter, it gives the function access to the original argument. Any changes made to the reference variable are actually performed on the argument referenced by the variable. Array parameters work very much like reference variables. They give the function direct access to the original array. Any changes made with the array parameter are actually made on the original array used as the argument. The function doubleArray in Program 7-19 uses this capability to double the contents of each element in the array. Program 7-19 1 // This program uses a function to double the value of 2 // each element of an array. 3 #include 4 using namespace std; 5 6 // Function prototypes 7 void doubleArray(int [], int); 8 void showValues(int [], int); 9 10 int main() 11 { 12 const int ARRAY_SIZE = 7; 13 int set[ARRAY_SIZE] = {1, 2, 3, 4, 5, 6, 7}; 14 15 // Display the initial values. 16 cout << "The array's values are:\n"; 17 showValues(set, ARRAY_SIZE); 18 19 // Double the values in the array. 20 doubleArray(set, ARRAY_SIZE); 21 22 // Display the resulting values. 23 cout << "After calling doubleArray the values are:\n"; 24 showValues(set, ARRAY_SIZE); 25 26 return 0; 27 } 28 (program continues) 412 Chapter 7 Arrays Program 7-19 (continued) 29 //***************************************************** 30 // Definition of function doubleArray * 31 // This function doubles the value of each element * 32 // in the array passed into nums. The value passed * 33 // into size is the number of elements in the array. * 34 //***************************************************** 35 36 void doubleArray(int nums[], int size) 37 { 38 for (int index = 0; index < size; index++) 39 nums[index] *= 2; 40 } 41 42 //*************************************************** 43 // Definition of function showValues. * 44 // This function accepts an array of integers and * 45 // the array's size as its arguments. The contents * 46 // of the array are displayed. * 47 //*************************************************** 48 49 void showValues(int nums[], int size) 50 { 51 for (int index = 0; index < size; index++) 52 cout << nums[index] << " "; 53 cout << endl; 54 } Program Output The array's values are: 1234567 After calling doubleArray the values are: 2 4 6 8 10 12 14 Using const Array Parameters Sometimes you want a function to be able to modify the contents of an array that is passed to it as an argument, and sometimes you don’t. You can prevent a function from making changes to an array argument by using the const key word in the parameter declaration. Here is an example of the showValues function, shown previously, rewritten with a const array parameter: void showValues(const int nums[], int size) { for (int index = 0; index < size; index++) cout << nums[index] << " "; cout << endl; } When an array parameter is declared as const, the function is not allowed to make changes to the array’s contents. If a statement in the function attempts to modify the array, an error 7.8 Arrays as Function Arguments 413 will occur at compile time. As a precaution, you should always use const array parameters in any function that is not intended to modify its array argument. That way, the function will fail to compile if you inadvertently write code in it that modifies the array. Some Useful Array Functions Section 7.6 introduced you to algorithms such as summing an array and finding the highest and lowest values in an array. Now that you know how to pass an array as an argument to a function, you can write general purpose functions that perform those operations. The following In the Spotlight section shows an example. In the Spotlight: Processing an Array Dr. LaClaire gives four exams during the semester in her chemistry class. At the end of the semester she drops each student’s lowest test score before averaging the scores. She has asked you to write a program that will read a student’s four test scores as input, and calculate the average with the lowest score dropped. Here is the pseudocode algorithm that you developed: Read the student’s four test scores. Calculate the total of the scores. Find the lowest score. Subtract the lowest score from the total. This gives the adjusted total. Divide the adjusted total by 3. This is the average. Display the average. Program 7-20 shows the program, which is modularized. Rather than presenting the entire program at once, let’s first examine the main function, and then each additional function separately. Here is the first part of the program, including the main function: Program 7-20 (main function) 1 // This program gets a series of test scores and 2 // calculates the average of the scores with the 3 // lowest score dropped. 4 #include 5 #include 6 using namespace std; 7 8 // Function prototypes 9 void getTestScores(double[], int); 10 double getTotal(const double[], int); 11 double getLowest(const double[], int); 12 (program continues) 414 Chapter 7 Arrays Program 7-20 (continued) 13 int main() 14 { 15 const int SIZE = 4; // Array size 16 double testScores[SIZE], // Array of test scores 17 total, // Total of the scores 18 lowestScore, // Lowest test score 19 average; // Average test score 20 21 // Set up numeric output formatting. 22 cout << fixed << showpoint << setprecision(1); 23 24 // Get the test scores from the user. 25 getTestScores(testScores, SIZE); 26 27 // Get the total of the test scores. 28 total = getTotal(testScores, SIZE); 29 30 // Get the lowest test score. 31 lowestScore = getLowest(testScores, SIZE); 32 33 // Subtract the lowest score from the total. 34 total −= lowestScore; 35 36 // Calculate the average. Divide by 3 because 37 // the lowest test score was dropped. 38 average = total / (SIZE − 1); 39 40 // Display the average. 41 cout << "The average with the lowest score " 42 << "dropped is " << average << ".\n"; 43 44 return 0; 45 } 46 Lines 15 through 19 define the following items: • SIZE, an int constant that is used as an array size declarator • testScores, a double array to hold the test scores • tOtal, a double variable that will hold the test score totals • lowestScore, a double variable that will hold the lowest test score • average, a double variable that will hold the average of the test scores Line 25 calls the getTestScores function, passing the testScores array and the value of the SIZE constant as arguments. The function gets the test scores from the user and stores them in the array. Line 28 calls the getTotal function, passing the testScores array and the value of the SIZE constant as arguments. The function returns the total of the values in the array. This value is assigned to the total variable. 7.8 Arrays as Function Arguments 415 Line 31 calls the getLowest function, passing the testScores array and the value of the SIZE constant as arguments. The function returns the lowest value in the array. This value is assigned to the lowestScore variable. Line 34 subtracts the lowest test score from the total variable. Then, line 38 calculates the average by dividing total by SIZE – 1. (The program divides by SIZE – 1 because the lowest test score was dropped.) Lines 41 and 42 display the average. The getTestScores function appears next, as shown here: Program 7-20 (getTestScores function) 47 //*********************************************************** 48 // The getTestScores function accepts an array and its size * 49 // as arguments. It prompts the user to enter test scores, * 50 // which are stored in the array. * 51 //*********************************************************** 52 53 void getTestScores(double scores[], int size) 54 { 55 // Loop counter 56 int index; 57 58 // Get each test score. 59 for(index = 0; index <= size − 1; index++) 60 { 61 cout << "Enter test score number " 62 << (index + 1) << ": "; 63 cin >> scores[index]; 64 } 65 } 66 The getTestScores function has two parameters: • scores[]—A double array • size—An int specifying the size of the array that is passed into the scores[] parameter The purpose of this function is to get a student’s test scores from the user and store them in the array that is passed as an argument into the scores[] parameter. The getTotal function appears next, as shown here: Program 7-20 (getTotal function) 67 //***************************************************** 68 // The getTotal function accepts a double array * 69 // and its size as arguments. The sum of the array's * 70 // elements is returned as a double. * 71 //***************************************************** 72 (program continues) 416 Chapter 7 Arrays Program 7-20 (continued) 73 double getTotal(const double numbers[], int size) 74 { 75 double total = 0; // Accumulator 76 77 // Add each element to total. 78 for (int count = 0; count < size; count++) 79 total += numbers[count]; 80 81 // Return the total. 82 return total; 83 } 84 The getTotal function has two parameters: • numbers[] —A const double array • size —An int specifying the size of the array that is passed into the numbers[] parameter This function returns the total of the values in the array that is passed as an argument into the numbers[] parameter. The getLowest function appears next, as shown here: Program 7-20 (getLowest function) 85 //***************************************************** 86 // The getLowest function accepts a double array and * 87 // its size as arguments. The lowest value in the * 88 // array is returned as a double. * 89 //***************************************************** 90 91 double getLowest(const double numbers[], int size) 92 { 93 double lowest; // To hold the lowest value 94 95 // Get the first array's first element. 96 lowest = numbers[0]; 97 98 // Step through the rest of the array. When a 99 // value less than lowest is found, assign it 100 // to lowest. 101 for (int count = 1; count < size; count++) 102 { 103 if (numbers[count] < lowest) 104 lowest = numbers[count]; 105 } 106 107 // Return the lowest value. 108 return lowest; 109 } 7.8 Arrays as Function Arguments 417 The getLowest function has two parameters: • numbers[]—A const double array • size—An int specifying the size of the array that is passed into the numbers[] parameter This function returns the lowest value in the array that is passed as an argument into the numbers[] parameter. Here is an example of the program’s output: Program 7-20 Program Output with Example Input Shown in Bold Enter test score number 1: 92 [Enter] Enter test score number 2: 67 [Enter] Enter test score number 3: 75 [Enter] Enter test score number 4: 88 [Enter] The average with the lowest score dropped is 85.0. Checkpoint 7.14 Given the following array definitions double array1[4] = {1.2, 3.2, 4.2, 5.2}; double array2[4]; will the following statement work? If not, why? array2 = array1; 7.15 When an array name is passed to a function, what is actually being passed? 7.16 When used as function arguments, are arrays passed by value? 7.17 What is the output of the following program? (You may need to consult the ASCII table in Appendix B.) #include using namespace std; // Function prototypes void fillArray(char [], int); void showArray(const char [], int); int main () { const int SIZE = 8; char prodCode[SIZE] = {'0', '0', '0', '0', '0', '0', '0', '0'}; fillArray(prodCode, SIZE); showArray(prodCode, SIZE); return 0; } // Definition of function fillArray. // (Hint: 65 is the ASCII code for 'A') 418 Chapter 7 Arrays 7.18 void fillArray(char arr[], int size) { char code = 65; for (int k = 0; k < size; code++, k++) arr[k] = code; } // Definition of function showArray. void showArray(const char codes[], int size) { for (int k = 0; k < size; k++) cout << codes[k]; cout << endl; } The following program skeleton, when completed, will ask the user to enter 10 integers, which are stored in an array. The function avgArray, which you must write, is to calculate and return the average of the numbers entered. #include using namespace std; // Write your function prototype here int main() { const int SIZE = 10; int userNums[SIZE]; cout << "Enter 10 numbers: "; for (int count = 0; count < SIZE; count++) { cout << "#" << (count + 1) << " "; cin >> userNums[count]; } cout << "The average of those numbers is "; cout << avgArray(userNums, SIZE) << endl; return 0; } // // Write the function avgArray here. // 7.9 Two-Dimensional Arrays CONCEPT: A two-dimensional array is like several identical arrays put together. It is useful for storing multiple sets of data. An array is useful for storing and working with a set of data. Sometimes, though, it’s necessary to work with multiple sets of data. For example, in a grade-averaging program a teacher might record all of one student’s test scores in an array of doubles. If the teacher 7.9 Two-Dimensional Arrays 419 has 30 students, that means she’ll need 30 arrays of doubles to record the scores for the entire class. Instead of defining 30 individual arrays, however, it would be better to define a two-dimensional array. The arrays that you have studied so far are one-dimensional arrays. They are called onedimensional because they can only hold one set of data. Two-dimensional arrays, which are sometimes called 2D arrays, can hold multiple sets of data. It’s best to think of a twodimensional array as having rows and columns of elements, as shown in Figure 7-15. This figure shows an array of test scores, having three rows and four columns. Figure 7-15 Row 0 Row 1 Row 2 Column 0 Column 1 Column 2 Column 3 scores[0] [0] scores[0] [1] scores[0] [2] scores[0] [3] scores[1] [0] scores[1] [1] scores[1] [2] scores[1] [3] scores[2] [0] scores[2] [1] scores[2] [2] scores[2] [3] The array depicted in Figure 7-15 has three rows (numbered 0 through 2), and four columns (numbered 0 through 3). There are a total of 12 elements in the array. To define a two-dimensional array, two size declarators are required. The first one is for the number of rows, and the second one is for the number of columns. Here is an example definition of a two-dimensional array with three rows and four columns: double scores[3][4]; Rows Columns The first size declarator specifies the number of rows, and the second size declarator specifies the number of columns. Notice that each number is enclosed in its own set of brackets. When processing the data in a two-dimensional array, each element has two subscripts: one for its row and another for its column. In the scores array defined above, the elements in row 0 are referenced as scores[0][0] scores[0][1] scores[0][2] scores[0][3] The elements in row 1 are scores[1][0] scores[1][1] scores[1][2] scores[1][3] And the elements in row 2 are scores[2][0] scores[2][1] scores[2][2] scores[2][3] 420 Chapter 7 Arrays The subscripted references are used in a program just like the references to elements in a single-dimensional array, except now you use two subscripts. The first subscript represents the row position, and the second subscript represents the column position. For example, the following statement assigns the value 92.25 to the element at row 2, column 1 of the scores array: scores[2][1] = 92.25; And the following statement displays the element at row 0, column 2: cout << scores[0][2]; Programs that cycle through each element of a two-dimensional array usually do so with nested loops. Program 7-21 is an example. Program 7-21 1 // This program demonstrates a two-dimensional array. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int NUM_DIVS = 3; // Number of divisions 9 const int NUM_QTRS = 4; // Number of quarters 10 double sales[NUM_DIVS][NUM_QTRS]; // Array with 3 rows and 4 columns. 11 double totalSales = 0; // To hold the total sales. 12 int div, qtr; // Loop counters. 13 14 cout << "This program will calculate the total sales of\n"; 15 cout << "all the company's divisions.\n"; 16 cout << "Enter the following sales information:\n\n"; 17 18 // Nested loops to fill the array with quarterly 19 // sales figures for each division. 20 for (div = 0; div < NUM_DIVS; div++) 21 { 22 for (qtr = 0; qtr < NUM_QTRS; qtr++) 23 { 24 cout << "Division " << (div + 1); 25 cout << ", Quarter " << (qtr + 1) << ": $"; 26 cin >> sales[div][qtr]; 27 } 28 cout << endl; // Print blank line. 29 } 30 31 // Nested loops used to add all the elements. 32 for (div = 0; div < NUM_DIVS; div++) 33 { 34 for (qtr = 0; qtr < NUM_QTRS; qtr++) 35 totalSales += sales[div][qtr]; 36 } 37 7.9 Two-Dimensional Arrays 421 38 39 40 41 42 } cout << fixed << showpoint << setprecision(2); cout << "The total sales for the company are: $"; cout << totalSales << endl; return 0; Program Output with Example Input Shown in Bold This program will calculate the total sales of all the company's divisions. Enter the following sales data: Division 1, Quarter 1: $31569.45 [Enter] Division 1, Quarter 2: $29654.23 [Enter] Division 1, Quarter 3: $32982.54 [Enter] Division 1, Quarter 4: $39651.21 [Enter] Division 2, Quarter 1: $56321.02 [Enter] Division 2, Quarter 2: $54128.63 [Enter] Division 2, Quarter 3: $41235.85 [Enter] Division 2, Quarter 4: $54652.33 [Enter] Division 3, Quarter 1: $29654.35 [Enter] Division 3, Quarter 2: $28963.32 [Enter] Division 3, Quarter 3: $25353.55 [Enter] Division 3, Quarter 4: $32615.88 [Enter] The total sales for the company are: $456782.34 When initializing a two-dimensional array, it helps to enclose each row’s initialization list in a set of braces. Here is an example: int hours[3][2] = {{8, 5}, {7, 9}, {6, 3}}; The same definition could also be written as: int hours[3][2] = {{8, 5}, {7, 9}, {6, 3}}; In either case, the values are assigned to hours in the following manner: hours[0][0] is set to 8 hours[0][1] is set to 5 hours[1][0] is set to 7 hours[1][1] is set to 9 hours[2][0] is set to 6 hours[2][1] is set to 3 Figure 7-16 illustrates the initialization. Figure 7-16 Row 0 Row 1 Row 2 Column 0 8 7 6 Column 1 5 9 3 422 Chapter 7 Arrays The extra braces that enclose each row’s initialization list are optional. Both of the following statements perform the same initialization: int hours[3][2] = {{8, 5}, {7, 9}, {6, 3}}; int hours[3][2] = {8, 5, 7, 9, 6, 3}; Because the extra braces visually separate each row, however, it’s a good idea to use them. In addition, the braces give you the ability to leave out initializers within a row without omitting the initializers for the rows that follow it. For instance, look at the following array definition: int table[3][2] = {{1}, {3, 4}, {5}}; table[0][0] is initialized to 1, table[1][0] is initialized to 3, table[1][1] is initialized to 4, and table[2][0] is initialized to 5. table[0][1] and table[2][1] are not initialized. Because some of the array elements are initialized, these two initialized elements are automatically set to zero. Passing Two-Dimensional Arrays to Functions Program 7-22 demonstrates passing a two-dimensional array to a function. When a twodimensional array is passed to a function, the parameter type must contain a size declarator for the number of columns. Here is the header for the function showArray, from Program 7-22: void showArray(const int numbers[][COLS], int rows) COLS is a global named constant which is set to 4. The function can accept any two-dimensional integer array, as long as it consists of four columns. In the program, the contents of two separate arrays are displayed by the function. Program 7-22 1 // This program demonstrates accepting a 2D array argument. 2 #include 3 #include 4 using namespace std; 5 6 // Global constants 7 const int COLS = 4; // Number of columns in each array 8 const int TBL1_ROWS = 3; // Number of rows in table1 9 const int TBL2_ROWS = 4; // Number of rows in table2 10 11 void showArray(const int [][COLS], int); // Function prototype 12 13 int main() 14 { 15 int table1[TBL1_ROWS][COLS] = {{1, 2, 3, 4}, 16 {5, 6, 7, 8}, 17 {9, 10, 11, 12}}; 18 int table2[TBL2_ROWS][COLS] = {{10, 20, 30, 40}, 19 {50, 60, 70, 80}, 20 {90, 100, 110, 120}, 21 {130, 140, 150, 160}}; 7.9 Two-Dimensional Arrays 423 22 23 cout << "The contents of table1 are:\n"; 24 showArray(table1, TBL1_ROWS); 25 cout << "The contents of table2 are:\n"; 26 showArray(table2, TBL2_ROWS); 27 return 0; 28 } 29 30 //****************************************************************** 31 // Function Definition for showArray * 32 // The first argument is a two-dimensional int array with COLS * 33 // columns. The second argument, rows, specifies the number of * 34 // rows in the array. The function displays the array's contents. * 35 //****************************************************************** 36 37 void showArray(const int numbers[][COLS], int rows) 38 { 39 for (int x = 0; x < rows; x++) 40 { 41 for (int y = 0; y < COLS; y++) 42 { 43 cout << setw(4) << numbers[x][y] << " "; 44 } 45 cout << endl; 46 } 47 } Program Output The contents of table1 are: 1234 5678 9 10 11 12 The contents of table2 are: 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 C++ requires the columns to be specified in the function prototype and header because of the way two-dimensional arrays are stored in memory. One row follows another, as shown in Figure 7-17. Figure 7-17 When the compiler generates code for accessing the elements of a two-dimensional array, it needs to know how many bytes separate the rows in memory. The number of columns is a critical factor in this calculation. 424 Chapter 7 Arrays Summing All the Elements of a Two-Dimensional Array To sum all the elements of a two-dimensional array, you can use a pair of nested loops to add the contents of each element to an accumulator. The following code is an example. const int NUM_ROWS = 5; // Number of rows const int NUM_COLS = 5; // Number of columns int total = 0; // Accumulator int numbers[NUM_ROWS][NUM_COLS] = {{2, 7, 9, 6, 4}, {6, 1, 8, 9, 4}, {4, 3, 7, 2, 9}, {9, 9, 0, 3, 1}, {6, 2, 7, 4, 1}}; // Sum the array elements. for (int row = 0; row < NUM_ROWS; row++) { for (int col = 0; col < NUM_COLS; col++) total += numbers[row][col]; } // Display the sum. cout << "The total is " << total << endl; Summing the Rows of a Two-Dimensional Array Sometimes you may need to calculate the sum of each row in a two-dimensional array. For example, suppose a two-dimensional array is used to hold a set of test scores for a set of students. Each row in the array is a set of test scores for one student. To get the sum of a student’s test scores (perhaps so an average may be calculated), you use a loop to add all the elements in one row. The following code shows an example. const int NUM_STUDENTS = 3; // Number of students const int NUM_SCORES = 5; // Number of test scores double total; // Accumulator is set in the loops double average; // To hold each student's average double scores[NUM_STUDENTS][NUM_SCORES] = {{88, 97, 79, 86, 94}, {86, 91, 78, 79, 84}, {82, 73, 77, 82, 89}}; // Get each student's average score. for (int row = 0; row < NUM_STUDENTS; row++) { // Set the accumulator. total = 0; // Sum a row. for (int col = 0; col < NUM_SCORES; col++) total += scores[row][col]; // Get the average. average = total / NUM_SCORES; // Display the average. cout << "Score average for student " << (row + 1) << " is " << average < 4 using namespace std; 5 6 // Function Prototype 7 bool testPIN(const int [], const int [], int); 8 9 int main () 10 { 11 const int NUM_DIGITS = 7; // Number of digits in a PIN 12 int pin1[NUM_DIGITS] = {2, 4, 1, 8, 7, 9, 0}; // Base set of values. 13 int pin2[NUM_DIGITS] = {2, 4, 6, 8, 7, 9, 0}; // Only 1 element is 14 // different from pin1. 15 int pin3[NUM_DIGITS] = {1, 2, 3, 4, 5, 6, 7}; // All elements are 16 // different from pin1. 17 if (testPIN(pin1, pin2, NUM_DIGITS)) 18 cout << "ERROR: pin1 and pin2 report to be the same.\n"; 19 else 20 cout << "SUCCESS: pin1 and pin2 are different.\n"; 21 22 if (testPIN(pin1, pin3, NUM_DIGITS)) 23 cout << "ERROR: pin1 and pin3 report to be the same.\n"; 24 else 25 cout << "SUCCESS: pin1 and pin3 are different.\n"; 26 27 if (testPIN(pin1, pin1, NUM_DIGITS)) 28 cout << "SUCCESS: pin1 and pin1 report to be the same.\n"; 29 else 30 cout << "ERROR: pin1 and pin1 report to be different.\n"; 31 return 0; 32 } 33 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 429 34 //****************************************************************** 35 // The following function accepts two int arrays. The arrays are * 36 // compared. If they contain the same values, true is returned. * 37 // If they contain different values, false is returned. * 38 //****************************************************************** 39 40 bool testPIN(const int custPIN[], const int databasePIN[], int size) 41 { 42 for (int index = 0; index < size; index++) 43 { 44 if (custPIN[index] != databasePIN[index]) 45 return false; // We've found two different values. 46 } 47 return true; // If we make it this far, the values are the same. 48 } Program Output SUCCESS: pin1 and pin2 are different. SUCCESS: pin1 and pin3 are different. SUCCESS: pin1 and pin1 report to be the same. Case Study: See the Intersection of Sets Case Study on the book’s companion Web site at www.pearsonhighered.com/gaddis. 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector CONCEPT: The Standard Template Library offers a vector data type, which in many ways, is superior to standard arrays. The Standard Template Library (STL) is a collection of data types and algorithms that you may use in your programs. These data types and algorithms are programmer-defined. They are not part of the C++ language, but were created in addition to the built-in data types. If you plan to continue your studies in the field of computer science, you should become familiar with the STL. This section introduces one of the STL data types. For more information on the STL, see Chapter 16. The data types that are defined in the STL are commonly called containers. They are called containers because they store and organize data. There are two types of containers in the STL: sequence containers and associative containers. A sequence container organizes data in a sequential fashion, similar to an array. Associative containers organize data with keys, which allow rapid, random access to elements stored in the container. In this section you will learn to use the vector data type, which is a sequence container. A vector is like an array in the following ways: • A vector holds a sequence of values, or elements. • A vector stores its elements in contiguous memory locations. • You can use the array subscript operator [] to read the individual elements in the vector. 430 Chapter 7 Arrays However, a vector offers several advantages over arrays. Here are just a few: • You do not have to declare the number of elements that the vector will have. • If you add a value to a vector that is already full, the vector will automatically increase its size to accommodate the new value. • vectors can report the number of elements they contain. Defining a vector To use vectors in your program, you must include the vector header file with the following statement: #include N O T E : To use the vector data type, you must have the using namespace std; statement in your program. Now you are ready to define an actual vector object. The syntax for defining a vector is somewhat different from the syntax used in defining a regular variable or array. Here is an example: vector numbers; This statement defines numbers as a vector of ints. Notice that the data type is enclosed in angled brackets, immediately after the word vector. Because the vector expands in size as you add values to it, there is no need to declare a size. You can define a starting size, if you prefer. Here is an example: vector numbers(10); This statement defines numbers as a vector of 10 ints. This is only a starting size, however. Although the vector has 10 elements, its size will expand if you add more than 10 values to it. N O T E : If you specify a starting size for a vector, the size declarator is enclosed in parentheses, not square brackets. When you specify a starting size for a vector, you may also specify an initialization value. The initialization value is copied to each element. Here is an example: vector numbers(10, 2); In this statement, numbers is defined as a vector of 10 ints. Each element in numbers is initialized to the value 2. You may also initialize a vector with the values in another vector. For example, look at the following statement. Assume that set1 is a vector of ints that already has values stored in it. vector set2(set1); After this statement executes, set2 will be a copy of set1. Table 7-3 summarizes the vector definition procedures we have discussed. 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 431 Table 7-3 Definition Format vector amounts; vector names; vector scores(15); vector letters(25, 'A'); vector values2(values1); Description Defines amounts as an empty vector of floats. Defines names as an empty vector of string objects. Defines scores as a vector of 15 ints. Defines letters as a vector of 25 characters. Each element is initialized with 'A'. Defines values2 as a vector of doubles. All the elements of values1, which is also a vector of doubles, are copied to value2. Using an Initialization List with a vector in C++ 11 11 If you are using C++ 11, you can initialize a vector with a list of values, as shown in this example: vector numbers { 10, 20, 30, 40 }; This statement defines a vector of ints named numbers. The vector will have 4 elements, initialized with the values 10, 20, 30, and 40. Notice that the initialization list is enclosed in a set of braces, but you do not use an = operator before the list. Storing and Retrieving Values in a vector To store a value in an element that already exists in a vector, you may use the array subscript operator []. For example, look at Program 7-24. Program 7-24 1 // This program stores, in two vectors, the hours worked by 5 2 // employees, and their hourly pay rates. 3 #include 4 #include 5 #include // Needed to define vectors 6 using namespace std; 7 8 int main() 9{ 10 const int NUM_EMPLOYEES = 5; // Number of employees 11 vector hours(NUM_EMPLOYEES); // A vector of integers 12 vector payRate(NUM_EMPLOYEES); // A vector of doubles 13 int index; // Loop counter 14 15 // Input the data. 16 cout << "Enter the hours worked by " << NUM_EMPLOYEES; 17 cout << " employees and their hourly rates.\n"; 18 for (index = 0; index < NUM_EMPLOYEES; index++) (program continues) 432 Chapter 7 Arrays Program 7-24 (continued) 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 } { cout << "Hours worked by employee #" << (index + 1); cout << ": "; cin >> hours[index]; cout << "Hourly pay rate for employee #"; cout << (index + 1) << ": "; cin >> payRate[index]; } // Display each employee's gross pay. cout << "\nHere is the gross pay for each employee:\n"; cout << fixed << showpoint << setprecision(2); for (index = 0; index < NUM_EMPLOYEES; index++) { double grossPay = hours[index] * payRate[index]; cout << "Employee #" << (index + 1); cout << ": $" << grossPay << endl; } return 0; Program Output with Example Input Shown in Bold Enter the hours worked by 5 employees and their hourly rates. Hours worked by employee #1: 10 [Enter] Hourly pay rate for employee #1: 9.75 [Enter] Hours worked by employee #2: 15 [Enter] Hourly pay rate for employee #2: 8.62 [Enter] Hours worked by employee #3: 20 [Enter] Hourly pay rate for employee #3: 10.50 [Enter] Hours worked by employee #4: 40 [Enter] Hourly pay rate for employee #4: 18.75 [Enter] Hours worked by employee #5: 40 [Enter] Hourly pay rate for employee #5: 15.65 [Enter] Here is the gross pay for each employee: Employee #1: $97.50 Employee #2: $129.30 Employee #3: $210.00 Employee #4: $750.00 Employee #5: $626.00 Notice that Program 7-24 uses the following statements in lines 11 and 12 to define two vectors. vector hours(NUM_EMPLOYEES); // A vector of integers vector payRate(NUM_EMPLOYEES); // A vector of doubles 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 433 Both of the vectors are defined with the starting size 5, which is the value of the named constant NUM_EMPLOYEES. The program uses the following loop in lines 18 through 26 to store a value in each element of both vectors: for (index = 0; index < NUM_EMPLOYEES; index++) { cout << "Hours worked by employee #" << (index + 1); cout << ": "; cin >> hours[index]; cout << "Hourly pay rate for employee #"; cout << (index + 1) << ": "; cin >> payRate[index]; } Because the values entered by the user are being stored in vector elements that already exist, the program uses the array subscript operator [], as shown in the following statements, which appear in lines 22 and 25: cin >> hours[index]; cin >> payRate[index]; Using the Range-Based for Loop with a vector in C++ 11 11 With C++ 11, you can use a range-based for loop to step through the elements of a vector, as shown in Program 7-25. Program 7-25 1 // This program demonstrates the range-based for loop with a vector. 2 include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Define and initialize a vector. 9 vector numbers { 10, 20, 30, 40, 50 }; 10 11 // Display the vector elements. 12 for (int val : numbers) 13 cout << val << endl; 14 15 return 0; 16 } Program Output 10 20 30 40 50 434 Chapter 7 Arrays Program 7-26 shows how you can use a reference variable with the range-based for loop to store items in a vector. Program 7-26 1 // This program demonstrates the range-based for loop with a vector. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Define and initialize a vector. 9 vector numbers(5); 10 11 // Get values for the vector elements. 12 for (int &val : numbers) 13 { 14 cout << "Enter an integer value: "; 15 cin >> val; 16 } 17 18 // Display the vector elements. 19 cout << "Here are the values you entered:\n"; 20 for (int val : numbers) 21 cout << val << endl; 22 23 return 0; 24 } Program Output with Example Input Shown in Bold Enter an integer value: 1 [Enter] Enter an integer value: 2 [Enter] Enter an integer value: 3 [Enter] Enter an integer value: 4 [Enter] Enter an integer value: 5 [Enter] Here are the values you entered: 1 2 3 4 5 In line 9, we define numbers as a vector of ints, with 5 elements. Notice that in line 12 the range variable, val, has an ampersand (&) written in front of its name. This declares val as a reference variable. As the loop executes, the val variable will be an alias for a vector element. Any changes made to the val variable will actually be made to the vector element it references. 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 435 Also notice that in line 20 we did not declare val as a reference variable (there is no ampersand written in front of the variable’s name). Because the loop is simply displaying the vector elements, and does not need to change the vector’s contents, there is no need to make val a reference variable. Using the push_back Member Function You cannot use the [] operator to access a vector element that does not exist. To store a value in a vector that does not have a starting size, or that is already full, use the push_back member function. The push_back member function accepts a value as an argument and stores that value after the last element in the vector. (It pushes the value onto the back of the vector.) Here is an example: numbers.push_back(25); Assuming numbers is a vector of ints, this statement stores 25 as the last element. If numbers is full, the statement creates a new last element and stores 25 in it. If there are no elements in numbers, this statement creates an element and stores 25 in it. Program 7-27 is a modification of Program 7-24. This version, however, allows the user to specify the number of employees. The two vectors, hours and payRate, are defined without starting sizes. Because these vectors have no starting elements, the push_back member function is used to store values in the vectors. Program 7-27 1 // This program stores, in two arrays, the hours worked by 5 2 // employees, and their hourly pay rates. 3 #include 4 #include 5 #include // Needed to define vectors 6 using namespace std; 7 8 int main() 9{ 10 vector hours; // hours is an empty vector 11 vector payRate; // payRate is an empty vector 12 int numEmployees; // The number of employees 13 int index; // Loop counter 14 15 // Get the number of employees. 16 cout << "How many employees do you have? "; 17 cin >> numEmployees; 18 19 // Input the payroll data. 20 cout << "Enter the hours worked by " << numEmployees; 21 cout << " employees and their hourly rates.\n"; 22 for (index = 0; index < numEmployees; index++) 23 { 24 int tempHours; // To hold the number of hours entered 25 double tempRate; // To hold the pay rate entered 26 (program continues) 436 Chapter 7 Arrays Program 7-27 (continued) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 } cout << "Hours worked by employee #" << (index + 1); cout << ": "; cin >> tempHours; hours.push_back(tempHours); // Add an element to hours cout << "Hourly pay rate for employee #"; cout << (index + 1) << ": "; cin >> tempRate; payRate.push_back(tempRate); // Add an element to payRate } // Display each employee's gross pay. cout << "Here is the gross pay for each employee:\n"; cout << fixed << showpoint << setprecision(2); for (index = 0; index < numEmployees; index++) { double grossPay = hours[index] * payRate[index]; cout << "Employee #" << (index + 1); cout << ": $" << grossPay << endl; } return 0; Program Output with Example Input Shown in Bold How many employees do you have? 3 [Enter] Enter the hours worked by 3 employees and their hourly rates. Hours worked by employee #1: 40 [Enter] Hourly pay rate for employee #1: 12.63 [Enter] Hours worked by employee #2: 25 [Enter] Hourly pay rate for employee #2: 10.35 [Enter] Hours worked by employee #3: 45 [Enter] Hourly pay rate for employee #3: 22.65 [Enter] Here is the gross pay for each employee: Employee #1: $505.20 Employee #2: $258.75 Employee #3: $1019.2 Notice that in lines 40 through 45 the second loop, which calculates and displays each employee’s gross pay, uses the [] operator to access the elements of the hours and payRate vectors: for (index = 0; index < numEmployees; index++) { double grossPay = hours[index] * payRate[index]; cout << "Employee #" << (index + 1); cout << ": $" << grossPay << endl; } This is possible because the first loop in lines 22 through 35 uses the push_back member function to create the elements in the two vectors. 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 437 Determining the Size of a vector Unlike arrays, vectors can report the number of elements they contain. This is accomplished with the size member function. Here is an example of a statement that uses the size member function: numValues = set.size(); In this statement, assume that numValues is an int and set is a vector. After the statement executes, numValues will contain the number of elements in set. The size member function is especially useful when you are writing functions that accept vectors as arguments. For example, look at the following code for the showValues function: void showValues(vector vect) { for (int count = 0; count < vect.size(); count++) cout << vect[count] << endl; } Because the vector can report its size, this function does not need to accept a second argument indicating the number of elements in the vector. Program 7-28 demonstrates this function. Program 7-28 1 // This program demonstrates the vector size 2 // member function. 3 #include 4 #include 5 using namespace std; 6 7 // Function prototype 8 void showValues(vector); 9 10 int main() 11 { 12 vector values; 13 14 // Put a series of numbers in the vector. 15 for (int count = 0; count < 7; count++) 16 values.push_back(count * 2); 17 18 // Display the numbers. 19 showValues(values); 20 return 0; 21 } 22 23 //************************************************** 24 // Definition of function showValues. * 25 // This function accepts an int vector as its * 26 // argument. The value of each of the vector's * 27 // elements is displayed. * 28 //************************************************** (program continues) 438 Chapter 7 Arrays Program 7-28 (continued) 29 30 void showValues(vector vect) 31 { 32 for (int count = 0; count < vect.size(); count++) 33 cout << vect[count] << endl; 34 } Program Output 0 2 4 6 8 10 12 Removing Elements from a vector Use the pop_back member function to remove the last element from a vector. In the following statement, assume that collection is the name of a vector. collection.pop_back(); This statement removes the last element from the collection vector. Program 7-29 demonstrates the function. Program 7-29 1 // This program demonstrates the vector pop_back member function. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 vector values; 9 10 // Store values in the vector. 11 values.push_back(1); 12 values.push_back(2); 13 values.push_back(3); 14 cout << "The size of values is " << values.size() << endl; 15 16 // Remove a value from the vector. 17 cout << "Popping a value from the vector...\n"; 18 values.pop_back(); 19 cout << "The size of values is now " << values.size() << endl; 20 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 439 21 22 23 24 25 26 27 28 29 30 31 } // Now remove another value from the vector. cout << "Popping a value from the vector...\n"; values.pop_back(); cout << "The size of values is now " << values.size() << endl; // Remove the last value from the vector. cout << "Popping a value from the vector...\n"; values.pop_back(); cout << "The size of values is now " << values.size() << endl; return 0; Program Output The size of values is 3 Popping a value from the vector... The size of values is now 2 Popping a value from the vector... The size of values is now 1 Popping a value from the vector... The size of values is now 0 Clearing a vector To completely clear the contents of a vector, use the clear member function, as shown in the following statement: numbers.clear(); After this statement executes, numbers will be cleared of all its elements. Program 7-30 demonstrates the function. Program 7-30 1 // This program demonstrates the vector clear member function. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 vector values(100); 9 10 cout << "The values vector has " 11 << values.size() << " elements.\n"; 12 cout << "I will call the clear member function...\n"; 13 values.clear(); 14 cout << "Now, the values vector has " 15 << values.size() << " elements.\n"; 16 return 0; 17 } (program output continues) 440 Chapter 7 Arrays Program 7-30 (continued) Program Output The values vector has 100 elements. I will call the clear member function... Now, the values vector has 0 elements. Detecting an Empty vector To determine if a vector is empty, use the empty member function. The function returns true if the vector is empty and false if the vector has elements stored in it. Assuming numberVector is a vector, here is an example of its use: if (numberVector.empty()) cout << "No values in numberVector.\n"; Program 7-31 uses a function named avgVector, which demonstrates the empty member function. Program 7-31 1 // This program demonstrates the vector's empty member function. 2 #include 3 #include 4 using namespace std; 5 6 // Function prototype 7 double avgVector(vector); 8 9 int main() 10 { 11 vector values; // A vector to hold values 12 int numValues; // The number of values 13 double average; // To hold the average 14 15 // Get the number of values to average. 16 cout << "How many values do you wish to average? "; 17 cin >> numValues; 18 19 // Get the values and store them in the vector. 20 for (int count = 0; count < numValues; count++) 21 { 22 int tempValue; 23 cout << "Enter a value: "; 24 cin >> tempValue; 25 values.push_back(tempValue); 26 } 27 28 // Get the average of the values and display it. 29 average = avgVector(values); 30 cout << "Average: " << average << endl; 7.12 If You Plan to Continue in Computer Science: Introduction to the STL vector 441 31 return 0; 32 } 33 34 //************************************************************* 35 // Definition of function avgVector. * 36 // This function accepts an int vector as its argument. If * 37 // the vector contains values, the function returns the * 38 // average of those values. Otherwise, an error message is * 39 // displayed and the function returns 0.0. * 40 //************************************************************* 41 42 double avgVector(vector vect) 43 { 44 int total = 0; // accumulator 45 double avg; // average 46 47 if (vect.empty()) // Determine if the vector is empty 48 { 49 cout << "No values to average.\n"; 50 avg = 0.0; 51 } 52 else 53 { 54 for (int count = 0; count < vect.size(); count++) 55 total += vect[count]; 56 avg = total / vect.size(); 57 } 58 return avg; 59 } Program Output with Example Input Shown in Bold How many values do you wish to average? 5 [Enter] Enter a value: 12 Enter a value: 18 Enter a value: 3 Enter a value: 7 Enter a value: 9 Average: 9 Program Output with Different Example Input Shown in Bold How many values do you wish to average? 0 [Enter] No values to average. Average: 0 Summary of vector Member Functions Table 7-4 provides a summary of the vector member function we have discussed, as well as some additional ones. 442 Chapter 7 Arrays Table 7-4 Member Function at(element) capacity() clear() empty() pop_back() push_back(value) reverse() Description Returns the value of the element located at element in the vector. Example: x = vect.at(5); This statement assigns the value of the fifth element of vect to x. Returns the maximum number of elements that may be stored in the vector without additional memory being allocated. (This is not the same value as returned by the size member function). Example: x = vect.capacity(); This statement assigns the capacity of vect to x. Clears a vector of all its elements. Example: vect.clear(); This statement removes all the elements from vect. Returns true if the vector is empty. Otherwise, it returns false. Example: if (vect.empty()) cout << "The vector is empty."; This statement displays the message if vect is empty. Removes the last element from the vector. Example: vect.pop_back(); This statement removes the last element of vect, thus reducing its size by 1. Stores a value in the last element of the vector. If the vector is full or empty, a new element is created. Example: vect.push_back(7); This statement stores 7 in the last element of vect. Reverses the order of the elements in the vector. (The last element becomes the first element, and the first element becomes the last element.) Example: vect.reverse(); This statement reverses the order of the element in vect. Review Questions and Exercises 443 Table 7-4 (continued) Member Function Description resize(elements, value) Resizes a vector by elements elements. Each of the new elements is initialized with the value in value. Example: swap(vector2) vect.resize(5, 1); This statement increases the size of vect by five elements. The five new elements are initialized to the value 1. Swaps the contents of the vector with the contents of vector2. Example: vect1.swap(vect2); This statement swaps the contents of vect1 and vect2 Checkpoint 7.27 What header file must you #include in order to define vector objects? 7.28 Write a definition statement for a vector named frogs. frogs should be an empty vector of ints. 7.29 Write a definition statement for a vector named lizards. lizards should be a vector of 20 floats. 7.30 Write a definition statement for a vector named toads. toads should be a vector of 100 chars, with each element initialized to 'Z'. 7.31 gators is an empty vector of ints. Write a statement that stores the value 27 in gators. 7.32 snakes is a vector of doubles, with 10 elements. Write a statement that stores the value 12.897 in element 4 of the snakes vector. Review Questions and Exercises Short Answer 1. What is the difference between a size declarator and a subscript? 2. Look at the following array definition. int values[10]; How many elements does the array have? What is the subscript of the first element in the array? What is the subscript of the last element in the array? Assuming that an int uses four bytes of memory, how much memory does the array use? 444 Chapter 7 Arrays 3. Why should a function that accepts an array as an argument, and processes that array, also accept an argument specifying the array’s size? 4. Consider the following array definition: int values[5] = { 4, 7, 6, 8, 2 }; What does each of the following statements display? cout << values[4] << endl; __________ cout << (values[2] + values[3]) << endl; __________ cout << ++values[1] << endl; __________ 5. How do you define an array without providing a size declarator? 6. Look at the following array definition. int numbers[5] = { 1, 2, 3 }; What value is stored in numbers[2]? What value is stored in numbers[4]? 7. Assuming that array1 and array2 are both arrays, why is it not possible to assign the contents of array2 to array1 with the following statement? array1 = array2; 8. Assuming that numbers is an array of doubles, will the following statement display the contents of the array? cout << numbers << endl; 9. Is an array passed to a function by value or by reference? 10. When you pass an array name as an argument to a function, what is actually being passed? 11. How do you establish a parallel relationship between two or more arrays? 12. Look at the following array definition. double sales[8][10]; How many rows does the array have? How many columns does the array have? How many elements does the array have? Write a statement that stores a number in the last column of the last row in the array. 13. When writing a function that accepts a two-dimensional array as an argument, which size declarator must you provide in the parameter for the array? 14. What advantages does a vector offer over an array? Fill-in-the-Blank 15. The _________ indicates the number of elements, or values, an array can hold. 16. The size declarator must be a(n) _________ with a value greater than _________. 17. Each element of an array is accessed and indexed by a number known as a(n) _________. 18. Subscript numbering in C++ always starts at _________. 19. The number inside the brackets of an array definition is the _________, but the num- ber inside an array’s brackets in an assignment statement, or any other statement that works with the contents of the array, is the _________. 20. C++ has no array _________ checking, which means you can inadvertently store data past the end of an array. Review Questions and Exercises 445 21. Starting values for an array may be specified with a(n) _________ list. 22. If an array is partially initialized, the uninitialized elements will be set to _________. 23. If the size declarator of an array definition is omitted, C++ counts the number of items in the _________ to determine how large the array should be. 24. By using the same _________ for multiple arrays, you can build relationships between the data stored in the arrays. 25. You cannot use the _________ operator to copy data from one array to another in a single statement. 26. Any time the name of an array is used without brackets and a subscript, it is seen as _________. 27. To pass an array to a function, pass the _________ of the array. 28. A(n) _________ array is like several arrays of the same type put together. 29. It’s best to think of a two-dimensional array as having _________ and _________. 30. To define a two-dimensional array, _________ size declarators are required. 31. When initializing a two-dimensional array, it helps to enclose each row’s initialization list in _________. 32. When a two-dimensional array is passed to a function the _________ size must be specified. 33. The ____________________ is a collection of programmer-defined data types and algo- rithms that you may use in your programs. 34. The two types of containers defined by the STL are ___________ and ______________. 35. The vector data type is a(n) ______________ container. 36. To define a vector in your program, you must #include the ____________ header file. 37. To store a value in a vector that does not have a starting size, or that is already full, use the ________________ member function. 38. To determine the number of elements in a vector, use the _____________ member function. 39. Use the ________________ member function to remove the last element from a vector. 40. To completely clear the contents of a vector, use the ___________ member function. Algorithm Workbench 41. names is an integer array with 20 elements. Write a regular for loop, as well as a range-based for loop that prints each element of the array. 42. The arrays numberArray1 and numberArray2 have 100 elements. Write code that copies the values in numberArray1 to numberArray2. 43. In a program you need to store the identification numbers of 10 employees (as ints) and their weekly gross pay (as doubles). A) Define two arrays that may be used in parallel to store the 10 employee identifica- tion numbers and gross pay amounts. B) Write a loop that uses these arrays to print each employee’s identification number and weekly gross pay. 446 Chapter 7 Arrays 44. Define a two-dimensional array of integers named grades. It should have 30 rows and 10 columns. 45. In a program you need to store the populations of 12 countries. A) Define two arrays that may be used in parallel to store the names of the countries and their populations. B) Write a loop that uses these arrays to print each country’s name and its population. 46. The following code totals the values in two arrays: numberArray1 and numberArray2. Both arrays have 25 elements. Will the code print the correct sum of values for both arrays? Why or why not? int total = 0; // Accumulator int count; // Loop counter // Calculate and display the total of the first array. for (count = 0; count < 24; count++) total += numberArray1[count]; cout << "The total for numberArray1 is " << total << endl; // Calculate and display the total of the second array. for (count = 0; count < 24; count++) total += numberArray2[count]; cout << "The total for numberArray2 is " << total << endl; 47. Look at the following array definition. int numberArray[9][11]; Write a statement that assigns 145 to the first column of the first row of this array. Write a statement that assigns 18 to the last column of the last row of this array. 48. values is a two-dimensional array of floats with 10 rows and 20 columns. Write code that sums all the elements in the array and stores the sum in the variable total. 49. An application uses a two-dimensional array defined as follows. int days[29][5]; Write code that sums each row in the array and displays the results. Write code that sums each column in the array and displays the results. True or False 50. T F An array’s size declarator can be either a literal, a named constant, or a variable. 51. T F To calculate the amount of memory used by an array, multiply the number of elements by the number of bytes each element uses. 52. T F The individual elements of an array are accessed and indexed by unique numbers. 53. T F The first element in an array is accessed by the subscript 1. 54. T F The subscript of the last element in a single-dimensional array is one less than the total number of elements in the array. 55. T F The contents of an array element cannot be displayed with cout. 56. T F Subscript numbers may be stored in variables. 57. T F You can write programs that use invalid subscripts for an array. Review Questions and Exercises 447 58. T 59. T 60. T 61. T 62. T 63. T 64. T 65. T 66. T 67. T 68. T 69. T 70. T 71. T 72. T 73. T 74. T 75. T 76. T 77. T 78. T 79. T F Arrays cannot be initialized when they are defined. A loop or other means must be used. F The values in an initialization list are stored in the array in the order they appear in the list. F C++ allows you to partially initialize an array. F If an array is partially initialized, the uninitialized elements will contain “garbage.” F If you leave an element uninitialized, you do not have to leave all the ones that follow it uninitialized. F If you leave out the size declarator of an array definition, you do not have to include an initialization list. F The uninitialized elements of a string array will automatically be set to the value "0". F You cannot use the assignment operator to copy one array’s contents to another in a single statement. F When an array name is used without brackets and a subscript, it is seen as the value of the first element in the array. F To pass an array to a function, pass the name of the array. F When defining a parameter variable to hold a single-dimensional array argument, you do not have to include the size declarator. F When an array is passed to a function, the function has access to the original array. F A two-dimensional array is like several identical arrays put together. F It’s best to think of two-dimensional arrays as having rows and columns. F The first size declarator (in the declaration of a two-dimensional array) represents the number of columns. The second size definition represents the number of rows. F Two-dimensional arrays may be passed to functions, but the row size must be specified in the definition of the parameter variable. F C++ allows you to create arrays with three or more dimensions. F A vector is an associative container. F To use a vector, you must include the vector header file. F vectors can report the number of elements they contain. F You can use the [] operator to insert a value into a vector that has no elements. F If you add a value to a vector that is already full, the vector will automatically increase its size to accommodate the new value. Find the Error Each of the following definitions and program segments has errors. Locate as many as you can. 80. int size; double values[size]; 81. int collection[-20]; 448 Chapter 7 Arrays 82. int table[10]; for (int x = 0; x < 20; x++) { cout << "Enter the next value: "; cin >> table[x]; } 83. int hours[3] = 8, 12, 16; 84. int numbers[8] = {1, 2, , 4, , 5}; 85. float ratings[]; 86. char greeting[] = {'H', 'e', 'l', 'l', 'o'}; cout << greeting; 87. int array1[4], array2[4] = {3, 6, 9, 12}; array1 = array2; 88. void showValues(int nums) { for (int count = 0; count < 8; count++) cout << nums[count]; } 89. void showValues(int nums[4][]) { for (rows = 0; rows < 4; rows++) for (cols = 0; cols < 5; cols++) cout << nums[rows][cols]; } 90. vector numbers = { 1, 2, 3, 4 }; VideoNote Solving the Chips and Salsa Problem Programming Challenges 1. Largest/Smallest Array Values Write a program that lets the user enter 10 values into an array. The program should then display the largest and smallest values stored in the array. 2. Rainfall Statistics Write a program that lets the user enter the total rainfall for each of 12 months into an array of doubles. The program should calculate and display the total rainfall for the year, the average monthly rainfall, and the months with the highest and lowest amounts. Input Validation: Do not accept negative numbers for monthly rainfall figures. 3. Chips and Salsa Write a program that lets a maker of chips and salsa keep track of sales for five different types of salsa: mild, medium, sweet, hot, and zesty. The program should use two parallel 5-element arrays: an array of strings that holds the five salsa names and an array of integers that holds the number of jars sold during the past month for each salsa type. The salsa names should be stored using an initialization list at the time the name array is created. The program should prompt the user to enter the number of jars Programming Challenges 449 sold for each type. Once this sales data has been entered, the program should produce a report that displays sales for each salsa type, total sales, and the names of the highest selling and lowest selling products. Input Validation: Do not accept negative values for number of jars sold. 4. Larger Than n In a program, write a function that accepts three arguments: an array, the size of the array, and a number n. Assume that the array contains integers. The function should display all of the numbers in the array that are greater than the number n. 5. Monkey Business A local zoo wants to keep track of how many pounds of food each of its three monkeys eats each day during a typical week. Write a program that stores this information in a two-dimensional 3 × 5 array, where each row represents a different monkey and each column represents a different day of the week. The program should first have the user input the data for each monkey. Then it should create a report that includes the following information: • Average amount of food eaten per day by the whole family of monkeys. • The least amount of food eaten during the week by any one monkey. • The greatest amount of food eaten during the week by any one monkey. Input Validation: Do not accept negative numbers for pounds of food eaten. 6. Rain or Shine An amateur meteorologist wants to keep track of weather conditions during the past year’s three-month summer season and has designated each day as either rainy (‘R’), cloudy (‘C’), or sunny (‘S’). Write a program that stores this information in a 3 × 30 array of characters, where the row indicates the month (0 = June, 1 = July, 2 = August) and the column indicates the day of the month. Note that data are not being collected for the 31st of any month. The program should begin by reading the weather data in from a file. Then it should create a report that displays, for each month and for the whole three-month period, how many days were rainy, how many were cloudy, and how many were sunny. It should also report which of the three months had the largest number of rainy days. Data for the program can be found in the RainOrShine.txt file. 7. Number Analysis Program Write a program that asks the user for a file name. Assume the file contains a series of numbers, each written on a separate line. The program should read the contents of the file into an array and then display the following data: • The lowest number in the array • The highest number in the array • The total of the numbers in the array • The average of the numbers in the array If you have downloaded this book’s source code from the companion Web site, you will find a file named numbers.txt in the Chapter 07 folder. You can use the file to test the program. (The companion Web site is at www.pearsonhighered.com/gaddis.) 450 Chapter 7 Arrays 8. Lo Shu Magic Square The Lo Shu Magic Square is a grid with 3 rows and 3 columns shown in Figure 7-19. The Lo Shu Magic Square has the following properties: • The grid contains the numbers 1 through 9 exactly. • The sum of each row, each column, and each diagonal all add up to the same number. This is shown in Figure 7-20. In a program you can simulate a magic square using a two-dimensional array. Write a function that accepts a two-dimensional array as an argument, and determines whether the array is a Lo Shu Magic Square. Test the function in a program. Figure 7-19 4 9 2 3 5 7 8 1 6 Figure 7-20 15 4 9 2 15 3 5 7 15 8 1 6 15 15 15 15 15 9. Payroll Write a program that uses the following arrays: • empId: an array of seven long integers to hold employee identification numbers. The array should be initialized with the following numbers: 5658845 4520125 7895122 8777541 8451277 1302850 7580489 • hours: an array of seven integers to hold the number of hours worked by each employee • payRate: an array of seven doubles to hold each employee’s hourly pay rate • wages: an array of seven doubles to hold each employee’s gross wages The program should relate the data in each array through the subscripts. For example, the number in element 0 of the hours array should be the number of hours worked by Programming Challenges 451 the employee whose identification number is stored in element 0 of the empId array. That same employee’s pay rate should be stored in element 0 of the payRate array. The program should display each employee number and ask the user to enter that employee’s hours and pay rate. It should then calculate the gross wages for that employee (hours times pay rate) and store them in the wages array. After the data has been entered for all the employees, the program should display each employee’s identification number and gross wages. Input Validation: Do not accept negative values for hours or numbers less than 15.00 for pay rate. 10. Driver’s License Exam The local Driver’s License Office has asked you to write a program that grades the written portion of the driver’s license exam. The exam has 20 multiple choice questions. Here are the correct answers: 1. A 2. D 3. B 4. B 5. C 6. B 7. A 8. B 9. C 10. D 11. A 12. C 13. D 14. B 15. D 16. C 17. C 18. A 19. D 20. B Your program should store the correct answers shown above in an array. It should ask the user to enter the student’s answers for each of the 20 questions, and the answers should be stored in another array. After the student’s answers have been entered, the program should display a message indicating whether the student passed or failed the exam. (A student must correctly answer 15 of the 20 questions to pass the exam.) It should then display the total number of correctly answered questions, the total number of incorrectly answered questions, and a list showing the question numbers of the incorrectly answered questions. Input Validation: Only accept the letters A, B, C, or D as answers. 11. Exam Grader One of your professors has asked you to write a program to grade her final exams, which consist of only 20 multiple-choice questions. Each question has one of four possible answers: A, B, C, or D. The file CorrectAnswers.txt contains the correct answers for all of the questions, with each answer written on a separate line. The first line contains the answer to the first question, the second line contains the answer to the second question, and so forth. (Download the book’s source code from the companion Web site at www.pearsonhighered.com/gaddis. You will find the file in the Chapter 07 folder.) Write a program that reads the contents of the CorrectAnswers.txt file into a char array, and then reads the contents of another file, containing a student’s answers, into a second char array. (You can use the file StudentAnswers.txt for testing purposes. This file is also in the Chapter 07 source code folder, available on the book’s companion Web site.) The program should determine the number of questions that the student missed and then display the following: • A list of the questions missed by the student, showing the correct answer and the incorrect answer provided by the student for each missed question 452 Chapter 7 Arrays • The total number of questions missed • The percentage of questions answered correctly. This can be calculated as Correctly Answered Questions ÷ Total Number of Questions • If the percentage of correctly answered questions is 70% or greater, the program should indicate that the student passed the exam. Otherwise, it should indicate that the student failed the exam. 12. Grade Book A teacher has five students who have taken four tests. The teacher uses the following grading scale to assign a letter grade to a student, based on the average of his or her four test scores. Test Score 90–100 80–89 70–79 60–69 0–59 Letter Grade A B C D F Write a program that uses an array of string objects to hold the five student names, an array of five characters to hold the five students’ letter grades, and five arrays of four doubles to hold each student’s set of test scores. The program should allow the user to enter each student’s name and his or her four test scores. It should then calculate and display each student’s average test score and a letter grade based on the average. Input Validation: Do not accept test scores less than 0 or greater than 100. 13. Grade Book Modification Modify the grade book application in Programming Challenge 13 so it drops each student’s lowest score when determining the test score averages and letter grades. 14. Lottery Application Write a program that simulates a lottery. The program should have an array of five integers named lottery and should generate a random number in the range of 0 through 9 for each element in the array. The user should enter five digits, which should be stored in an integer array named user. The program is to compare the corresponding elements in the two arrays and keep a count of the digits that match. For example, the following shows the lottery array and the user array with sample numbers stored in each. There are two matching digits (elements 2 and 4). lottery array: 7 4 9 13 user array: 4 2 9 73 The program should display the random numbers stored in the lottery array and the number of matching digits. If all of the digits match, display a message proclaiming the user as a grand prize winner. Programming Challenges 453 15. vector Modification Modify the National Commerce Bank case study presented in Program 7-23 so pin1, pin2, and pin3 are vectors instead of arrays. You must also modify the testPIN function to accept a vector instead of an array. 16. World Series Champions If you have downloaded this book’s source code from the companion Web site, you will find the following files in this chapter’s folder: • Teams.txt—This file contains a list of several Major League baseball teams in alphabetical order. Each team listed in the file has won the World Series at least once. • WorldSeriesWinners.txt—This file contains a chronological list of the World Series’ winning teams from 1903 to 2012. (The first line in the file is the name of the team that won in 1903, and the last line is the name of the team that won in 2012. Note that the World Series was not played in 1904 or 1994.) Write a program that displays the contents of the Teams.txt file on the screen and prompts the user to enter the name of one of the teams. The program should then display the number of times that team has won the World Series in the time period from 1903 to 2012. TIP: Read the contents of the WorldSeriesWinners.txt file into an array or vector. When the user enters the name of a team, the program should step through the array or vector counting the number of times the selected team appears. 17. Name Search If you have downloaded this book’s source code from the companion Web site, you will find the following files in this chapter’s folder: • GirlNames.txt—This file contains a list of the 200 most popular names given to girls born in the United States from 2000 to 2009. • BoyNames.txt—This file contains a list of the 200 most popular names given to boys born in the United States from 2000 to 2009. Write a program that reads the contents of the two files into two separate arrays or vectors. The user should be able to enter a boy’s name, a girl’s name, or both, and the application should display messages indicating whether the names were among the most popular. 18. Tic-Tac-Toe Game Write a program that allows two players to play a game of tic-tac-toe. Use a twodimensional char array with three rows and three columns as the game board. Each element of the array should be initialized with an asterisk (*). The program should run a loop that • Displays the contents of the board array • Allows player 1 to select a location on the board for an X. The program should ask the user to enter the row and column number. 454 Chapter 7 Arrays • Allows player 2 to select a location on the board for an O. The program should ask the user to enter the row and column number. • Determines whether a player has won, or a tie has occurred. If a player has won, the program should declare that player the winner and end. If a tie has occurred, the program should say so and end. Player 1 wins when there are three Xs in a row on the game board. The Xs can appear in a row, in a column, or diagonally across the board. A tie occurs when all of the locations on the board are full, but there is no winner. 19. 2D Array Operations Write a program that creates a two-dimensional array initialized with test data. Use any data type you wish. The program should have the following functions: • getTotal. This function should accept a two-dimensional array as its argument and return the total of all the values in the array. • getAverage. This function should accept a two-dimensional array as its argument and return the average of all the values in the array. • getRowTotal. This function should accept a two-dimensional array as its first argument and an integer as its second argument. The second argument should be the subscript of a row in the array. The function should return the total of the values in the specified row. • getColumnTotal. This function should accept a two-dimensional array as its first argument and an integer as its second argument. The second argument should be the subscript of a column in the array. The function should return the total of the values in the specified column. • getHighestInRow. This function should accept a two-dimensional array as its first argument and an integer as its second argument. The second argument should be the subscript of a row in the array. The function should return the highest value in the specified row of the array. • getLowestInRow. This function should accept a two-dimensional array as its first argument and an integer as its second argument. The second argument should be the subscript of a row in the array. The function should return the lowest value in the specified row of the array. Demonstrate each of the functions in this program. Group Project 20. Theater Seating This program should be designed and written by a team of students. Here are some suggestions: • One student should design function main, which will call the other functions in the program. The remainder of the functions will be designed by other members of the team. • The requirements of the program should be analyzed so each student is given about the same work load. • The parameters and return types of each function should be decided in advance. • The program can be implemented as a multi-file program, or all the functions can be cut and pasted into the main file. Programming Challenges 455 Here is the assignment: Write a program that can be used by a small theater to sell tickets for performances. The theater’s auditorium has 15 rows of seats, with 30 seats in each row. The program should display a screen that shows which seats are available and which are taken. For example, the following screen shows a chart depicting each seat in the theater. Seats that are taken are represented by an * symbol, and seats that are available are represented by a # symbol: Row 1 Row 2 Row 3 Row 4 Row 5 Row 6 Row 7 Row 8 Row 9 Row 10 Row 11 Row 12 Row 13 Row 14 Row 15 Seats 123456789012345678901234567890 ***###***###*########*****#### ####*************####*******## **###**********########****### **######**************##****** ********#####*********######## ##############************#### #######************########### ************##****############ #########*****############**** #####*************############ #**********#################** #############********########* ###***********########**###### ############################## ############################## Here is a list of tasks this program must perform: • When the program begins, it should ask the user to enter the seat prices for each row. The prices can be stored in a separate array. (Alternatively, the prices may be read from a file.) • Once the prices are entered, the program should display a seating chart similar to the one shown above. The user may enter the row and seat numbers for tickets being sold. Every time a ticket or group of tickets is purchased, the program should display the total ticket prices and update the seating chart. • The program should keep a total of all ticket sales. The user should be given an option of viewing this amount. • The program should also give the user an option to see a list of how many seats have been sold, how many seats are available in each row, and how many seats are available in the entire auditorium. Input Validation: When tickets are being sold, do not accept row or seat numbers that do not exist. When someone requests a particular seat, the program should make sure that seat is available before it is sold. This page intentionally left blank CHAPTER 8 Searching and Sorting Arrays TOPICS 8.1 Focus on Software Engineering: Introduction to Search Algorithms 8.2 Focus on Problem Solving and Program Design: A Case Study 8.3 Focus on Software Engineering: Introduction to Sorting Algorithms 8.4 Focus on Problem Solving and Program Design: A Case Study 8.5 If You Plan to Continue in Computer Science: Sorting and Searching vectors 8.1 Focus on Software Engineering: Introduction to Search Algorithms CONCEPT: A search algorithm is a method of locating a specific item in a larger collection of data. This section discusses two algorithms for searching the contents of an array. It’s very common for programs not only to store and process data stored in arrays, but to search arrays for specific items. This section will show you two methods of searching an array: the linear search and the binary search. Each has its advantages and disadvantages. The Linear Search The linear search is a very simple algorithm. Sometimes called a sequential search, it uses a loop to sequentially step through an array, starting with the first element. It compares each element with the value being searched for and stops when either the value is found or the end of the array is encountered. If the value being searched for is not in the array, the algorithm will unsuccessfully search to the end of the array. Here is the pseudocode for a function that performs the linear search: Set found to false. Set position to −1. Set index to 0. 457 458 Chapter 8 Searching and Sorting Arrays While found is false and index < number of elements If list[index] is equal to search value found = true. position = index. End If Add 1 to index. End While. Return position. The function searchList shown below is an example of C++ code used to perform a linear search on an integer array. The array list, which has a maximum of numElems elements, is searched for an occurrence of the number stored in value. If the number is found, its array subscript is returned. Otherwise, −1 is returned indicating the value did not appear in the array. int searchList(const int list[], int numElems, int value) { int index = 0; // Used as a subscript to search array int position = −1; // To record position of search value bool found = false; // Flag to indicate if the value was found while (index < numElems && !found) { if (list[index] == value) // If the value is found { found = true; // Set the flag position = index; // Record the value's subscript } index++; // Go to the next element } return position; // Return the position, or −1 } N OTE: The reason −1 is returned when the search value is not found in the array is because −1 is not a valid subscript. Program 8-1 is a complete program that uses the searchList function. It searches the fiveelement array tests to find a score of 100. Program 8-1 1 // This program demonstrates the searchList function, which 2 // performs a linear search on an integer array. 3 #include 4 using namespace std; 5 6 // Function prototype 7 int searchList(const int [], int, int); 8 const int SIZE = 5; 9 10 int main() 11 { 12 int tests[SIZE] = {87, 75, 98, 100, 82}; 13 int results; 8.1 Focus on Software Engineering: Introduction to Search Algorithms 459 14 15 // Search the array for 100. 16 results = searchList(tests, SIZE, 100); 17 18 // If searchList returned −1, then 100 was not found. 19 if (results == −1) 20 cout << "You did not earn 100 points on any test\n"; 21 else 22 { 23 // Otherwise results contains the subscript of 24 // the first 100 found in the array. 25 cout << "You earned 100 points on test "; 26 cout << (results + 1) << endl; 27 } 28 return 0; 29 } 30 31 //***************************************************************** 32 // The searchList function performs a linear search on an * 33 // integer array. The array list, which has a maximum of numElems * 34 // elements, is searched for the number stored in value. If the * 35 // number is found, its array subscript is returned. Otherwise, * 36 // −1 is returned indicating the value was not in the array. * 37 //****************************************************************** 38 39 int searchList(const int list[], int numElems, int value) 40 { 41 int index = 0; // Used as a subscript to search array 42 int position = −1; // To record position of search value 43 bool found = false; // Flag to indicate if the value was found 44 45 while (index < numElems && !found) 46 { 47 if (list[index] == value) // If the value is found 48 { 49 found = true; // Set the flag 50 position = index; // Record the value's subscript 51 } 52 index++; // Go to the next element 53 } 54 return position; // Return the position, or −1 55 } Program Output You earned 100 points on test 4 Inefficiency of the Linear Search The advantage of the linear search is its simplicity. It is very easy to understand and implement. Furthermore, it doesn’t require the data in the array to be stored in any particular order. Its disadvantage, however, is its inefficiency. If the array being searched contains 20,000 elements, the algorithm will have to look at all 20,000 elements in order to find 460 Chapter 8 Searching and Sorting Arrays a value stored in the last element (so the algorithm actually reads an element of the array 20,000 times). In an average case, an item is just as likely to be found near the beginning of the array as near the end. Typically, for an array of N items, the linear search will locate an item in N/2 attempts. If an array has 50,000 elements, the linear search will make a comparison with 25,000 of them in a typical case. This is assuming, of course, that the search item is consistently found in the array. (N/2 is the average number of comparisons. The maximum number of comparisons is always N.) When the linear search fails to locate an item, it must make a comparison with every element in the array. As the number of failed search attempts increases, so does the average number of comparisons. Obviously, the linear search should not be used on large arrays if the speed is important. VideoNote The Binary Search The Binary Search The binary search is a clever algorithm that is much more efficient than the linear search. Its only requirement is that the values in the array be sorted in order. Instead of testing the array’s first element, this algorithm starts with the element in the middle. If that element happens to contain the desired value, then the search is over. Otherwise, the value in the middle element is either greater than or less than the value being searched for. If it is greater, then the desired value (if it is in the list) will be found somewhere in the first half of the array. If it is less, then the desired value (again, if it is in the list) will be found somewhere in the last half of the array. In either case, half of the array’s elements have been eliminated from further searching. If the desired value wasn’t found in the middle element, the procedure is repeated for the half of the array that potentially contains the value. For instance, if the last half of the array is to be searched, the algorithm immediately tests its middle element. If the desired value isn’t found there, the search is narrowed to the quarter of the array that resides before or after that element. This process continues until either the value being searched for is found or there are no more elements to test. Here is the pseudocode for a function that performs a binary search on an array: Set first index to 0. Set last index to the last subscript in the array. Set found to false. Set position to −1. While found is not true and first is less than or equal to last Set middle to the subscript halfway between array[first] and array[last]. If array[middle] equals the desired value Set found to true. Set position to middle. Else If array[middle] is greater than the desired value Set last to middle − 1. Else Set first to middle + 1. End If. End While. Return position. 8.1 Focus on Software Engineering: Introduction to Search Algorithms 461 This algorithm uses three index variables: first, last, and middle. The first and last variables mark the boundaries of the portion of the array currently being searched. They are initialized with the subscripts of the array’s first and last elements. The subscript of the element halfway between first and last is calculated and stored in the middle variable. If the element in the middle of the array does not contain the search value, the first or last variables are adjusted so that only the top or bottom half of the array is searched during the next iteration. This cuts the portion of the array being searched in half each time the loop fails to locate the search value. The function binarySearch shown in the following example is used to perform a binary search on an integer array. The first parameter, array, which has a maximum of numElems elements, is searched for an occurrence of the number stored in value. If the number is found, its array subscript is returned. Otherwise, –1 is returned indicating the value did not appear in the array. int binarySearch(const int array[], int numElems, int value) { int first = 0, // First array element last = numElems − 1, // Last array element middle, // Midpoint of search position = −1; // Position of search value bool found = false; // Flag while (!found && first <= last) { middle = (first + last) / 2; // Calculate midpoint if (array[middle] == value) // If value is found at mid { found = true; position = middle; } else if (array[middle] > value) // If value is in lower half last = middle − 1; else first = middle + 1; // If value is in upper half } return position; } Program 8-2 is a complete program using the binarySearch function. It searches an array of employee ID numbers for a specific value. Program 8-2 1 // This program demonstrates the binarySearch function, which 2 // performs a binary search on an integer array. 3 #include 4 using namespace std; 5 6 // Function prototype 7 int binarySearch(const int [], int, int); 8 const int SIZE = 20; (program continues) 462 Chapter 8 Searching and Sorting Arrays Program 8-2 (continued) 9 10 int main() 11 { 12 // Array with employee IDs sorted in ascending order. 13 int idNums[SIZE] = {101, 142, 147, 189, 199, 207, 222, 14 234, 289, 296, 310, 319, 388, 394, 15 417, 429, 447, 521, 536, 600}; 16 int results; // To hold the search results 17 int empID; // To hold an employee ID 18 19 // Get an employee ID to search for. 20 cout << "Enter the employee ID you wish to search for: "; 21 cin >> empID; 22 23 // Search for the ID. 24 results = binarySearch(idNums, SIZE, empID); 25 26 // If results contains −1 the ID was not found. 27 if (results == −1) 28 cout << "That number does not exist in the array. \n"; 29 else 30 { 31 // Otherwise results contains the subscript of 32 // the specified employee ID in the array. 33 cout << "That ID is found at element " << results; 34 cout << " in the array.\n"; 35 } 36 return 0; 37 } 38 39 //*************************************************************** 40 // The binarySearch function performs a binary search on an * 41 // integer array. array, which has a maximum of size elements, * 42 // is searched for the number stored in value. If the number is * 43 // found, its array subscript is returned. Otherwise, −1 is * 44 // returned indicating the value was not in the array. * 45 //*************************************************************** 46 47 int binarySearch(const int array[], int size, int value) 48 { 49 int first = 0, // First array element 50 last = size − 1, // Last array element 51 middle, // Midpoint of search 52 position = −1; // Position of search value 53 bool found = false; // Flag 54 55 while (!found && first <= last) 56 { 57 middle = (first + last) / 2; // Calculate midpoint 58 if (array[middle] == value) // If value is found at mid 8.2 Focus on Problem Solving and Program Design: A Case Study 463 59 60 61 62 63 64 65 66 67 68 69 } { found = true; position = middle; } else if (array[middle] > value) last = middle − 1; else first = middle + 1; } return position; // If value is in lower half // If value is in upper half Program Output with Example Input Shown in Bold Enter the employee ID you wish to search for: 199 [Enter] That ID is found at element 4 in the array. WARN IN G ! Notice that the array in Program 8-2 is initialized with its values already sorted in ascending order. The binary search algorithm will not work properly unless the values in the array are sorted. The Efficiency of the Binary Search Obviously, the binary search is much more efficient than the linear search. Every time it makes a comparison and fails to find the desired item, it eliminates half of the remaining portion of the array that must be searched. For example, consider an array with 1,000 elements. If the binary search fails to find an item on the first attempt, the number of elements that remains to be searched is 500. If the item is not found on the second attempt, the number of elements that remains to be searched is 250. This process continues until the binary search has either located the desired item or determined that it is not in the array. With 1,000 elements, this takes no more than 10 comparisons. (Compare this to the linear search, which would make an average of 500 comparisons!) Powers of 2 are used to calculate the maximum number of comparisons the binary search will make on an array of any size. (A power of 2 is 2 raised to the power of some number.) Simply find the smallest power of 2 that is greater than or equal to the number of elements in the array. For example, a maximum of 16 comparisons will be made on an array of 50,000 elements (216 = 65,536), and a maximum of 20 comparisons will be made on an array of 1,000,000 elements (220 = 1,048,576). 8.2 Focus on Problem Solving and Program Design: A Case Study The Demetris Leadership Center (DLC, Inc.) publishes the books, DVDs, and audio CDs listed in Table 8-1. 464 Chapter 8 Searching and Sorting Arrays Table 8-1 Product Title Six Steps to Leadership Six Steps to Leadership The Road to Excellence Seven Lessons of Quality Seven Lessons of Quality Seven Lessons of Quality Teams Are Made, Not Born Leadership for the Future Leadership for the Future Product Description Book Audio CD DVD Book Audio CD DVD Book Book Audio CD Product Number 914 915 916 917 918 919 920 921 922 Unit Price $12.95 $14.95 $18.95 $16.95 $21.95 $31.95 $14.95 $14.95 $16.95 The manager of the Telemarketing Group has asked you to write a program that will help order-entry operators look up product prices. The program should prompt the user to enter a product number and will then display the title, description, and price of the product. Variables Table 8-2 lists the variables needed: Table 8-2 Variable NUM_PRODS MIN_PRODNUM MAX_PRODNUM id title description prices Description A constant integer initialized with the number of products the Demetris Leadership Center sells. This value will be used in the definition of the program’s array. A constant integer initialized with the lowest product number. A constant integer initialized with the highest product number. Array of integers. Holds each product’s number. Array of strings, initialized with the titles of products. Array of strings, initialized with the descriptions of each product. Array of doubles. Holds each product’s price. Modules The program will consist of the functions listed in Table 8-3. Table 8-3 Function main getProdNum binarySearch displayProd Description The program’s main function. It calls the program’s other functions. Prompts the user to enter a product number. The function validates input and rejects any value outside the range of correct product numbers. A standard binary search routine. Searches an array for a specified value. If the value is found, its subscript is returned. If the value is not found, –1 is returned. Uses a common subscript into the title, description, and prices arrays to display the title, description, and price of a product. 8.2 Focus on Problem Solving and Program Design: A Case Study 465 Function main Function main contains the variable definitions and calls the other functions. Here is its pseudocode: do Call getProdNum. Call binarySearch. If binarySearch returned −1 Inform the user that the product number was not found. else Call displayProd. End If. Ask the user if the program should repeat. While the user wants to repeat the program. Here is its actual C++ code. do { // Get the desired product number. prodNum = getProdNum(); // Search for the product number. index = binarySearch(id, NUM_PRODS, prodNum); // Display the results of the search. if (index == −1) cout << "That product number was not found.\n"; else displayProd(title, description, prices, index); // Does the user want to do this again? cout << "Would you like to look up another product? (y/n) "; cin >> again; } while (again == 'y' || again == 'Y'); The named constant NUM_PRODS is defined globally and initialized with the value 9. The arrays id, title, description, and prices will already be initialized with data. The getProdNum Function The getProdNum function prompts the user to enter a product number. It tests the value to ensure it is in the range of 914–922 (which are the valid product numbers). If an invalid value is entered, it is rejected and the user is prompted again. When a valid product number is entered, the function returns it. The pseudocode is shown below. Display a prompt to enter a product number. Read prodNum. While prodNum is invalid Display an error message. Read prodNum. End While. Return prodNum. 466 Chapter 8 Searching and Sorting Arrays Here is the actual C++ code. int getProdNum() { int prodNum; cout << "Enter the item's product number: "; cin >> prodNum; // Validate input. while (prodNum < MIN_PRODNUM || prodNum > MAX_PRODNUM) { cout << "Enter a number in the range of " << MIN_PRODNUM; cout <<" through " << MAX_PRODNUM << ".\n"; cin >> prodNum; } return prodNum; } The binarySearch Function The binarySearch function is identical to the function discussed earlier in this chapter. The displayProd Function The displayProd function has parameter variables named title, desc, price, and index. These accept as arguments (respectively) the title, description, and price arrays, and a subscript value. The function displays the data stored in each array at the subscript passed into index. Here is the C++ code. void displayProd(const string title[], const string desc[], const double price[], int index) { cout << "Title: " << title[index] << endl; cout << "Description: " << desc[index] << endl; cout << "Price: $" << price[index] << endl; } The Entire Program Program 8-3 shows the entire program’s source code. Program 8-3 1 // Demetris Leadership Center (DLC) product lookup program 2 // This program allows the user to enter a product number 3 // and then displays the title, description, and price of 4 // that product. 5 #include 6 #include 7 using namespace std; 8 8.2 Focus on Problem Solving and Program Design: A Case Study 467 9 const int NUM_PRODS = 9; // The number of products produced 10 const int MIN_PRODNUM = 914; // The lowest product number 11 const int MAX_PRODNUM = 922; // The highest product number 12 13 // Function prototypes 14 int getProdNum(); 15 int binarySearch(const int [], int, int); 16 void displayProd(const string [], const string [], const double [], int); 17 18 int main() 19 { 20 // Array of product IDs 21 int id[NUM_PRODS] = {914, 915, 916, 917, 918, 919, 920, 22 921, 922}; 23 24 // Array of product titles 25 string title[NUM_PRODS] = 26 { "Six Steps to Leadership", 27 "Six Steps to Leadership", 28 "The Road to Excellence", 29 "Seven Lessons of Quality", 30 "Seven Lessons of Quality", 31 "Seven Lessons of Quality", 32 "Teams Are Made, Not Born", 33 "Leadership for the Future", 34 "Leadership for the Future" 35 }; 36 37 // Array of product descriptions 38 string description[NUM_PRODS] = 39 { "Book", "Audio CD", "DVD", 40 "Book", "Audio CD", "DVD", 41 "Book", "Book", "Audio CD" 42 }; 43 44 // Array of product prices 45 double prices[NUM_PRODS] = {12.95, 14.95, 18.95, 16.95, 21.95, 46 31.95, 14.95, 14.95, 16.95}; 47 48 int prodNum; // To hold a product number 49 int index; // To hold search results 50 char again; // To hold a Y or N answer 51 52 do 53 { 54 // Get the desired product number. 55 prodNum = getProdNum(); 56 57 // Search for the product number. 58 index = binarySearch(id, NUM_PRODS, prodNum); 59 60 // Display the results of the search. 61 if (index == −1) (program continues) 468 Chapter 8 Searching and Sorting Arrays Program 8-3 (continued) 62 cout << "That product number was not found.\n"; 63 else 64 displayProd(title, description, prices, index); 65 66 // Does the user want to do this again? 67 cout << "Would you like to look up another product? (y/n) "; 68 cin >> again; 69 } while (again == 'y' || again == 'Y'); 70 return 0; 71 } 72 73 //*************************************************** 74 // Definition of getProdNum function * 75 // The getProdNum function asks the user to enter a * 76 // product number. The input is validated, and when * 77 // a valid number is entered, it is returned. * 78 //*************************************************** 79 80 int getProdNum() 81 { 82 int prodNum; // Product number 83 84 cout << "Enter the item's product number: "; 85 cin >> prodNum; 86 // Validate input 87 while (prodNum < MIN_PRODNUM || prodNum > MAX_PRODNUM) 88 { 89 cout << "Enter a number in the range of " << MIN_PRODNUM; 90 cout <<" through " << MAX_PRODNUM << ".\n"; 91 cin >> prodNum; 92 } 93 return prodNum; 94 } 95 96 //*************************************************************** 97 // Definition of binarySearch function * 98 // The binarySearch function performs a binary search on an * 99 // integer array. array, which has a maximum of numElems * 100 // elements, is searched for the number stored in value. If the * 101 // number is found, its array subscript is returned. Otherwise, * 102 // −1 is returned indicating the value was not in the array. * 103 //*************************************************************** 104 105 int binarySearch(const int array[], int numElems, int value) 106 { 107 int first = 0, // First array element 108 last = numElems − 1, // Last array element 109 middle, // Midpoint of search 110 position = −1; // Position of search value 111 bool found = false; // Flag 112 8.2 Focus on Problem Solving and Program Design: A Case Study 469 113 while (!found && first <= last) 114 { 115 middle = (first + last) / 2; // Calculate midpoint 116 if (array[middle] == value) // If value is found at mid 117 { 118 found = true; 119 position = middle; 120 } 121 else if (array[middle] > value) // If value is in lower half 122 last = middle − 1; 123 else 124 first = middle + 1; // If value is in upper half 125 } 126 return position; 127 } 128 129 //************************************************************ 130 // The displayProd function accepts three arrays and an int. * 131 // The arrays parameters are expected to hold the title, * 132 // description, and prices arrays defined in main. The index * 133 // parameter holds a subscript. This function displays the * 134 // information in each array contained at the subscript. * 135 //************************************************************ 136 137 void displayProd(const string title[], const string desc[], 138 const double price[], int index) 139 { 140 cout << "Title: " << title[index] << endl; 141 cout << "Description: " << desc[index] << endl; 142 cout << "Price: $" << price[index] << endl; 143 } Program Output with Example Input Shown in Bold Enter the item's product number: 916 [Enter] Title: The Road to Excellence Description: DVD Price: $18.95 Would you like to look up another product? (y/n) y [Enter] Enter the item's product number: 920 [Enter] Title: Teams Are Made, Not Born Description: Book Price: $14.95 Would you like to look up another product? (y/n) n [Enter] Checkpoint 8.1 Describe the difference between the linear search and the binary search. 8.2 On average, with an array of 20,000 elements, how many comparisons will the linear search perform? (Assume the items being searched for are consistently found in the array.) 470 Chapter 8 Searching and Sorting Arrays 8.3 With an array of 20,000 elements, what is the maximum number of comparisons the binary search will perform? 8.4 If a linear search is performed on an array, and it is known that some items are searched for more frequently than others, how can the contents of the array be reordered to improve the average performance of the search? 8.3 Focus on Software Engineering: Introduction to Sorting Algorithms CONCEPT: Sorting algorithms are used to arrange data into some order. Often the data in an array must be sorted in some order. Customer lists, for instance, are commonly sorted in alphabetical order. Student grades might be sorted from highest to lowest. Product codes could be sorted so all the products of the same color are stored together. To sort the data in an array, the programmer must use an appropriate sorting algorithm. A sorting algorithm is a technique for scanning through an array and rearranging its contents in some specific order. This section will introduce two simple sorting algorithms: the bubble sort and the selection sort. The Bubble Sort The bubble sort is an easy way to arrange data in ascending or descending order. If an array is sorted in ascending order, it means the values in the array are stored from lowest to highest. If the values are sorted in descending order, they are stored from highest to lowest. Let’s see how the bubble sort is used in arranging the following array’s elements in ascending order: 7 2 3 8 9 1 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 The bubble sort starts by comparing the first two elements in the array. If element 0 is greater than element 1, they are exchanged. After the exchange, the array shown above would appear as: 2 7 3 8 9 1 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 This method is repeated with elements 1 and 2. If element 1 is greater than element 2, they are exchanged. The array above would then appear as: 2 3 7 8 9 1 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 Next, elements 2 and 3 are compared. In this array, these two elements are already in the proper order (element 2 is less than element 3), so no exchange takes place. As the cycle continues, elements 3 and 4 are compared. Once again, no exchange is necessary because they are already in the proper order. 8.3 Focus on Software Engineering: Introduction to Sorting Algorithms 471 When elements 4 and 5 are compared, however, an exchange must take place because element 4 is greater than element 5. The array now appears as: 2 3 7 8 1 9 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 At this point, the entire array has been scanned, but its contents aren’t quite in the right order yet. So, the sort starts over again with elements 0 and 1. Because those two are in the proper order, no exchange takes place. Elements 1 and 2 are compared next, but once again, no exchange takes place. This continues until elements 3 and 4 are compared. Because element 3 is greater than element 4, they are exchanged. The array now appears as 2 3 7 1 8 9 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 By now you should see how the sort will eventually cause the elements to appear in the correct order. The sort repeatedly passes through the array until no exchanges are made. Ultimately, the array will appear as 1 Element 0 2 Element 1 3 Element 2 7 Element 3 8 9 Element 4 Element 5 Here is the bubble sort in pseudocode: Do Set swap flag to false. For count is set to each subscript in array from 0 through the next-to-last subscript If array[count] is greater than array[count + 1] Swap the contents of array[count] and array[count + 1]. Set swap flag to true. End If. End For. While any elements have been swapped. The C++ code below implements the bubble sort as a function. The parameter array is an integer array to be sorted. size contains the number of elements in array. void sortArray(int array[], int size) { bool swap; int temp; do { swap = false; for (int count = 0; count < (size − 1); count++) { if (array[count] > array[count + 1]) { temp = array[count]; array[count] = array[count + 1]; array[count + 1] = temp; swap = true; 472 Chapter 8 Searching and Sorting Arrays } } } while (swap); } Inside the function is a for loop nested inside a do-while loop. The for loop sequences through the entire array, comparing each element with its neighbor and swapping them if necessary. Anytime two elements are exchanged, the flag variable swap is set to true. The for loop must be executed repeatedly until it can sequence through the entire array without making any exchanges. This is why it is nested inside a do-while loop. The dowhile loop sets swap to false, and then executes the for loop. If swap is set to true after the for loop has finished, the do-while loop repeats. Here is the starting line of the for loop: for (int count = 0; count < (size − 1); count++) The variable count holds the array subscript values. It starts at zero and is incremented as long as it is less than size − 1. The value of size is the number of elements in the array, and count stops just short of reaching this value because the following line compares each element with the one after it: if (array[count] > array[count + 1]) When array[count] is the next-to-last element, it will be compared to the last element. If the for loop were allowed to increment count past size − 1, the last element in the array would be compared to a value outside the array. Let’s look at the if statement in its entirety: if (array[count] > array[count + 1]) { temp = array[count]; array[count] = array[count + 1]; array[count + 1] = temp; swap = true; } If array[count] is greater than array[count + 1], the two elements must be exchanged. First, the contents of array[count] are copied into the variable temp. Then the contents of array[count + 1] is copied into array[count]. The exchange is made complete when the contents of temp (the previous contents of array[count]) are copied to array[count + 1]. Last, the swap flag variable is set to true. This indicates that an exchange has been made. Program 8-4 demonstrates the bubble sort function in a complete program. Program 8-4 1 // This program uses the bubble sort algorithm to sort an 2 / array in ascending order. 3 #include 4 using namespace std; 5 6 // Function prototypes 7 void sortArray(int [], int); 8 void showArray(const int [], int); 8.3 Focus on Software Engineering: Introduction to Sorting Algorithms 473 9 10 int main() 11 { 12 // Array of unsorted values 13 int values[6] = {7, 2, 3, 8, 9, 1}; 14 15 // Display the values. 16 cout << "The unsorted values are: \n"; 17 showArray(values, 6); 18 19 // Sort the values. 20 sortArray(values, 6); 21 22 // Display them again. 23 cout << "The sorted values are:\n"; 24 showArray(values, 6); 25 26 } return 0; 27 28 //*********************************************************** 29 // Definition of function sortArray * 30 // This function performs an ascending order bubble sort on * 31 // array. size is the number of elements in the array. * 32 //*********************************************************** 33 34 void sortArray(int array[], int size) 35 { 36 bool swap; 37 int temp; 38 39 do 40 { 41 swap = false; 42 for (int count = 0; count < (size − 1); count++) 43 { 44 if (array[count] > array[count + 1]) 45 { 46 temp = array[count]; 47 array[count] = array[count + 1]; 48 array[count + 1] = temp; 49 swap = true; 50 } 51 } 52 } while (swap); 53 } 54 55 //************************************************************* 56 // Definition of function showArray. * 57 // This function displays the contents of array. size is the * 58 // number of elements. * 59 //************************************************************* 60 (program continues) 474 Chapter 8 Searching and Sorting Arrays Program 8-4 (continued) 61 void showArray(const int array[], int size) 62 { 63 for (int count = 0; count < size; count++) 64 cout << array[count] << " "; 65 cout << endl; 66 } Program Output The unsorted values are: 723891 The sorted values are: 123789 VideoNote The Selection Sort The Selection Sort The bubble sort is inefficient for large arrays because items only move by one element at a time. The selection sort, however, usually performs fewer exchanges because it moves items immediately to their final position in the array. It works like this: The smallest value in the array is located and moved to element 0. Then the next smallest value is located and moved to element 1. This process continues until all of the elements have been placed in their proper order. Let’s see how the selection sort works when arranging the elements of the following array: 5 7 2 8 9 1 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 The selection sort scans the array, starting at element 0, and locates the element with the smallest value. The contents of this element are then swapped with the contents of element 0. In this example, the 1 stored in element 5 is swapped with the 5 stored in element 0. After the exchange, the array would appear as 1 7 2 8 9 5 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 The algorithm then repeats the process, but because element 0 already contains the smallest value in the array, it can be left out of the procedure. This time, the algorithm begins the scan at element 1. In this example, the contents of element 2 are exchanged with those of element 1. The array would then appear as 1 2 7 8 9 5 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 Once again the process is repeated, but this time the scan begins at element 2. The algorithm will find that element 5 contains the next smallest value. This element’s contents are exchanged with those of element 2, causing the array to appear as 1 Element 0 2 Element 1 5 Element 2 8 Element 3 9 Element 4 7 Element 5 8.3 Focus on Software Engineering: Introduction to Sorting Algorithms 475 Next, the scanning begins at element 3. Its contents are exchanged with those of element 5, causing the array to appear as 1 2 5 7 9 8 Element 0 Element 1 Element 2 Element 3 Element 4 Element 5 At this point there are only two elements left to sort. The algorithm finds that the value in element 5 is smaller than that of element 4, so the two are swapped. This puts the array in its final arrangement: 1 2 5 7 Element 0 Element 1 Element 2 Element 3 Here is the selection sort algorithm in pseudocode: 8 Element 4 9 Element 5 For startScan is set to each subscript in array from 0 through the next-to-last subscript Set index variable to startScan. Set minIndex variable to startScan. Set minValue variable to array[startScan]. For index is set to each subscript in array from (startScan + 1) through the last subscript If array[index] is less than minValue Set minValue to array[index]. Set minIndex to index. End If. End For. Set array[minIndex] to array[startScan]. Set array[startScan] to minValue. End For. The following C++ code implements the selection sort in a function. It accepts two arguments: array and size. array is an integer array, and size is the number of elements in the array. The function uses the selection sort to arrange the values in the array in ascending order. void selectionSort(int array[], int size) { int startScan, minIndex, minValue; for (startScan = 0; startScan < (size − 1); startScan++) { minIndex = startScan; minValue = array[startScan]; for(int index = startScan + 1; index < size; index++) { if (array[index] < minValue) { minValue = array[index]; minIndex = index; } } array[minIndex] = array[startScan]; array[startScan] = minValue; } } 476 Chapter 8 Searching and Sorting Arrays Inside the function are two for loops, one nested inside the other. The inner loop sequences through the array, starting at array[startScan + 1], searching for the element with the smallest value. When the element is found, its subscript is stored in the variable minIndex and its value is stored in minValue. The outer loop then exchanges the contents of this element with array[startScan] and increments startScan. This procedure repeats until the contents of every element have been moved to their proper location. Program 8-5 demonstrates the selection sort function in a complete program. Program 8-5 1 // This program uses the selection sort algorithm to sort an 2 // array in ascending order. 3 #include 4 using namespace std; 5 6 // Function prototypes 7 void selectionSort(int [], int); 8 void showArray(const int [], int); 9 10 int main() 11 { 12 // Define an array with unsorted values 13 const int SIZE = 6; 14 int values[SIZE] = {5, 7, 2, 8, 9, 1}; 15 16 // Display the values. 17 cout << "The unsorted values are\n"; 18 showArray(values, SIZE); 19 20 // Sort the values. 21 selectionSort(values, SIZE); 22 23 // Display the values again. 24 cout << "The sorted values are\n"; 25 showArray(values, SIZE); 26 return 0; 27 } 28 29 //************************************************************** 30 // Definition of function selectionSort. * 31 // This function performs an ascending order selection sort on * 32 // array. size is the number of elements in the array. * 33 //************************************************************** 34 35 void selectionSort(int array[], int size) 36 { 37 int startScan, minIndex, minValue; 38 39 for (startScan = 0; startScan < (size − 1); startScan++) 40 { 41 minIndex = startScan; 42 minValue = array[startScan]; 43 for(int index = startScan + 1; index < size; index++) 8.4 Focus on Problem Solving and Program Design: A Case Study 477 44 { 45 if (array[index] < minValue) 46 { 47 minValue = array[index]; 48 minIndex = index; 49 } 50 } 51 array[minIndex] = array[startScan]; 52 array[startScan] = minValue; 53 } 54 } 55 56 //************************************************************** 57 // Definition of function showArray. * 58 // This function displays the contents of array. size is the * 59 // number of elements. * 60 //************************************************************** 61 62 void showArray(const int array[], int size) 63 { 64 for (int count = 0; count < size; count++) 65 cout << array[count] << " "; 66 cout << endl; 67 } Program Output The unsorted values are 572891 The sorted values are 125789 8.4 Focus on Problem Solving and Program Design: A Case Study Like the previous case study, this is a program developed for the Demetris Leadership Center. Recall that DLC, Inc., publishes books, DVDs, and audio CDs. (See Table 8-1 for a complete list of products, with title, description, product number, and price.) Table 8-4 shows the number of units of each product sold during the past six months. Table 8-4 Product Number 914 915 916 917 918 919 920 921 922 Units Sold 842 416 127 514 437 269 97 492 212 478 Chapter 8 Searching and Sorting Arrays The vice president of sales has asked you to write a sales reporting program that displays the following information: • A list of the products in the order of their sales dollars (NOT units sold), from highest to lowest • The total number of all units sold • The total sales for the six-month period Variables Table 8-5 lists the variables needed: Table 8-5 Variable NUM_PRODS prodNum units prices sales Description A constant integer initialized with the number of products that DLC, Inc., sells. This value will be used in the definition of the program’s array. Array of ints. Holds each product’s number. Array of ints. Holds each product’s number of units sold. Array of doubles. Holds each product’s price. Array of doubles. Holds the computed sales amounts (in dollars) of each product. The elements of the four arrays, prodNum, units, prices, and sales, will correspond with each other. For example, the product whose number is stored in prodNum[2] will have sold the number of units stored in units[2]. The sales amount for the product will be stored in sales[2]. Modules The program will consist of the functions listed in Table 8-6. Table 8-6 Function main calcSales dualSort showOrder showTotals Description The program’s main function. It calls the program’s other functions. Calculates each product’s sales. Sorts the sales array so the elements are ordered from highest to lowest. The prodNum array is ordered so the product numbers correspond with the correct sales figures in the sorted sales array. Displays a list of the product numbers and sales amounts from the sorted sales and prodNum arrays. Displays the total number of units sold and the total sales amount for the period. Function main Function main is very simple. It contains the variable definitions and calls the other functions. Here is the pseudocode for its executable statements: 8.4 Focus on Problem Solving and Program Design: A Case Study 479 Call calcSales. Call dualSort. Set display mode to fixed point with two decimal places of precision. Call showOrder. Call showTotals. Here is its actual C++ code: // Calculate each product's sales. calcSales(units, prices, sales, NUM_PRODS); // Sort the elements in the sales array in descending // order and shuffle the ID numbers in the id array to // keep them in parallel. dualSort(id, sales, NUM_PRODS); // Set the numeric output formatting. cout << setprecision(2) << fixed << showpoint; // Display the products and sales amounts. showOrder(sales, id, NUM_PRODS); // Display total units sold and total sales. showTotals(sales, units, NUM_PRODS); The named constant NUM_PRODS will be defined globally and initialized to the value 9. The arrays id, units, and prices will already be initialized with data. (It will be left as an exercise for you to modify this program so the user may enter these values.) The calcSales Function The calcSales function multiplies each product’s units sold by its price. The resulting amount is stored in the sales array. Here is the function’s pseudocode: For index is set to each subscript in the arrays from 0 through the last subscript. Set sales[index] to units[index] times prices[index]. End For. And here is the function’s actual C++ code: void calcSales(const int units[], const double prices[], double sales[], int num) { for (int index = 0; index < num; index++) sales[index] = units[index] * prices[index]; } The dualSort Function The dualSort function is a modified version of the selection sort algorithm shown in Program 8-5. The dualSort function accepts two arrays as arguments: the sales array and the id array. The function actually performs the selection sort on the sales array. When the function moves an element in the sales array, however, it also moves the corresponding 480 Chapter 8 Searching and Sorting Arrays element in the id array. This is to ensure that the product numbers in the id array still have subscripts that match their sales figures in the sales array. The dualSort function is also different in another way: It sorts the array in descending order. Here is the pseudocode for the dualSort function: For startScan variable is set to each subscript in array from 0 through the next-to-last subscript Set index variable to startScan. Set maxIndex variable to startScan. Set tempId variable to id[startScan]. Set maxValue variable to sales[startScan]. For index variable is set to each subscript in array from (startScan + 1) through the last subscript If sales[index] is greater than maxValue Set maxValue to sales[index]. Set tempId to tempId[index]. Set maxIndex to index. End If. End For. Set sales[maxIndex] to sales[startScan]. Set id[maxIndex] = id[startScan]. Set sales[startScan] to maxValue. Set id[startScan] = tempId. End For. Here is the actual C++ code for the dualSort function: void dualSort(int id[], double sales[], int size) { int startScan, maxIndex, tempId; double maxValue; for (startScan = 0; startScan < (size − 1); startScan++) { maxIndex = startScan; maxValue = sales[startScan]; tempId = id[startScan]; for(int index = startScan + 1; index < size; index++) { if (sales[index] > maxValue) { maxValue = sales[index]; tempId = id[index]; maxIndex = index; } } sales[maxIndex] = sales[startScan]; id[maxIndex] = id[startScan]; sales[startScan] = maxValue; id[startScan] = tempId; } } 8.4 Focus on Problem Solving and Program Design: A Case Study 481 N O T E : Once the dualSort function is called, the id and sales arrays are no longer synchronized with the units and prices arrays. Because this program doesn’t use units and prices together with id and sales after this point, it will not be noticed in the final output. However, it is never a good programming practice to sort parallel arrays in such a way that they are out of synchronization. It will be left as an exercise for you to modify the program so all the arrays are synchronized and used in the final output of the program. The showOrder Function The showOrder function displays a heading and the sorted list of product numbers and their sales amounts. It accepts the id and sales arrays as arguments. Here is its pseudocode: Display heading. For index variable is set to each subscript of the arrays from 0 through the last subscript Display id[index]. Display sales[index]. End For. Here is the function’s actual C++ code: void showOrder(const double sales[], const int id[], int num) { cout << "Product Number\tSales\n"; cout << "----------------------------------\n"; for (int index = 0; index < num; index++) { cout << id[index] << "\t\t$"; cout << setw(8) << sales[index] << endl; } cout << endl; } The showTotals Function The showTotals function displays the total number of units of all products sold and the total sales for the period. It accepts the units and sales arrays as arguments. Here is its pseudocode: Set totalUnits variable to 0. Set totalSales variable to 0.0. For index variable is set to each subscript in the arrays from 0 through the last subscript Add units[index] to totalUnits[index]. Add sales[index] to totalSales. End For. Display totalUnits with appropriate heading. Display totalSales with appropriate heading. 482 Chapter 8 Searching and Sorting Arrays Here is the function’s actual C++ code: void showTotals(const double sales[], const int units[], int num) { int totalUnits = 0; double totalSales = 0.0; for (int index = 0; index < num; index++) { totalUnits += units[index]; totalSales += sales[index]; } cout << "Total Units Sold: " << totalUnits << endl; cout << "Total Sales: $" << totalSales << endl; } The Entire Program Program 8-6 shows the entire program’s source code. Program 8-6 1 // This program produces a sales report for DLC, Inc. 2 #include 3 #include 4 using namespace std; 5 6 // Function prototypes 7 void calcSales(const int [], const double [], double [], int); 8 void showOrder(const double [], const int [], int); 9 void dualSort(int [], double [], int); 10 void showTotals(const double [], const int [], int); 11 12 // NUM_PRODS is the number of products produced. 13 const int NUM_PRODS = 9; 14 15 int main() 16 { 17 // Array with product ID numbers 18 int id[NUM_PRODS] = {914, 915, 916, 917, 918, 919, 920, 19 921, 922}; 20 21 // Array with number of units sold for each product 22 int units[NUM_PRODS] = {842, 416, 127, 514, 437, 269, 97, 23 492, 212}; 24 25 // Array with product prices 26 double prices[NUM_PRODS] = {12.95, 14.95, 18.95, 16.95, 21.95, 27 31.95, 14.95, 14.95, 16.95}; 28 29 // Array to hold the computed sales amounts 30 double sales[NUM_PRODS]; 31 8.4 Focus on Problem Solving and Program Design: A Case Study 483 32 // Calculate each product's sales. 33 calcSales(units, prices, sales, NUM_PRODS); 34 35 // Sort the elements in the sales array in descending 36 // order and shuffle the ID numbers in the id array to 37 // keep them in parallel. 38 dualSort(id, sales, NUM_PRODS); 39 40 // Set the numeric output formatting. 41 cout << setprecision(2) << fixed << showpoint; 42 43 // Display the products and sales amounts. 44 showOrder(sales, id, NUM_PRODS); 45 46 // Display total units sold and total sales. 47 showTotals(sales, units, NUM_PRODS); 48 49 } return 0; 50 51 //**************************************************************** 52 // Definition of calcSales. Accepts units, prices, and sales * 53 // arrays as arguments. The size of these arrays is passed * 54 // into the num parameter. This function calculates each * 55 // product's sales by multiplying its units sold by each unit's * 56 // price. The result is stored in the sales array. * 57 //**************************************************************** 58 59 void calcSales(const int units[], const double prices[], double sales[], int num) 60 { 61 for (int index = 0; index < num; index++) 62 63 } sales[index] = units[index] * prices[index]; 64 65 //*************************************************************** 66 // Definition of function dualSort. Accepts id and sales arrays * 67 // as arguments. The size of these arrays is passed into size. * 68 // This function performs a descending order selection sort on * 69 // the sales array. The elements of the id array are exchanged * 70 // identically as those of the sales array. size is the number * 71 // of elements in each array. * 72 //*************************************************************** 73 74 void dualSort(int id[], double sales[], int size) 75 { 76 int startScan, maxIndex, tempid; 77 double maxValue; 78 79 for (startScan = 0; startScan < (size − 1); startScan++) 80 { 81 maxIndex = startScan; 82 maxValue = sales[startScan]; 83 tempid = id[startScan]; (program continues) 484 Chapter 8 Searching and Sorting Arrays Program 8-6 (continued) 84 for(int index = startScan + 1; index < size; index++) 85 { 86 if (sales[index] > maxValue) 87 { 88 maxValue = sales[index]; 89 tempid = id[index]; 90 maxIndex = index; 91 } 92 } 93 sales[maxIndex] = sales[startScan]; 94 id[maxIndex] = id[startScan]; 95 sales[startScan] = maxValue; 96 id[startScan] = tempid; 97 } 98 } 99 100 //**************************************************************** 101 // Definition of showOrder function. Accepts sales and id arrays * 102 // as arguments. The size of these arrays is passed into num. * 103 // The function first displays a heading, then the sorted list * 104 // of product numbers and sales. * 105 //**************************************************************** 106 107 void showOrder(const double sales[], const int id[], int num) 108 { 109 cout << "Product Number\tSales\n"; 110 cout << "----------------------------------\n"; 111 for (int index = 0; index < num; index++) 112 { 113 cout << id[index] << "\t\t$"; 114 cout << setw(8) << sales[index] << endl; 115 } 116 cout << endl; 117 } 118 119 //***************************************************************** 120 // Definition of showTotals function. Accepts sales and id arrays * 121 // as arguments. The size of these arrays is passed into num. * 122 // The function first calculates the total units (of all * 123 // products) sold and the total sales. It then displays these * 124 // amounts. * 125 //***************************************************************** 126 127 void showTotals(const double sales[], const int units[], int num) 128 { 129 int totalUnits = 0; 130 double totalSales = 0.0; 131 132 for (int index = 0; index < num; index++) 133 { 8.5 If You Plan to Continue in Computer Science: Sorting and Searching vectors 485 134 135 136 137 138 139 } totalUnits += units[index]; totalSales += sales[index]; } cout << "Total Units Sold: " << totalUnits << endl; cout << "Total Sales: $" << totalSales << endl; Program Output Product Number Sales ---------------------------------- 914 $10903.90 918 $ 9592.15 917 $ 8712.30 919 $ 8594.55 921 $ 7355.40 915 $ 6219.20 922 $ 3593.40 916 $ 2406.65 920 $ 1450.15 Total Units Sold: 3406 Total Sales: $58827.70 8.5 If You Plan to Continue in Computer Science: Sorting and Searching vectors (Continued from Section 7.12) CONCEPT: The sorting and searching algorithms you have studied in this chapter may be applied to STL vectors as well as arrays. Once you have properly defined an STL vector and populated it with values, you may sort and search the vector with the algorithms presented in this chapter. Simply substitute the vector syntax for the array syntax when necessary. Program 8-7, which illustrates this, is a modification of the case study in Program 8-6. Program 8-7 1 // This program produces a sales report for DLC, Inc. 2 // This version of the program uses STL vectors instead of arrays. 3 #include 4 #include 5 #include 6 using namespace std; 7 8 // Function prototypes 9 void initVectors(vector &, vector &, vector &); 10 void calcSales(vector, vector, vector &); 11 void showOrder(vector, vector); (program continues) 486 Chapter 8 Searching and Sorting Arrays Program 8-7 (continued) 12 void dualSort(vector &, vector &); 13 void showTotals(vector, vector); 14 15 int main() 16 { 17 vector id; // Product ID numbers 18 vector units; // Units sold 19 vector prices; // Product prices 20 vector sales; // To hold product sales 21 22 // Must provide an initialization routine. 23 initVectors(id, units, prices); 24 25 // Calculate each product's sales. 26 calcSales(units, prices, sales); 27 28 // Sort the elements in the sales array in descending 29 // order and shuffle the ID numbers in the id array to 30 // keep them in parallel. 31 dualSort(id, sales); 32 33 // Set the numeric output formatting. 34 cout << fixed << showpoint << setprecision(2); 35 36 // Display the products and sales amounts. 37 showOrder(sales, id); 38 39 // Display total units sold and total sales. 40 showTotals(sales, units); 41 return 0; 42 } 43 44 //****************************************************************** 45 // Definition of initVectors. Accepts id, units, and prices * 46 // vectors as reference arguments. This function initializes each * 47 // vector to a set of starting values. * 48 //****************************************************************** 49 50 void initVectors(vector &id, vector &units, 51 vector &prices) 52 { 53 // Initialize the id vector with the ID numbers 54 // 914 through 922. 55 for (int value = 914; value <= 922; value++) 56 id.push_back(value); 57 58 // Initialize the units vector with data. 59 units.push_back(842); 60 units.push_back(416); 61 units.push_back(127); 8.5 If You Plan to Continue in Computer Science: Sorting and Searching vectors 487 62 units.push_back(514); 63 units.push_back(437); 64 units.push_back(269); 65 units.push_back(97); 66 units.push_back(492); 67 units.push_back(212); 68 69 // Initialize the prices vector. 70 prices.push_back(12.95); 71 prices.push_back(14.95); 72 prices.push_back(18.95); 73 prices.push_back(16.95); 74 prices.push_back(21.95); 75 prices.push_back(31.95); 76 prices.push_back(14.95); 77 prices.push_back(14.95); 78 79 } prices.push_back(16.95); 80 81 82 //**************************************************************** 83 // Definition of calcSales. Accepts units, prices, and sales * 84 // vectors as arguments. The sales vector is passed into a * 85 // reference parameter. This function calculates each product's * 86 // sales by multiplying its units sold by each unit's price. The * 87 // result is stored in the sales vector. * 88 //**************************************************************** 89 90 void calcSales(vector units, vector prices, 91 vector &sales) 92 { 93 for (int index = 0; index < units.size(); index++) 94 95 } sales.push_back(units[index] * prices[index]); 96 97 //**************************************************************** 98 // Definition of function dualSort. Accepts id and sales vectors * 99 // as reference arguments. This function performs a descending * 100 // order selection sort on the sales vector. The elements of the * 101 // id vector are exchanged identically as those of the sales * 102 // vector. * 103 //**************************************************************** 104 105 void dualSort(vector &id, vector &sales) 106 { 107 int startScan, maxIndex, tempid, size; 108 double maxValue; 109 110 size = id.size(); 111 for (startScan = 0; startScan < (size − 1); startScan++) 112 { 113 maxIndex = startScan; (program continues) 488 Chapter 8 Searching and Sorting Arrays Program 8-7 (continued) 114 maxValue = sales[startScan]; 115 tempid = id[startScan]; 116 for(int index = startScan + 1; index < size; index++) 117 { 118 if (sales[index] > maxValue) 119 { 120 maxValue = sales[index]; 121 tempid = id[index]; 122 maxIndex = index; 123 } 124 } 125 sales[maxIndex] = sales[startScan]; 126 id[maxIndex] = id[startScan]; 127 sales[startScan] = maxValue; 128 id[startScan] = tempid; 129 } 130 } 131 132 //***************************************************************** 133 // Definition of showOrder function. Accepts sales and id vectors * 134 // as arguments. The function first displays a heading, then the * 135 // sorted list of product numbers and sales. * 136 //***************************************************************** 137 138 void showOrder(vector sales, vector id) 139 { 140 cout << "Product Number\tSales\n"; 141 cout << "----------------------------------\n"; 142 for (int index = 0; index < id.size(); index++) 143 { 144 cout << id[index] << "\t\t$"; 145 cout << setw(8) << sales[index] << endl; 146 } 147 cout << endl; 148 } 149 150 //******************************************************************* 151 // Definition of showTotals function. Accepts sales and id vectors * 152 // as arguments. The function first calculates the total units (of * 153 // all products) sold and the total sales. It then displays these * 154 // amounts. * 155 //******************************************************************* 156 157 void showTotals(vector sales, vector units) 158 { 159 int totalUnits = 0; 160 double totalSales = 0.0; 161 162 for (int index = 0; index < units.size(); index++) 163 { 8.5 If You Plan to Continue in Computer Science: Sorting and Searching vectors 489 164 165 166 167 168 169 } totalUnits += units[index]; totalSales += sales[index]; } cout << "Total Units Sold: " << totalUnits << endl; cout << "Total Sales: $" << totalSales << endl; Program Output Product Number Sales ---------------------------------- 914 $10903.90 918 $ 9592.15 917 $ 8712.30 919 $ 8594.55 921 $ 7355.40 915 $ 6219.20 922 $ 3593.40 916 $ 2406.65 920 $ 1450.15 Total Units Sold: 3406 Total Sales: $58827.70 There are some differences between this program and Program 8-6. First, the initVectors function was added. In Program 8-6, this was not necessary because the id, units, and prices arrays had initialization lists. vectors do not accept initialization lists, so this function stores the necessary initial values in the id, units, and prices vectors. Now, look at the function header for initVectors: void initVectors(vector &id, vector &units, vector &prices) Notice that the vector parameters are references (as indicated by the & that precedes the parameter name). This brings up an important difference between vectors and arrays: By default, vectors are passed by value, whereas arrays are only passed by reference. If you want to change a value in a vector argument, it must be passed into a reference parameter. Reference vector parameters are also used in the calcSales and dualSort functions. Also, notice that each time a value is added to a vector, the push_back member function is called. This is because the [] operator cannot be used to store a new element in a vector. It can only be used to store a value in an existing element or read a value from an existing element. The code in this function appears cumbersome because it calls each vector’s push_back member function once for each value that is to be stored in the vector. This code can be simplified by storing the vector initialization values in arrays and then using loops to call the push_back member function, storing the values in the arrays in the vectors. The following code shows an alternative initVectors function that takes this approach. void initVectors(vector &id, vector &units, vector &prices) { const int NUM_PRODS = 9; int count; 490 Chapter 8 Searching and Sorting Arrays int unitsSold[NUM_PRODS] = {842, 416, 127, 514, 437, 269, 97, 492, 212}; double productPrices[NUM_PRODS] = {12.95, 14.95, 18.95, 16.95, 21.95, 31.95, 14.95, 14.95, 16.95}; // Initialize the id vector for (int value = 914; value <= 922; value++) id.push_back(value); // Initialize the units vector for (count = 0; count < NUM_PRODS; count++) units.push_back(unitsSold[count]); // Initialize the prices vector for (count = 0; count < NUM_PRODS; count++) prices.push_back(productPrices[count]); } Next, notice that the calcSales, showOrder, dualSort, and showTotals functions do not accept an argument indicating the number of elements in the vectors. This is not necessary because vectors have the size member function, which returns the number of elements in the vector. The following code segment, which is taken from the calcSales function, shows the units.size() member function being used to control the number of loop iterations. for (int index = 0; index < units.size(); index++) sales.push_back(units[index] * prices[index]); Review Questions and Exercises Short Answer 1. Why is the linear search also called “sequential search”? 2. If a linear search function is searching for a value that is stored in the last element of a 10,000-element array, how many elements will the search code have to read to locate the value? 3. In an average case involving an array of N elements, how many times will a linear search function have to read the array to locate a specific value? 4. A binary search function is searching for a value that is stored in the middle element of an array. How many times will the function read an element in the array before finding the value? 5. What is the maximum number of comparisons that a binary search function will make when searching for a value in a 1,000-element array? 6. Why is the bubble sort inefficient for large arrays? 7. Why is the selection sort more efficient than the bubble sort on large arrays? Fill-in-the-Blank 8. The _________ search algorithm steps sequentially through an array, comparing each item with the search value. Programming Challenges 491 9. The _________ search algorithm repeatedly divides the portion of an array being searched in half. 10. The _________ search algorithm is adequate for small arrays but not large arrays. 11. The _________ search algorithm requires that the array’s contents be sorted. 12. If an array is sorted in _________ order, the values are stored from lowest to highest. 13. If an array is sorted in _________ order, the values are stored from highest to lowest. True or False 14. T F If data are sorted in ascending order, it means they are ordered from lowest value to highest value. 15. T F If data are sorted in descending order, it means they are ordered from lowest value to highest value. 16. T F The average number of comparisons performed by the linear search on an array of N elements is N/2 (assuming the search values are consistently found). 17. T F The maximum number of comparisons performed by the linear search on an array of N elements is N/2 (assuming the search values are consistently found). 18. Complete the following table calculating the average and maximum number of comparisons the linear search will perform, and the maximum number of comparisons the binary search will perform. Array Size ➞ Linear Search (Average Comparisons) Linear Search (Maximum Comparisons) Binary Search (Maximum Comparisons) 50 Elements 500 Elements 10,000 Elements 100,000 10,000,000 Elements Elements Programming Challenges 1. Charge Account Validation Write a program that lets the user enter a charge account number. The program should determine if the number is valid by checking for it in the following list: 5658845 4520125 7895122 8777541 8451277 1302850 8080152 4562555 5552012 5050552 7825877 1250255 1005231 6545231 3852085 7576651 7881200 4581002 The list of numbers above should be initialized in a single-dimensional array. A simple linear search should be used to locate the number entered by the user. If the user enters a number that is in the array, the program should display a message saying that the number is valid. If the user enters a number that is not in the array, the program should display a message indicating that the number is invalid. 492 Chapter 8 Searching and Sorting Arrays VideoNote Solving the Charge Account Validation Modification Problem 2. Lottery Winners A lottery ticket buyer purchases 10 tickets a week, always playing the same 10 5-digit “lucky” combinations. Write a program that initializes an array or a vector with these numbers and then lets the player enter this week’s winning 5-digit number. The program should perform a linear search through the list of the player’s numbers and report whether or not one of the tickets is a winner this week. Here are the numbers: 13579 26791 26792 33445 55555 62483 77777 79422 85647 93121 3. Lottery Winners Modification Modify the program you wrote for Programming Challenge 2 (Lottery Winners) so it performs a binary search instead of a linear search. 4. Charge Account Validation Modification Modify the program you wrote for Problem 1 (Charge Account Validation) so it performs a binary search to locate valid account numbers. Use the selection sort algorithm to sort the array before the binary search is performed. 5. Rainfall Statistics Modification Modify the Rainfall Statistics program you wrote for Programming Challenge 2 of Chapter 7. The program should display a list of months, sorted in order of rainfall, from highest to lowest. 6. String Selection Sort Modify the selectionSort function presented in this chapter so it sorts an array of strings instead of an array of ints. Test the function with a driver program. Use Program 8-8 as a skeleton to complete. Program 8-8 #include #include using namespace std; int main() { const int NUM_NAMES = 20; string names[NUM_NAMES] = {"Collins, Bill", "Smith, Bart", "Allen, Jim", "Griffin, Jim", "Stamey, Marty", "Rose, Geri", "Taylor, Terri", "Johnson, Jill", "Allison, Jeff", "Looney, Joe", "Wolfe, Bill", "James, Jean", "Weaver, Jim", "Pore, Bob", "Rutherford, Greg", "Javens, Renee", "Harrison, Rose", "Setzer, Cathy", "Pike, Gordon", "Holland, Beth" }; // Insert your code to complete this program return 0; } Programming Challenges 493 7. Binary String Search Modify the binarySearch function presented in this chapter so it searches an array of strings instead of an array of ints. Test the function with a driver program. Use Program 8-8 as a skeleton to complete. (The array must be sorted before the binary search will work.) 8. Search Benchmarks Write a program that has an array of at least 20 integers. It should call a function that uses the linear search algorithm to locate one of the values. The function should keep a count of the number of comparisons it makes until it finds the value. The program then should call a function that uses the binary search algorithm to locate the same value. It should also keep count of the number of comparisons it makes. Display these values on the screen. 9. Sorting Benchmarks Write a program that uses two identical arrays of at least 20 integers. It should call a function that uses the bubble sort algorithm to sort one of the arrays in ascending order. The function should keep a count of the number of exchanges it makes. The program then should call a function that uses the selection sort algorithm to sort the other array. It should also keep count of the number of exchanges it makes. Display these values on the screen. 10. Sorting Orders Write a program that uses two identical arrays of just eight integers. It should display the contents of the first array, then call a function to sort the array using an ascending order bubble sort modified to print out the array contents after each pass of the sort. Next, the program should display the contents of the second array, then call a function to sort the array using an ascending order selection sort modified to print out the array contents after each pass of the sort. 11. Using Files—String Selection Sort Modification Modify the program you wrote for Programming Challenge 6 so it reads in 20 strings from a file. The data can be found in the names.txt file. This page intentionally left blank CHAPTER 9 Pointers TOPICS 9.1 Getting the Address of a Variable 9.2 Pointer Variables 9.3 The Relationship Between Arrays and Pointers 9.4 Pointer Arithmetic 9.5 Initializing Pointers 9.6 Comparing Pointers 9.7 Pointers as Function Parameters 9.8 Focus on Software Engineering: Dynamic Memory Allocation 9.9 Focus on Software Engineering: Returning Pointers from Functions 9.10 Using Smart Pointers to Avoid Memory Leaks 9.11 Focus on Problem Solving and Program Design: A Case Study 9.1 Getting the Address of a Variable CONCEPT: The address operator (&) returns the memory address of a variable. Every variable is allocated a section of memory large enough to hold a value of the variable’s data type. On a PC, for instance, it’s common for one byte to be allocated for chars, two bytes for shorts, four bytes for ints, longs, and floats, and eight bytes for doubles. Each byte of memory has a unique address. A variable’s address is the address of the first byte allocated to that variable. Suppose the following variables are defined in a program: char letter; short number; float amount; Figure 9-1 illustrates how they might be arranged in memory and shows their addresses. 495 496 Chapter 9 Pointers Figure 9-1 letter number amount 1200 1201 1203 In Figure 9-1, the variable letter is shown at address 1200, number is at address 1201, and amount is at address 1203. N OTE: The addresses of the variables shown in Figure 9-1 are arbitrary values used only for illustration purposes. Getting the address of a variable is accomplished with an operator in C++. When the address operator (&) is placed in front of a variable name, it returns the address of that variable. Here is an expression that returns the address of the variable amount: &amount And here is a statement that displays the variable’s address on the screen: cout << &amount; N O T E : Do not confuse the address operator with the & symbol used when defining a reference variable. Program 9-1 demonstrates the use of the address operator to display the address, size, and contents of a variable. Program 9-1 1 // This program uses the & operator to determine a variable's 2 // address and the sizeof operator to determine its size. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int x = 25; 9 10 cout << "The address of x is " << &x << endl; 11 cout << "The size of x is " << sizeof(x) << " bytes\n"; 12 cout << "The value in x is " << x << endl; 13 return 0; 14 } Program Output The address of x is 0x8f05 The size of x is 4 bytes The value in x is 25 9.2 Pointer Variables 497 N O T E : The address of the variable x is displayed in hexadecimal. This is the way addresses are normally shown in C++. 9.2 Pointer Variables CONCEPT: Pointer variables, which are often just called pointers, are designed to hold memory addresses. With pointer variables you can indirectly manipulate data stored in other variables. A pointer variable, which often is just called a pointer, is a special variable that holds a memory address. Just as int variables are designed to hold integers, and double variables are designed to hold floating-point numbers, pointer variables are designed to hold memory addresses. Memory addresses identify specific locations in the computer’s memory. Because a pointer variable holds a memory address, it can be used to hold the location of some other piece of data. This should give you a clue as to why it is called a pointer: It “points” to some piece of data that is stored in the computer’s memory. Pointer variables also allow you to work with the data that they point to. We’ve already used memory addresses in this book to work with data. Recall from Chapter 6 that when we pass an array as an argument to a function, we are actually passing the array’s beginning address. For example, suppose we have an array named numbers and we call the showValues function as shown here. const int SIZE = 5; int numbers[SIZE] = { 1, 2, 3, 4, 5 }; showValues(numbers, SIZE); In this code we are passing the name of the array, numbers, and its size as arguments to the showValues function. Here is the definition for the showValues function: void showValues(int values[], int size) { for (int count = 0; count < size; count++) cout << values[count] << endl; } In the function, the values parameter receives the address of the numbers array. It works like a pointer because it “points” to the numbers array, as shown in Figure 9-2. 498 Chapter 9 Pointers Figure 9-2 numbers array 1 2 345 showValues(numbers, SIZE); address 5 void showValues(int values[], int size) { for (int count = 0; count < size; count++) cout << values[count] << endl; } Inside the showValues function, anything that is done to the values parameter is actually done to the numbers array. We can say that the values parameter references the numbers array. Also recall from Chapter 6 that we discussed reference variables. A reference variable acts as an alias for another variable. It is called a reference variable because it references another variable in the program. Anything that you do to the reference variable is actually done to the variable it references. For example, suppose we have the variable jellyDonuts and we pass the variable to the getOrder function, as shown here: int jellyDonuts; getOrder(jellyDonuts); Here is the definition for the getOrder function: void getOrder(int &donuts) { cout << "How many doughnuts do you want? "; cin >> donuts; } In the function, the donuts parameter is a reference variable, and it receives the address of the jellyDonuts variable. It works like a pointer because it “points” to the jellyDonuts variable as shown in Figure 9-3. Inside the getOrder function, the donuts parameter references the jellyDonuts variable. Anything that is done to the donuts parameter is actually done to the jellyDonuts variable. When the user enters a value, the cin statement uses the donuts reference variable to indirectly store the value in the jellyDonuts variable. Notice that the connection between the donuts reference variable and the jellyDonuts argument is automatically established by C++ when the function is called. When you are writing this code, you don’t have go to the trouble of finding the memory address of the jellyDonuts variable and then properly storing that address in the donuts reference Figure 9-3 jellyDonuts variable 9.2 Pointer Variables 499 getOrder(jellyDonuts); address void getOrder(int &donuts) { cout << "How many doughnuts do you want? "; cin >> donuts; } variable. When you are storing a value in the donuts variable, you don’t have to specify that the value should actually be stored in the jellyDonuts variable. C++ handles all of that automatically. In C++, pointer variables are yet another mechanism for using memory addresses to work with pieces of data. Pointer variables are similar to reference variables, but pointer variables operate at a lower level. By this, I mean that C++ does not automatically do as much work for you with pointer variables as it does with reference variables. In order to make a pointer variable reference another item in memory, you have to write code that fetches the memory address of that item and assigns the address to the pointer variable. Also, when you use a pointer variable to store a value in the memory location that the pointer references, your code has to specify that the value should be stored in the location referenced by the pointer variable, and not in the pointer variable itself. Because reference variables are easier to work with, you might be wondering why you would ever use pointers at all. In C++, pointers are useful, and even necessary, for many operations. One such operation is dynamic memory allocation. When you are writing a program that will need to work with an unknown amount of data, dynamic memory allocation allows you to create variables, arrays, and more complex data structures in memory while the program is running. We will discuss dynamic memory allocation in greater detail in this chapter. Pointers are also very useful in algorithms that manipulate arrays and work with certain types of strings. In object-oriented programming, which you will learn about in Chapters 13, 14, and 15, pointers are very useful for creating and working with objects and for sharing access to those objects. Creating and Using Pointer Variables The definition of a pointer variable looks pretty much like any other definition. Here is an example: int *ptr; 500 Chapter 9 Pointers The asterisk in front of the variable name indicates that ptr is a pointer variable. The int data type indicates that ptr can be used to hold the address of an integer variable. The definition statement above would read “ptr is a pointer to an int.” N O T E : In this definition, the word int does not mean that ptr is an integer variable. It means that ptr can hold the address of an integer variable. Remember, pointers only hold one kind of value: an address. Some programmers prefer to define pointers with the asterisk next to the type name, rather than the variable name. For example, the previous definition shown above could be written as: int* ptr; This style of definition might visually reinforce the fact that ptr’s data type is not int, but pointer-to-int. Both definition styles are correct. 11 It is never a good idea to define a pointer variable without initializing it with a valid memory address. If you inadvertently use an uninitialized pointer variable, you will be affecting some unknown location in memory. For that reason, it is a good idea to initialize pointer variables with the special value nullptr. In C++ 11, the nullptr key word was introduced to represent the address 0. So, assigning nullptr to a pointer variable makes the variable point to the address 0. When a pointer is set to the address 0, it is referred to as a null pointer because it points to “nothing.” Here is an example of how you define a pointer variable and initialize it with the value nullptr: int *ptr = nullptr; NOTE: If you are using an older compiler that does not support the C++ 11 standard, you should initialize pointers with the integer 0, or the value NULL. The value NULL is defined in the iostream header file (as well as other header files) to represent the value 0. Program 9-2 demonstrates a very simple usage of a pointer: storing and printing the address of another variable. Program 9-2 1 // This program stores the address of a variable in a pointer. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int x = 25; // int variable 8 int *ptr = nullptr; // Pointer variable, can point to an int 9 10 ptr = &x; // Store the address of x in ptr 11 cout << "The value in x is " << x << endl; 12 cout << "The address of x is " << ptr << endl; 13 return 0; 14 } 9.2 Pointer Variables 501 Program Output The value in x is 25 The address of x is 0x7e00 In Program 9-2, two variables are defined: x and ptr. The variable x is an int, and the variable ptr is a pointer to an int. The variable x is initialized with the value 25, and ptr is initialized with nullptr. Then, the variable ptr is assigned the address of x with the following statement in line 10: ptr = &x; Figure 9-4 illustrates the relationship between ptr and x. Figure 9-4 ptr 0x7e00 x 25 Address of x: 0x7e00 As shown in Figure 9-4, x, which is located at memory address 0x7e00, contains the number 25. ptr contains the address 0x7e00. In essence, it “points” to the variable x. The real benefit of pointers is that they allow you to indirectly access and modify the variable being pointed to. In Program 9-2, for instance, ptr could be used to change the contents of the variable x. This is done with the indirection operator, which is an asterisk (*). When the indirection operator is placed in front of a pointer variable name, it dereferences the pointer. When you are working with a dereferenced pointer, you are actually working with the value the pointer is pointing to. This is demonstrated in Program 9-3. Program 9-3 1 // This program demonstrates the use of the indirection operator. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 int x = 25; // int variable 8 int *ptr = nullptr; // Pointer variable, can point to an int 9 10 ptr = &x; // Store the address of x in ptr 11 12 // Use both x and ptr to display the value in x. 13 cout << "Here is the value in x, printed twice:\n"; 14 cout << x << endl; // Displays the contents of x 15 cout << *ptr << endl; // Displays the contents of x 16 17 // Assign 100 to the location pointed to by ptr. This 18 // will actually assign 100 to x. 19 *ptr = 100; (program continues) 502 Chapter 9 Pointers Program 9-3 (continued) 20 21 22 23 24 25 26 } // Use both x and ptr to display the value in x. cout << "Once again, here is the value in x:\n"; cout << x << endl; // Displays the contents of x cout << *ptr << endl; // Displays the contents of x return 0; Program Output Here is the value in x, printed twice: 25 25 Once again, here is the value in x: 100 100 Take a closer look at the statement in line 10: ptr = &x; This statement assigns the address of the x variable to the ptr variable. Now look at line 15: cout << *ptr << endl; // Displays the contents of x When you apply the indirection operator (*) to a pointer variable, you are working, not with the pointer variable itself, but with the item it points to. Because this statement sends the expression *ptr to the cout object, it does not display the value in ptr, but the value that ptr points to. Since ptr points to the x variable, this statement displays the contents of the x variable. Suppose the statement did not use the indirection operator. Suppose that statement had been written as: cout << ptr << endl; // Displays an address Because the indirection operator is not applied to ptr in this statement, it works directly with the ptr variable. This statement would display the address that is stored in ptr. Now take a look at the following statement, which appears in line 19: *ptr = 100; Notice the indirection operator being used with ptr. That means the statement is not affecting ptr, but the item that ptr points to. This statement assigns 100 to the item ptr points to, which is the x variable. After this statement executes, 100 will be stored in the x variable. Program 9-4 demonstrates that pointers can point to different variables. Program 9-4 1 // This program demonstrates a pointer variable referencing 2 // different variables. 3 #include 4 using namespace std; 9.2 Pointer Variables 503 5 6 int main() 7{ 8 int x = 25, y = 50, z = 75; // Three int variables 9 int *ptr = nullptr; // Pointer variable 10 11 // Display the contents of x, y, and z. 12 cout << "Here are the values of x, y, and z:\n"; 13 cout << x << " " << y << " " << z << endl; 14 15 // Use the pointer to manipulate x, y, and z. 16 17 ptr = &x; // Store the address of x in ptr. 18 *ptr += 100; // Add 100 to the value in x. 19 20 ptr = &y; // Store the address of y in ptr. 21 *ptr += 100; // Add 100 to the value in y. 22 23 ptr = &z; // Store the address of z in ptr. 24 *ptr += 100; // Add 100 to the value in z. 25 26 // Display the contents of x, y, and z. 27 cout << "Once again, here are the values of x, y, and z:\n"; 28 cout << x << " " << y << " " << z << endl; 29 return 0; 30 } Program Output Here are the values of x, y, and z: 25 50 75 Once again, here are the values of x, y, and z: 125 150 175 Take a closer look at the statement in line 17: ptr = &x; This statement assigns the address of the x variable to the ptr variable. Now look at line 18: *ptr += 100; In this statement notice that the indirection operator (*) is used with the ptr variable. When we apply the indirection operator to ptr, we are working, not with ptr, but with the item that ptr points to. When this statement executes, ptr is pointing at x, so the statement in line 18 adds 100 to the contents of x. Then the following statement, in line 20, executes: ptr = &y; This statement assigns the address of the y variable to the ptr variable. After this statement executes, ptr is no longer pointing at x. Rather, it will be pointing at y. The statement in line 21, shown here, adds 100 to the y variable. *ptr += 100; These steps are repeated with the z variable in lines 23 and 24. 504 Chapter 9 Pointers N OTE: So far you’ve seen three different uses of the asterisk in C++: • As the multiplication operator, in statements such as distance = speed * time; • In the definition of a pointer variable, such as int *ptr = nullptr; • As the indirection operator, in statements such as *ptr = 100; 9.3 The Relationship Between Arrays and Pointers CONCEPT: Array names can be used as constant pointers, and pointers can be used as array names. You learned in Chapter 7 that an array name, without brackets and a subscript, actually represents the starting address of the array. This means that an array name is really a pointer. Program 9-5 illustrates this by showing an array name being used with the indirection operator. Program 9-5 1 // This program shows an array name being dereferenced with the * 2 // operator. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 short numbers[] = {10, 20, 30, 40, 50}; 9 10 cout << "The first element of the array is "; 11 cout << *numbers << endl; 12 return 0; 13 } Program Output The first element of the array is 10 Because numbers works like a pointer to the starting address of the array, the first element is retrieved when numbers is dereferenced. So how could the entire contents of an array be retrieved using the indirection operator? Remember, array elements are stored together in memory, as illustrated in Figure 9-5. It makes sense that if numbers is the address of numbers[0], values could be added to numbers to get the addresses of the other elements in the array. It’s important to know, however, that pointers do not work like regular variables when used in mathematical statements. In C++, when you add a value to a pointer, you are actually adding that value times 9.3 The Relationship Between Arrays and Pointers 505 Figure 9-5 numbers[0] numbers[1] numbers[2] numbers[3] numbers[4] numbers the size of the data type being referenced by the pointer. In other words, if you add one to numbers, you are actually adding 1 * sizeof(short) to numbers. If you add two to numbers, the result is numbers + 2 * sizeof(short), and so forth. On a PC, this means the following are true, because short integers typically use two bytes: *(numbers + 1) is actually *(numbers + 1 * 2) *(numbers + 2) is actually *(numbers + 2 * 2) *(numbers + 3) is actually *(numbers + 3 * 2) and so forth. This automatic conversion means that an element in an array can be retrieved by using its subscript or by adding its subscript to a pointer to the array. If the expression *numbers, which is the same as *(numbers + 0), retrieves the first element in the array, then *(numbers + 1) retrieves the second element. Likewise, *(numbers + 2) retrieves the third element, and so forth. Figure 9-6 shows the equivalence of subscript notation and pointer notation. Figure 9-6 numbers[0] numbers[1] numbers[2] numbers[3] numbers[4] *numbers *(numbers+1) *(numbers+2) *(numbers+3) *(numbers+4) N O T E : The parentheses are critical when adding values to pointers. The * operator has precedence over the + operator, so the expression *number + 1 is not equivalent to *(number + 1). *number + 1 adds one to the contents of the first element of the array, while *(number + 1) adds one to the address in number, then dereferences it. Program 9-6 shows the entire contents of the array being accessed, using pointer notation. Program 9-6 1 // This program processes an array using pointer notation. 2 #include 3 using namespace std; 4 (program continues) 506 Chapter 9 Pointers Program 9-6 (continued) 5 int main() 6{ 7 const int SIZE = 5; // Size of the array 8 int numbers[SIZE]; // Array of integers 9 int count; // Counter variable 10 11 // Get values to store in the array. 12 // Use pointer notation instead of subscripts. 13 cout << "Enter " << SIZE << " numbers: "; 14 for (count = 0; count < SIZE; count++) 15 cin >> *(numbers + count); 16 17 // Display the values in the array. 18 // Use pointer notation instead of subscripts. 19 cout << "Here are the numbers you entered:\n"; 20 for (count = 0; count < SIZE; count++) 21 cout << *(numbers + count)<< " "; 22 cout << endl; 23 return 0; 24 } Program Output with Example Input Shown in Bold Enter 5 numbers: 5 10 15 20 25 [Enter] Here are the numbers you entered: 5 10 15 20 25 When working with arrays, remember the following rule: array[index] is equivalent to *(array + index) W ARN IN G ! Remember that C++ performs no bounds checking with arrays. When stepping through an array with a pointer, it’s possible to give the pointer an address outside of the array. To demonstrate just how close the relationship is between array names and pointers, look at Program 9-7. It defines an array of doubles and a double pointer, which is assigned the starting address of the array. Not only is pointer notation then used with the array name, but subscript notation is used with the pointer! Program 9-7 1 // This program uses subscript notation with a pointer variable and 2 // pointer notation with an array name. 3 #include 4 #include 5 using namespace std; 6 9.3 The Relationship Between Arrays and Pointers 507 7 int main() 8{ 9 const int NUM_COINS = 5; 10 double coins[NUM_COINS] = {0.05, 0.1, 0.25, 0.5, 1.0}; 11 double *doublePtr; // Pointer to a double 12 int count; // Array index 13 14 // Assign the address of the coins array to doublePtr. 15 doublePtr = coins; 16 17 // Display the contents of the coins array. Use subscripts 18 // with the pointer! 19 cout << "Here are the values in the coins array:\n"; 20 for (count = 0; count < NUM_COINS; count++) 21 cout << doublePtr[count] << " "; 22 23 // Display the contents of the array again, but this time 24 // use pointer notation with the array name! 25 cout << "\nAnd here they are again:\n"; 26 for (count = 0; count < NUM_COINS; count++) 27 cout << *(coins + count) << " "; 28 cout << endl; 29 return 0; 30 } Program Output Here are the values in the coins array: 0.05 0.1 0.25 0.5 1 And here they are again: 0.05 0.1 0.25 0.5 1 Notice that the address operator is not needed when an array’s address is assigned to a pointer. Because the name of an array is already an address, use of the & operator would be incorrect. You can, however, use the address operator to get the address of an individual element in an array. For instance, &numbers[1] gets the address of numbers[1]. This technique is used in Program 9-8. Program 9-8 1 // This program uses the address of each element in the array. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 const int NUM_COINS = 5; 9 double coins[NUM_COINS] = {0.05, 0.1, 0.25, 0.5, 1.0}; 10 double *doublePtr = nullptr; // Pointer to a double 11 int count; // Array index 12 (program continues) 508 Chapter 9 Pointers Program 9-8 (continued) 13 14 15 16 17 18 19 20 21 22 23 24 25 } // Use the pointer to display the values in the array. cout << "Here are the values in the coins array:\n"; for (count = 0; count < NUM_COINS; count++) { // Get the address of an array element. doublePtr = &coins[count]; // Display the contents of the element. cout << *doublePtr << " "; } cout << endl; return 0; Program Output Here are the values in the coins array: 0.05 0.1 0.25 0.5 1 The only difference between array names and pointer variables is that you cannot change the address an array name points to. For example, consider the following definitions: double readings[20], totals[20]; double *dptr = nullptr; These statements are legal: dptr = readings; // Make dptr point to readings. dptr = totals; // Make dptr point to totals. But these are illegal: readings = totals; // ILLEGAL! Cannot change readings. totals = dptr; // ILLEGAL! Cannot change totals. Array names are pointer constants. You can’t make them point to anything but the array they represent. 9.4 Pointer Arithmetic CONCEPT: Some mathematical operations may be performed on pointers. The contents of pointer variables may be changed with mathematical statements that perform addition or subtraction. This is demonstrated in Program 9-9. The first loop increments the pointer variable, stepping it through each element of the array. The second loop decrements the pointer, stepping it through the array backward. 9.4 Pointer Arithmetic 509 Program 9-9 1 // This program uses a pointer to display the contents of an array. 2 #include 3 using namespace std; 4 5 int main() 6{ 7 const int SIZE = 8; 8 int set[SIZE] = {5, 10, 15, 20, 25, 30, 35, 40}; 9 int *numPtr = nullptr; // Pointer 10 int count; // Counter variable for loops 11 12 // Make numPtr point to the set array. 13 numPtr = set; 14 15 // Use the pointer to display the array contents. 16 cout << "The numbers in set are:\n"; 17 for (count = 0; count < SIZE; count++) 18 { 19 cout << *numPtr << " "; 20 numPtr++; 21 } 22 23 // Display the array contents in reverse order. 24 cout << "\nThe numbers in set backward are:\n"; 25 for (count = 0; count < SIZE; count++) 26 { 27 numPtr−−; 28 cout << *numPtr << " "; 29 } 30 return 0; 31 } Program Output The numbers in set are: 5 10 15 20 25 30 35 40 The numbers in set backward are: 40 35 30 25 20 15 10 5 N O T E : Because numPtr is a pointer to an integer, the increment operator adds the size of one integer to numPtr, so it points to the next element in the array. Likewise, the decrement operator subtracts the size of one integer from the pointer. Not all arithmetic operations may be performed on pointers. For example, you cannot multiply or divide a pointer. The following operations are allowable: • The ++ and −− operators may be used to increment or decrement a pointer variable. • An integer may be added to or subtracted from a pointer variable. This may be per- formed with the + and − operators, or the += and −= operators. • A pointer may be subtracted from another pointer. 510 Chapter 9 Pointers 9.5 Initializing Pointers CONCEPT: Pointers may be initialized with the address of an existing object. Remember that a pointer is designed to point to an object of a specific data type. When a pointer is initialized with an address, it must be the address of an object the pointer can point to. For instance, the following definition of pint is legal because myValue is an integer: int myValue; int *pint = &myValue; The following is also legal because ages is an array of integers: int ages[20]; int *pint = ages; But the following definition of pint is illegal because myFloat is not an int: float myFloat; int *pint = &myFloat; // Illegal! Pointers may be defined in the same statement as other variables of the same type. The following statement defines an integer variable, myValue, and then defines a pointer, pint, which is initialized with the address of myValue: int myValue, *pint = &myValue; And the following statement defines an array, readings, and a pointer, marker, which is initialized with the address of the first element in the array: double readings[50], *marker = readings; Of course, a pointer can only be initialized with the address of an object that has already been defined. The following is illegal because pint is being initialized with the address of an object that does not exist yet: int *pint = &myValue; // Illegal! int myValue; Checkpoint 9.1 Write a statement that displays the address of the variable count. 9.2 Write the definition statement for a variable fltPtr. The variable should be a pointer to a float. 9.3 List three uses of the * symbol in C++. 9.4 What is the output of the following code? int x = 50, y = 60, z = 70; int *ptr = nullptr; cout << x << " " << y << " " << z << endl; ptr = &x; 9.6 Comparing Pointers 511 *ptr *= 10; ptr = &y; *ptr *= 5; ptr = &z; *ptr *= 2; cout << x << " " << y << " " << z << endl; 9.5 Rewrite the following loop so it uses pointer notation (with the indirection operator) instead of subscript notation. for (int x = 0; x < 100; x++) cout << arr[x] << endl; 9.6 Assume ptr is a pointer to an int and holds the address 12000. On a system with 4-byte integers, what address will be in ptr after the following statement? ptr += 10; 9.7 Assume pint is a pointer variable. Is each of the following statements valid or invalid? If any is invalid, why? A) pint++; B) −−pint; C) pint /= 2; D) pint *= 4; E) pint += x; // Assume x is an int. 9.8 Is each of the following definitions valid or invalid? If any is invalid, why? A) int ivar; int *iptr = &ivar; B) int ivar, *iptr = &ivar; C) float fvar; int *iptr = &fvar; D) int nums[50], *iptr = nums; E) int *iptr = &ivar; int ivar; 9.6 Comparing Pointers CONCEPT: If one address comes before another address in memory, the first address is considered “less than” the second. C++’s relational operators may be used to compare pointer values. Pointers may be compared by using any of C++’s relational operators: > < == != >= <= In an array, all the elements are stored in consecutive memory locations, so the address of element 1 is greater than the address of element 0. This is illustrated in Figure 9-7. 512 Chapter 9 Pointers Figure 9-7 An array of five integers arr[0] arr[1] arr[2] arr[3] arr[4] 0x5A00 0x5A04 (Addresses) 0x5A08 0x5A0C 0x5A10 Because the addresses grow larger for each subsequent element in the array, the following if statements are all true: if (&arr[1] > &arr[0]) if (arr < &arr[4]) if (arr == &arr[0]) if (&arr[2] != &arr[3]) N OTE: Comparing two pointers is not the same as comparing the values the two pointers point to. For example, the following if statement compares the addresses stored in the pointer variables ptr1 and ptr2: if (ptr1 < ptr2) The following statement, however, compares the values that ptr1 and ptr2 point to: if (*ptr1 < *ptr2) The capability of comparing addresses gives you another way to be sure a pointer does not go beyond the boundaries of an array. Program 9-10 initializes the pointer nums with the starting address of the array set. The nums pointer is then stepped through the array set until the address it contains is equal to the address of the last element of the array. Then the pointer is stepped backward through the array until it points to the first element. Program 9-10 1 // This program uses a pointer to display the contents 2 // of an integer array. 3 #include 4 using namespace std; 5 6 int main() 7{ 8 int set[8] = {5, 10, 15, 20, 25, 30, 35, 40}; 9 int *nums = set; // Make nums point to set 10 11 // Display the numbers in the array. 12 cout << "The numbers in set are:\n"; 13 cout << *nums << " "; // Display first element 9.7 Pointers as Function Parameters 513 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 } while (nums < &set[7]) { // Advance nums to point to the next element. nums++; // Display the value pointed to by nums. cout << *nums << " "; } // Display the numbers in reverse order. cout << "\nThe numbers in set backward are:\n"; cout << *nums << " "; // Display first element while (nums > set) { // Move backward to the previous element. nums−−; // Display the value pointed to by nums. cout << *nums << " "; } return 0; Program Output The numbers in set are: 5 10 15 20 25 30 35 40 The numbers in set backward are: 40 35 30 25 20 15 10 5 9.7 Pointers as Function Parameters CONCEPT: A pointer can be used as a function parameter. It gives the function access to the original argument, much like a reference parameter does. In Chapter 6 you were introduced to the concept of reference variables being used as function parameters. A reference variable acts as an alias to the original variable used as an argument. This gives the function access to the original argument variable, allowing it to change the variable’s contents. When a variable is passed into a reference parameter, the argument is said to be passed by reference. Another way to pass an argument by reference is to use a pointer variable as the parameter. Admittedly, reference variables are much easier to work with than pointers. Reference variables hide all the “mechanics” of dereferencing and indirection. You should still learn to use pointers as function arguments, however, because some tasks, especially when you are dealing with strings, are best done with pointers.* Also, the C++ library has many functions that use pointers as parameters. * It is also important to learn this technique in case you ever need to write a C program. In C, the only way to pass a variable by reference is to use a pointer. 514 Chapter 9 Pointers Here is the definition of a function that uses a pointer parameter: void doubleValue(int *val) { *val *= 2; } The purpose of this function is to double the variable pointed to by val with the following statement: *val *= 2; When val is dereferenced, the *= operator works on the variable pointed to by val. This statement multiplies the original variable, whose address is stored in val, by two. Of course, when the function is called, the address of the variable that is to be doubled must be used as the argument, not the variable itself. Here is an example of a call to the doubleValue function: doubleValue(&number); This statement uses the address operator (&) to pass the address of number into the val parameter. After the function executes, the contents of number will have been multiplied by two. The use of this function is illustrated in Program 9-11. Program 9-11 1 // This program uses two functions that accept addresses of 2 // variables as arguments. 3 #include 4 using namespace std; 5 6 // Function prototypes 7 void getNumber(int *); 8 void doubleValue(int *); 9 10 int main() 11 { 12 int number; 13 14 // Call getNumber and pass the address of number. 15 getNumber(&number); 16 17 // Call doubleValue and pass the address of number. 18 doubleValue(&number); 19 20 // Display the value in number. 21 cout << "That value doubled is " << number << endl; 22 return 0; 23 } 24 9.7 Pointers as Function Parameters 515 25 //**************************************************************** 26 // Definition of getNumber. The parameter, input, is a pointer. * 27 // This function asks the user for a number. The value entered * 28 // is stored in the variable pointed to by input. * 29 //**************************************************************** 30 31 void getNumber(int *input) 32 { 33 cout << "Enter an integer number: "; 34 cin >> *input; 35 } 36 37 //**************************************************************** 38 // Definition of doubleValue. The parameter, val, is a pointer. * 39 // This function multiplies the variable pointed to by val by * 40 // two. * 41 //**************************************************************** 42 43 void doubleValue(int *val) 44 { 45 *val *= 2; 46 } Program Output with Example Input Shown in Bold Enter an integer number: 10 [Enter] That value doubled is 20 Program 9-11 has two functions that use pointers as parameters. Notice the function prototypes: void getNumber(int *); void doubleValue(int *); Each one uses the notation int * to indicate the parameter is a pointer to an int. As with all other types of parameters, it isn’t necessary to specify the name of the variable in the prototype. The * is required, though. The getNumber function asks the user to enter an integer value. The following cin statement, in line 34, stores the value entered by the user in memory: cin >> *input; The indirection operator causes the value entered by the user to be stored, not in input, but in the variable pointed to by input. WARN IN G ! It’s critical that the indirection operator be used in the statement above. Without it, cin would store the value entered by the user in input, as if the value were an address. If this happens, input will no longer point to the number variable in function main. Subsequent use of the pointer will result in erroneous, if not disastrous, results. 516 Chapter 9 Pointers When the getNumber function is called in line 15, the address of the number variable in function main is passed as the argument. After the function executes, the value entered by the user is stored in number. Next, the doubleValue function is called in line 18, with the address of number passed as the argument. This causes number to be multiplied by two. Pointer variables can also be used to accept array addresses as arguments. Either subscript or pointer notation may then be used to work with the contents of the array. This is demonstrated in Program 9-12. Program 9-12 1 // This program demonstrates that a pointer may be used as a 2 // parameter to accept the address of an array. 3 #include 4 #include 5 using namespace std; 6 7 // Function prototypes 8 void getSales(double *, int); 9 double totalSales(double *, int); 10 11 int main() 12 { 13 const int QTRS = 4; 14 double sales[QTRS]; 15 16 // Get the sales data for all quarters. 17 getSales(sales, QTRS); 18 19 // Set the numeric output formatting. 20 cout << fixed << showpoint << setprecision(2); 21 22 // Display the total sales for the year. 23 cout << "The total sales for the year are $"; 24 cout << totalSales(sales, QTRS) << endl; 25 return 0; 26 } 27 28 //****************************************************************** 29 // Definition of getSales. This function uses a pointer to accept * 30 // the address of an array of doubles. The function asks the user * 31 // to enter sales figures and stores them in the array. * 32 //****************************************************************** 33 void getSales(double *arr, int size) 34 { 35 for (int count = 0; count < size; count++) 36 { 37 cout << "Enter the sales figure for quarter "; 38 cout << (count + 1) << ": "; 39 cin >> arr[count]; 40 } 41 } 42 9.7 Pointers as Function Parameters 517 43 //****************************************************************** 44 // Definition of totalSales. This function uses a pointer to * 45 // accept the address of an array. The function returns the total * 46 // of the elements in the array. * 47 //****************************************************************** 48 double totalSales(double *arr, int size) 49 { 50 double sum = 0.0; 51 52 for (int count = 0; count < size; count++) 53 { 54 sum += *arr; 55 arr++; 56 } 57 return sum; 58 } Program Output with Example Input Shown in Bold Enter the sales figure for quarter 1: 10263.98 [Enter] Enter the sales figure for quarter 2: 12369.69 [Enter] Enter the sales figure for quarter 3: 11542.13 [Enter] Enter the sales figure for quarter 4: 14792.06 [Enter] The total sales for the year are $48967.86 Notice that in the getSales function in Program 9-12, even though the parameter arr is defined as a pointer, subscript notation is used in the cin statement in line 39: cin >> arr[count]; In the totalSales function, arr is used with the indirection operator in line 54: sum += *arr; And in line 55, the address in arr is incremented to point to the next element: arr++; N OTE: The two previous statements could be combined into the following statement: sum += *arr++; The * operator will first dereference arr, then the ++ operator will increment the address in arr. Pointers to Constants You have seen how an item’s address can be passed into a pointer parameter, and how the pointer can be used to modify the item that was passed as an argument. Sometimes it is necessary to pass the address of a const item into a pointer. When this is the case, the pointer must be defined as a pointer to a const item. For example, consider the following array definition: 518 Chapter 9 Pointers const int SIZE = 6; const double payRates[SIZE] = { 18.55, 17.45, 12.85, 14.97, 10.35, 18.89 }; In this code, payRates is an array of const doubles. This means that each element in the array is a const double, and the compiler will not allow us to write code that changes the array’s contents. If we want to pass the payRates array into a pointer parameter, the parameter must be declared as a pointer to const double. The following function shows such an example: void displayPayRates(const double *rates, int size) { // Set numeric output formatting. cout << setprecision(2) << fixed << showpoint; // Display all the pay rates. for (int count = 0; count < size; count++) { cout << “Pay rate for employee ” << (count + 1) << “ is $" << *(rates + count) << endl; } } In the function header, notice that the rates parameter is defined as a pointer to const double. It should be noted that the word const is applied to the thing that rates points to, not rates itself. This is illustrated in Figure 9-8. Figure 9-8 The asterisk indicates that rates is a pointer. const double *rates This is what rates points to. Because rates is a pointer to a const, the compiler will not allow us to write code that changes the thing that rates points to. In passing the address of a constant into a pointer variable, the variable must be defined as a pointer to a constant. If the word const had been left out of the definition of the rates parameter, a compiler error would have resulted. Passing a Nonconstant Argument into a Pointer to a Constant Although a constant’s address can be passed only to a pointer to const, a pointer to const can also receive the address of a nonconstant item. For example, look at Program 9-13. 9.7 Pointers as Function Parameters 519 Program 9-13 1 // This program demonstrates a pointer to const parameter 2 #include 3 using namespace std; 4 5 void displayValues(const int *, int); 6 7 int main() 8{ 9 // Array sizes 10 const int SIZE = 6; 11 12 // Define an array of const ints. 13 const int array1[SIZE] = { 1, 2, 3, 4, 5, 6 }; 14 15 // Define an array of nonconst ints. 16 int array2[SIZE] = { 2, 4, 6, 8, 10, 12 }; 17 18 // Display the contents of the const array. 19 displayValues(array1, SIZE); 20 21 // Display the contents of the nonconst array. 22 displayValues(array2, SIZE); 23 return 0; 24 } 25 26 //*************************************************** 27 // The displayValues function uses a pointer to * 28 // parameter to display the contents of an array. * 29 //*************************************************** 30 31 void displayValues(const int *numbers, int size) 32 { 33 // Display all the values. 34 for (int count = 0; count < size; count++) 35 { 36 cout << *(numbers + count) << " "; 37 } 38 cout << endl; 39 } Program Output 123456 2 4 6 8 10 12 N OTE: When you are writing a function that uses a pointer parameter, and the function is not intended to change the data the parameter points to, it is always a good idea to make the parameter a pointer to const. Not only will this protect you from writing code in the function that accidentally changes the argument, but the function will be able to accept the addresses of both constant and nonconstant arguments. 520 Chapter 9 Pointers Constant Pointers In the previous section we discussed pointers to const. That is, pointers that point to const data. You can also use the const key word to define a constant pointer. Here is the difference between a pointer to const and a const pointer: • A pointer to const points to a constant item. The data that the pointer points to cannot change, but the pointer itself can change. • With a const pointer, it is the pointer itself that is constant. Once the pointer is initialized with an address, it cannot point to anything else. The following code shows an example of a const pointer. int value = 22; int * const ptr = &value; Notice in the definition of ptr the word const appears after the asterisk. This means that ptr is a const pointer. This is illustrated in Figure 9-9. In the code, ptr is initialized with the address of the value variable. Because ptr is a constant pointer, a compiler error will result if we write code that makes ptr point to anything else. An error will not result, however, if we use ptr to change the contents of value. This is because value is not constant, and ptr is not a pointer to const. Figure 9-9 * const indicates that ptr is a constant pointer. int * const ptr This is what ptr points to. Constant pointers must be initialized with a starting value, as shown in the previous example code. If a constant pointer is used as a function parameter, the parameter will be initialized with the address that is passed as an argument into it and cannot be changed to point to anything else while the function is executing. Here is an example that attempts to violate this rule: void setToZero(int * const ptr) { ptr = 0; // ERROR!! Cannot change the contents of ptr. } This function’s parameter, ptr, is a const pointer. It will not compile because we cannot have code in the function that changes the contents of ptr. However, ptr does not point to a const, so we can have code that changes the data that ptr points to. Here is an example of the function that will compile: void setToZero(int * const ptr) { *ptr = 0; } 9.7 Pointers as Function Parameters 521 Although the parameter is const pointer, we can call the function multiple times with different arguments. The following code will successfully pass the addresses of x, y, and z to the setToZero function: int x, y, z; // Set x, y, and z to 0. setToZero(&x); setToZero(&y); setToZero(&z); Constant Pointers to Constants So far, when using const with pointers we’ve seen pointers to constants and we’ve seen constant pointers. You can also have constant pointers to constants. For example, look at the following code: int value = 22; const int * const ptr = &value; In this code ptr is a const pointer to a const int. Notice the word const appears before int, indicating that ptr points to a const int, and it appears after the asterisk, indicating that ptr is a constant pointer. This is illustrated in Figure 9-10. Figure 9-10 * const indicates that ptr is a constant pointer. const int * const ptr This is what ptr points to. In the code, ptr is initialized with the address of value. Because ptr is a const pointer, we cannot write code that makes ptr point to anything else. Because ptr is a pointer to const, we cannot use it to change the contents of value. The following code shows one more example of a const pointer to a const. This is another version of the displayValues function in Program 9-13. void displayValues(const int * const numbers, int size) { // Display all the values. for (int count = 0; count < size; count++) { cout << *(numbers + count) << “ ”; } cout << endl; } In this code, the parameter numbers is a const pointer to a const int. Although we can call the function with different arguments, the function itself cannot change what numbers points to, and it cannot use numbers to change the contents of an argument. 522 Chapter 9 Pointers 9.8 Focus on Software Engineering: Dynamic Memory Allocation CONCEPT: Variables may be created and destroyed while a program is running. As long as you know how many variables you will need during the execution of a program, you can define those variables up front. For example, a program to calculate the area of a rectangle will need three variables: one for the rectangle’s length, one for the rectangle’s width, and one to hold the area. If you are writing a program to compute the payroll for 30 employees, you’ll probably create an array of 30 elements to hold the amount of pay for each person. But what about those times when you don’t know how many variables you need? For instance, suppose you want to write a test-averaging program that will average any number of tests. Obviously the program would be very versatile, but how do you store the individual test scores in memory if you don’t know how many variables to define? Quite simply, you allow the program to create its own variables “on the fly.” This is called dynamic memory allocation and is only possible through the use of pointers. To dynamically allocate memory means that a program, while running, asks the computer to set aside a chunk of unused memory large enough to hold a variable of a specific data type. Let’s say a program needs to create an integer variable. It will make a request to the computer that it allocate enough bytes to store an int. When the computer fills this request, it finds and sets aside a chunk of unused memory large enough for the variable. It then gives the program the starting address of the chunk of memory. The program can only access the newly allocated memory through its address, so a pointer is required to use those bytes. The way a C++ program requests dynamically allocated memory is through the new operator. Assume a program has a pointer to an int defined as int *iptr = nullptr; Here is an example of how this pointer may be used with the new operator: iptr = new int; This statement is requesting that the computer allocate enough memory for a new int variable. The operand of the new operator is the data type of the variable being created. Once the statement executes, iptr will contain the address of the newly allocated memory. This is illustrated in Figure 9-11. A value may be stored in this new variable by dereferencing the pointer: *iptr = 25; Any other operation may be performed on the new variable by simply using the dereferenced pointer. Here are some example statements: cout << *iptr; // Display the contents of the new variable. cin >> *iptr; // Let the user input a value. total += *iptr; // Use the new variable in a computation. Figure 9-11 iptr variable 9.8 Focus on Software Engineering: Dynamic Memory Allocation 523 Pool of unused memory This chunk of memory starts at address 0xA652 VideoNote Dynamically Allocating an Array Although the statements above illustrate the use of the new operator, there’s little purpose in dynamically allocating a single variable. A more practical use of the new operator is to dynamically create an array. Here is an example of how a 100-element array of integers may be allocated: iptr = new int[100]; Once the array is created, the pointer may be used with subscript notation to access it. For instance, the following loop could be used to store the value 1 in each element: for (int count = 0; count < 100; count++) iptr[count] = 1; But what if there isn’t enough free memory to accommodate the request? What if the program asks for a chunk large enough to hold a 100,000-element array of floats, and that much memory isn’t available? When memory cannot be dynamically allocated, C++ throws an exception and terminates the program. Throwing an exception means the program signals that an error has occurred. You will learn more about exceptions in Chapter 16. When a program is finished using a dynamically allocated chunk of memory, it should release it for future use. The delete operator is used to free memory that was allocated with new. Here is an example of how delete is used to free a single variable, pointed to by iptr: delete iptr; If iptr points to a dynamically allocated array, the [] symbol must be placed between delete and iptr: delete [] iptr; 524 Chapter 9 Pointers Failure to release dynamically allocated memory can cause a program to have a memory leak. For example, suppose the following grabMemory function is in a program: void grabMemory() { const int SIZE = 100; // Allocate space for a 100-element // array of integers. int *iptr = new int[SIZE]; // The function ends without deleting the memory! } Notice that the function dynamically allocates a 100-element int array and assigns its address to the iptr pointer variable. But then, the function ends without deleting the memory. Once the function has ended, the iptr pointer variable no longer exists. As a result, the program does not have a pointer to the dynamically allocated memory, therefore, the memory cannot be accessed or deleted. It will sit unused as long as the program is running. W A R N I N G ! Only use pointers with delete that were previously used with new. If you use a pointer with delete that does not reference dynamically allocated memory, unexpected problems could result! Program 9-14 demonstrates the use of new and delete. It asks for sales figures for any number of days. The figures are stored in a dynamically allocated array and then totaled and averaged. Program 9-14 1 // This program totals and averages the sales figures for any 2 // number of days. The figures are stored in a dynamically 3 // allocated array. 4 #include 5 #include 6 using namespace std; 7 8 int main() 9{ 10 double *sales = nullptr, // To dynamically allocate an array 11 total = 0.0, // Accumulator 12 average; // To hold average sales 13 int numDays, // To hold the number of days of sales 14 count; // Counter variable 15 16 // Get the number of days of sales. 17 cout << "How many days of sales figures do you wish "; 18 cout << "to process? "; 19 cin >> numDays; 9.8 Focus on Software Engineering: Dynamic Memory Allocation 525 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 } // Dynamically allocate an array large enough to hold // that many days of sales amounts. sales = new double[numDays]; // Get the sales figures for each day. cout << "Enter the sales figures below.\n"; for (count = 0; count < numDays; count++) { cout << "Day " << (count + 1) << ": "; cin >> sales[count]; } // Calculate the total sales for (count = 0; count < numDays; count++) { total += sales[count]; } // Calculate the average sales per day average = total / numDays; // Display the results cout << fixed << showpoint << setprecision(2); cout << "\n\nTotal Sales: $" << total << endl; cout << "Average Sales: $" << average << endl; // Free dynamically allocated memory delete [] sales; sales = nullptr; // Make sales a null pointer. return 0; Program Output with Example Input Shown in Bold How many days of sales figures do you wish to process? 5 [Enter] Enter the sales figures below. Day 1: 898.63 [Enter] Day 2: 652.32 [Enter] Day 3: 741.85 [Enter] Day 4: 852.96 [Enter] Day 5: 921.37 [Enter] Total Sales: $4067.13 Average Sales: $813.43 The statement in line 23 dynamically allocates memory for an array of doubles, using the value in numDays as the number of elements. The new operator returns the starting address of the chunk of memory, which is assigned to the sales pointer variable. The sales variable is then used throughout the program to store the sales amounts in the array and perform the necessary calculations. In line 48 the delete operator is used to free the allocated memory. 526 Chapter 9 Pointers Notice that in line 49 the value nullptr is assigned to the sales pointer. It is a good practice to set a pointer variable to nullptr after using delete on it. First, it prevents code from inadvertently using the pointer to access the area of memory that was freed. Second, it prevents errors from occurring if delete is accidentally called on the pointer again. The delete operator is designed to have no effect when used on a null pointer. 9.9 Focus on Software Engineering: Returning Pointers from Functions CONCEPT: Functions can return pointers, but you must be sure the item the pointer references still exists. Like any other data type, functions may return pointers. For example, the following function locates the null terminator that appears at the end of a string (such as a string literal) and returns a pointer to it. char *findNull(char *str) { char *ptr = str; while (*ptr != '\0') ptr++; return ptr; } The char * return type in the function header indicates the function returns a pointer to a char: char *findNull(char *str) When writing functions that return pointers, you should take care not to create elusive bugs. For instance, see if you can determine what’s wrong with the following function. string *getFullName() { string fullName[3]; cout << "Enter your first name: "; getline(cin, fullName[0]); cout << "Enter your middle name: "; getline(cin, fullName[1]); cout << "Enter your last name: "; getline(cin, fullName[2]); return fullName; } The problem, of course, is that the function returns a pointer to an array that no longer exists. Because the fullName array is defined locally, it is destroyed when the function terminates. Attempting to use the pointer will result in erroneous and unpredictable results. You should return a pointer from a function only if it is • A pointer to an item that was passed into the function as an argument • A pointer to a dynamically allocated chunk of memory 9.9 Focus on Software Engineering: Returning Pointers from Functions 527 For instance, the following function is acceptable: string *getFullName(string fullName[]) { cout << "Enter your first name: "; getline(cin, fullName[0]); cout << "Enter your middle name: "; getline(cin, fullName[1]); cout << "Enter your last name: "; getline(cin, fullName[2]); return fullName; } This function accepts a pointer to the memory location where the user’s input is to be stored. Because the pointer references a memory location that was valid prior to the function being called, it is safe to return a pointer to the same location. Here is another acceptable function: string *getFullName() { string *fullName = new string[3]; cout << "Enter your first name: "; getline(cin, fullName[0]); cout << "Enter your middle name: "; getline(cin, fullName[1]); cout << "Enter your last name: "; getline(cin, fullName[2]); return fullName; } This function uses the new operator to allocate a section of memory. This memory will remain allocated until the delete operator is used or the program ends, so it’s safe to return a pointer to it. Program 9-15 shows another example. This program uses a function, getRandomNumbers, to get a pointer to an array of random numbers. The function accepts an integer argument that is the number of random numbers in the array. The function dynamically allocates an array, uses the system clock to seed the random number generator, populates the array with random values, and then returns a pointer to the array. Program 9-15 1 // This program demonstrates a function that returns 2 // a pointer. 3 #include 4 #include // For rand and srand 5 #include // For the time function 6 using namespace std; 7 8 // Function prototype 9 int *getRandomNumbers(int); 10 (program continues) 528 Chapter 9 Pointers Program 9-15 (continued) 11 int main() 12 { 13 int *numbers = nullptr; // To point to the numbers 14 15 // Get an array of five random numbers. 16 numbers = getRandomNumbers(5); 17 18 // Display the numbers. 19 for (int count = 0; count < 5; count++) 20 cout << numbers[count] << endl; 21 22 // Free the memory. 23 delete [] numbers; 24 numbers = 0; 25 return 0; 26 } 27 28 //*************************************************** 29 // The getRandomNumbers function returns a pointer * 30 // to an array of random integers. The parameter * 31 // indicates the number of numbers requested. * 32 //*************************************************** 33 34 int *getRandomNumbers(int num) 35 { 36 int *arr = nullptr; // Array to hold the numbers 37 38 // Return a null pointer if num is zero or negative. 39 if (num <= 0) 40 return nullptr; 41 42 // Dynamically allocate the array. 43 arr = new int[num]; 44 45 // Seed the random number generator by passing 46 // the return value of time(0) to srand. 47 srand( time(0) ); 48 49 // Populate the array with random numbers. 50 for (int count = 0; count < num; count++) 51 arr[count] = rand(); 52 53 // Return a pointer to the array. 54 return arr; 55 } Program Output 2712 9656 24493 12483 7633 9.9 Focus on Software Engineering: Returning Pointers from Functions 529 In the Spotlight: Suppose you are developing a program that works with arrays of integers, and you find that you frequently need to duplicate the arrays. Rather than rewriting the array-duplicating code each time you need it, you decide to write a function that accepts an array and its size as arguments, creates a new array that is a copy of the argument array, and returns a pointer to the new array. The function will work as follows: Accept an array and its size as arguments. Dynamically allocate a new array that is the same size as the argument array. Copy the elements of the argument array to the new array. Return a pointer to the new array. Program 9-16 demonstrates the function, which is named duplicateArray. Program 9-16 1 // This program uses a function to duplicate 2 // an int array of any size. 3 #include 4 using namespace std; 5 6 // Function prototype 7 int *duplicateArray(const int *, int); 8 void displayArray(const int[], int); 9 10 int main() 11 { 12 // Define constants for the array sizes. 13 const int SIZE1 = 5, SIZE2 = 7, SIZE3 = 10; 14 15 // Define three arrays of different sizes. 16 int array1[SIZE1] = { 100, 200, 300, 400, 500 }; 17 int array2[SIZE2] = { 10, 20, 30, 40, 50, 60, 70 }; 18 int array3[SIZE3] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; 19 20 // Define three pointers for the duplicate arrays. 21 int *dup1 = nullptr, *dup2 = nullptr, *dup3 = nullptr; 22 23 // Duplicate the arrays. 24 dup1 = duplicateArray(array1, SIZE1); 25 dup2 = duplicateArray(array2, SIZE2); 26 dup3 = duplicateArray(array3, SIZE3); 27 28 // Display the original arrays. 29 cout << "Here are the original array contents:\n"; 30 displayArray(array1, SIZE1); 31 displayArray(array2, SIZE2); 32 displayArray(array3, SIZE3); 33 34 // Display the new arrays. (program continues) 530 Chapter 9 Pointers Program 9-16 (continued) 35 cout << "\nHere are the duplicate arrays: \n"; 36 displayArray(dup1, SIZE1); 37 displayArray(dup2, SIZE2); 38 displayArray(dup3, SIZE3); 39 40 // Free the dynamically allocated memory and 41 // set the pointers to 0. 42 delete [] dup1; 43 delete [] dup2; 44 delete [] dup3; 45 dup1 = nullptr; 46 dup2 = nullptr; 47 dup3 = nullptr; 48 return 0; 49 } 50 //****************************************************** 51 // The duplicateArray function accepts an int array * 52 // and an int that indicates the array's size. The * 53 // function creates a new array that is a duplicate * 54 // of the argument array and returns a pointer to the * 55 // new array. If an invalid size is passed the * 56 // function returns a null pointer. * 57 //****************************************************** 58 59 int *duplicateArray(const int *arr, int size) 60 { 61 int *newArray = nullptr; 62 63 // Validate the size. If 0 or a negative 64 // number was passed, return a null pointer. 65 if (size <= 0) 66 return nullptr; 67 68 // Allocate a new array. 69 newArray = new int[size]; 70 71 // Copy the array's contents to the 72 // new array. 73 for (int index = 0; index < size; index++) 74 newArray[index] = arr[index]; 75 76 // Return a pointer to the new array. 77 return newArray; 78 } 79 80 //************************************************** 81 // The displayArray function accepts an int array * 82 // and its size as arguments and displays the * 83 // contents of the array. * 84 //************************************************** 85 9.9 Focus on Software Engineering: Returning Pointers from Functions 531 86 void displayArray(const int arr[], int size) 87 { 88 for (int index = 0; index < size; index++) 89 cout << arr[index] << " "; 90 cout << endl; 91 } Program Output Here are the original array contents: 100 200 300 400 500 10 20 30 40 50 60 70 1 2 3 4 5 6 7 8 9 10 Here are the duplicate arrays: 100 200 300 400 500 10 20 30 40 50 60 70 1 2 3 4 5 6 7 8 9 10 The duplicateArray function appears in lines 59 through 78. The if statement in lines 65 through 66 validates that size contains a valid array size. If size is 0 or less, the function immediately returns nullptr to indicate that an invalid size was passed. Line 69 allocates a new array and assigns its address to the newArray pointer. Then the loop in lines 73 through 74 copies the elements of the arr parameter to the new array. Then the return statement in line 77 returns a pointer to the new array. Checkpoint 9.9 Assuming arr is an array of ints, will each of the following program segments display “True” or “False”? A) if (arr < &arr[1]) cout << "True"; else cout << "False"; B) if (&arr[4] < &arr[1]) cout << "True"; else cout << "False"; C) if (arr != &arr[2]) cout << "True"; else cout << "False"; D) if (arr != &arr[0]) cout << "True"; else cout << "False"; 9.10 Give an example of the proper way to call the following function: void makeNegative(int *val) { if (*val > 0) 532 Chapter 9 Pointers 9.11 *val = −(*val); } Complete the following program skeleton. When finished, the program will ask the user for a length (in inches), convert that value to centimeters, and display the result. You are to write the function convert. (Note: 1 inch = 2.54 cm. Do not modify function main.) #include #include using namespace std; // Write your function prototype here. int main() { double measurement; 9.12 9.13 9.14 9.15 9.16 9.17 cout << "Enter a length in inches, and I will convert\n"; cout << "it to centimeters: "; cin >> measurement; convert(&measurement); cout << fixed << setprecision(4); cout << "Value in centimeters: " << measurement << endl; return 0; } // // Write the function convert here. // Look at the following array definition: const int numbers[SIZE] = { 18, 17, 12, 14 }; Suppose we want to pass the array to the function processArray in the following manner: processArray(numbers, SIZE); Which of the following function headers is the correct one for the processArray function? A) void processArray(const int *arr, int size) B) void processArray(int * const arr, int size) Assume ip is a pointer to an int. Write a statement that will dynamically allocate an integer variable and store its address in ip. Write a statement that will free the memory allocated in the statement you wrote above. Assume ip is a pointer to an int. Then, write a statement that will dynamically allocate an array of 500 integers and store its address in ip. Write a statement that will free the memory allocated in the statement you just wrote. What is a null pointer? Give an example of a function that correctly returns a pointer. Give an example of a function that incorrectly returns a pointer. 9.10 Using Smart Pointers to Avoid Memory Leaks 533 9.10 Using Smart Pointers to Avoid Memory Leaks CONCEPT: C++ 11 introduces smart pointers, objects that work like pointers, but have the ability to automatically delete dynamically allocated memory that is no longer being used. 11 In C++ 11, you can use smart pointers to dynamically allocate memory and not worry about deleting the memory when you are finished using it. A smart pointer automatically deletes a chunk of dynamically allocated memory when the memory is no longer being used. This helps to prevent memory leaks from occurring. C++ 11 provides three types of smart pointer: unique_ptr, shared_ptr, and weak_ptr. To use any of the smart pointers, you must #include the memory header file with the following directive: #include In this book, we introduce unique_ptr. The syntax for defining a unique_ptr is somewhat different from the syntax used in defining a regular pointer. Here is an example: unique_ptr ptr( new int ); This statement defines a unique_ptr named ptr that points to a dynamically allocated int. Here are some details about the statement: • The notation that appears immediately after unique_ptr indicates that the pointer can point to an int. • The name of the pointer is ptr. • The expression new int that appears inside the parentheses allocates a chunk of memory to hold an int. The address of the chunk of memory will be assigned to the ptr pointer. Figure 9-12 shows the different parts of the definition statement. Figure 9-12 Name of the pointer unique_ptr ptr( new int ); Data type that the pointer will point to Expression that dynamically allocates the memory Once you have defined a unique_ptr, you can use it in the same way as a regular pointer. This is demonstrated in Program 9-17. 534 Chapter 9 Pointers Program 9-17 1 // This program demonstrates a unique_ptr. 2 #include 3 #include 4 using namespace std; 5 6 int main() 7{ 8 // Define a unique_ptr smart pointer, pointing 9 // to a dynamically allocated int. 10 unique_ptr ptr( new int ); 11 12 // Assign 99 to the dynamically allocated int. 13 *ptr = 99; 14 15 // Display the value of the dynamically allocated int. 16 cout << *ptr << endl; 17 return 0; 18 } Program Output 99 In line 3, we have a #include directive for the memory header file. Line 10 defines a unique_ ptr named ptr, pointing to a dynamically allocated int. Line 13 assigns the value 99 to the dynamically allocated int. Notice that the indirection operator (*) is used with the unique_ ptr, just as if it were a regular pointer. Line 16 displays the value stored in the dynamically allocated int, once again using the indirection operator with the unique_ptr. Notice there is no delete statement to free the dynamically allocated memory. It is unnecessary to delete the memory because the smart pointer will automatically delete it as the function comes to an end. Program 9-17 demonstrates a unique_ptr, but it isn’t very practical. Dynamically allocating an array is more useful than allocating a single integer. The following code shows an example of how to use a unique_ptr to dynamically allocate an array of integers. const int SIZE = 100; unique_ptr ptr( new int[SIZE] ); The first statement defines an int constant named SIZE, set to the value 100. The second statement defines a unique_ptr named ptr that points to a dynamically allocated array of 100 ints. Notice the following things about the definition statement: • Following unique_ptr, the notation indicates that the pointer will point to an array of ints. • The expression inside the parentheses, new int[SIZE], allocates an array of ints. The address of the dynamically allocated array of ints will be assigned to the ptr pointer. After the definition statement, you can use the [] operator with subscripts to access the array elements. Here is an example: ptr[0] = 99; cout << ptr[0] << endl; 9.10 Using Smart Pointers to Avoid Memory Leaks 535 The first statement assigns the value 99 to ptr[0], and the second statement displays the value of ptr[0]. Program 9-18 gives a more complete demonstration. Program 9-18 1 // This program demonstrates a unique_ptr pointing 2 // to a dynamically allocated array of integers. 3 #include 4 #include 5 using namespace std; 6 7 int main() 8{ 9 int max; // Max size of the array 10 11 // Get the number of values to store. 12 cout << "How many numbers do you want to enter? "; 13 cin >> max; 14 15 // Define a unique_ptr smart pointer, pointing 16 // to a dynamically allocated array of ints. 17 unique_ptr ptr( new int[max]); 18 19 // Get values for the array. 20 for (int index = 0; index < max; index++) 21 { 22 cout << "Enter an integer number: "; 23 cin >> ptr[index]; 24 } 25 26 // Display the values in the array. 27 cout << "Here are the values you entered:\n"; 28 for (int index = 0; index < max; index++) 29 cout << ptr[index] << endl; 30 31 return 0; 32 } Program Output with Example Input Shown in Bold How many numbers do you want to enter? 5 [Enter] Enter an integer number: 1 [Enter] Enter an integer number: 2 [Enter] Enter an integer number: 3 [Enter] Enter an integer number: 4 [Enter] Enter an integer number: 5 [Enter] Here are the values you entered: 1 2 3 4 5 536 Chapter 9 Pointers 9.11 Focus on Problem Solving and Program Design: A Case Study CONCEPT: This case study demonstrates how an array of pointers can be used to display the contents of a second array in sorted order, without sorting the second array. The United Cause, a charitable relief agency, solicits donations from businesses. The local United Cause office received the following donations from the employees of CK Graphics, Inc.: $5, $100, $5, $25, $10, $5, $25, $5, $5, $100, $10, $15, $10, $5, $10 The donations were received in the order they appear. The United Cause manager has asked you to write a program that displays the donations in ascending order, as well as in their original order. Variables Table 9-1 shows the major variables needed. Table 9-1 Variable NUM_DONATIONS donations arrPtr Description A constant integer initialized with the number of donations received from CK Graphics, Inc. This value will be used in the definition of the program’s arrays. An array of integers containing the donation amounts. An array of pointers to integers. This array has the same number of elements as the donations array. Each element of arrPtr will be initialized to point to an element of the donations array. Programming Strategy In this program the donations array will contain the donations in the order they were received. The elements of the arrPtr array are pointers to integers. They will point to the elements of the donations array, as illustrated in Figure 9-13. Figure 9-13 arrPtr Array [0] [1] [2] [3] [4] [5] [6] donations Array [0] [1] [2] [3] [4] [5] [6] 9.11 Focus on Problem Solving and Program Design: A Case Study 537 The arrPtr array will initially be set up to point to the elements of the donations array in their natural order. In other words, arrPtr[0] will point to donations[0], arrPtr[1] will point to donations[1], and so forth. In that arrangement, the following statement would cause the contents of donations[5] to be displayed: cout << *(arrPtr[5]) << endl; After the arrPtr array is sorted, however, arrPtr[0] will point to the smallest element of donations, arrPtr[1] will point to the next-to-smallest element of donations, and so forth. This is illustrated in Figure 9-14. Figure 9-14 arrPtr Array [0] [1] [2] [3] [4] [5] [6] donations Array [0] [1] [2] [3] [4] [5] [6] This technique gives us access to the elements of the donations array in a sorted order without actually disturbing the contents of the donations array itself. Modules The program will consist of the functions listed in Table 9-2. Table 9-2 Function main arrSelectSort showArray showArrPtr Description The program’s main function. It calls the program’s other functions. Performs an ascending order selection sort on its parameter, arr, which is an array of pointers. Each element of arr points to an element of a second array. After the sort, arr will point to the elements of the second array in ascending order. Displays the contents of its parameter, arr, which is an array of integers. This function is used to display the donations in their original order. Accepts an array of pointers to integers as an argument. Displays the contents of what each element of the array points to. This function is used to display the contents of the donations array in sorted order. Function main In addition to containing the variable definitions, function main sets up the arrPtr array to point to the elements of the donations array. Then the function arrSelectSort is called 538 Chapter 9 Pointers to sort the elements of arrPtr. Last, the functions showArrPtr and showArray are called to display the donations. Here is the pseudocode for main’s executable statements: For count is set to the values 0 through the number of donations Set arrPtr[count] to the address of donations[count]. End For Call arrSelectSort. Call showArrPtr. Call showArray. The arrSelectSort Function The arrSelectSort function is a modified version of the selection sort algorithm shown in Chapter 8. The only difference is that arr is now an array of pointers. Instead of sorting on the contents of arr’s elements, arr is sorted on the contents of what its elements point to. Here is the pseudocode: For startScan is set to the values 0 up to (but not including) the next-to-last subscript in arr Set index variable to startScan. Set minIndex variable to startScan. Set minElem pointer to arr[startScan]. For index variable is set to the values from (startScan + 1) through the last subscript in arr If *(arr[index]) is less than *minElem Set minElem to arr[index]. Set minIndex to index. End If. End For. Set arr[minIndex] to arr[startScan]. Set arr[startScan] to minElem. End For. The showArrPtr Function The showArrPtr function accepts an array of pointers as its argument. It displays the values pointed to by the elements of the array. Here is its pseudocode: For every element in the arr Dereference the element and display what it points to. End For. The showArray Function The showArray function simply displays the contents of arr sequentially. Here is its pseudocode: For every element in arr Display the element's contents End For. 9.11 Focus on Problem Solving and Program Design: A Case Study 539 The Entire Program Program 9-19 shows the entire program’s source code. Program 9-19 1 // This program shows the donations made to the United Cause 2 // by the employees of CK Graphics, Inc. It displays 3 // the donations in order from lowest to highest 4 // and in the original order they were received. 5 #include 6 using namespace std; 7 8 // Function prototypes 9 void arrSelectSort(int *[], int); 10 void showArray(const int [], int); 11 void showArrPtr(int *[], int); 12 13 int main() 14 { 15 const int NUM_DONATIONS = 15; // Number of donations 16 17 // An array containing the donation amounts. 18 int donations[NUM_DONATIONS] = { 5, 100, 5, 25, 10, 19 5, 25, 5, 5, 100, 20 10, 15, 10, 5, 10 }; 21 22 // An array of pointers to int. 23 int *arrPtr[NUM_DONATIONS] = { nullptr, nullptr, nullptr, nullptr, nullptr, 24 nullptr, nullptr, nullptr, nullptr, nullptr, 25 nullptr, nullptr, nullptr, nullptr, nullptr }; 26 27 // Each element of arrPtr is a pointer to int. Make each 28 // element point to an element in the donations array. 29 for (int count = 0; count < NUM_DONATIONS; count++) 30 arrPtr[count] = &donations[count]; 31 32 // Sort the elements of the array of pointers. 33 arrSelectSort(arrPtr, NUM_DONATIONS); 34 35 // Display the donations using the array of pointers. This 36 // will display them in sorted order. 37 cout << "The donations, sorted in ascending order are: \n"; 38 showArrPtr(arrPtr, NUM_DONATIONS); 39 40 // Display the donations in their original order. 41 cout << "The donations, in their original order are: \n"; 42 showArray(donations, NUM_DONATIONS); 43 return 0; 44 } 45 (program continues) 540 Chapter 9 Pointers Program 9-19 (continued) 46 //**************************************************************** 47 // Definition of function arrSelectSort. * 48 // This function performs an ascending order selection sort on * 49 // arr, which is an array of pointers. Each element of array * 50 // points to an element of a second array. After the sort, * 51 // arr will point to the elements of the second array in * 52 // ascending order. * 53 //**************************************************************** 54 55 void arrSelectSort(int *arr[], int size) 56 { 57 int startScan, minIndex; 58 int *minElem; 59 60 for (startScan = 0; startScan < (size - 1); startScan++) 61 { 62 minIndex = startScan; 63 minElem = arr[startScan]; 64 for(int index = startScan + 1; index < size; index++) 65 { 66 if (*(arr[index]) < *minElem) 67 { 68 minElem = arr[index]; 69 minIndex = index; 70 } 71 } 72 arr[minIndex] = arr[startScan]; 73 arr[startScan] = minElem; 74 } 75 } 76 77 //*********************************************************** 78 // Definition of function showArray. * 79 // This function displays the contents of arr. size is the * 80 // number of elements. * 81 //*********************************************************** 82 83 void showArray(const int arr[], int size) 84 { 85 for (int count = 0; count < size; count++) 86 cout << arr[count] << " "; 87 cout << endl; 88 } 89 90 //*************************************************************** 91 // Definition of function showArrPtr. * 92 // This function displays the contents of the array pointed to * 93 // by arr. size is the number of elements. * 94 //*************************************************************** 95 Review Questions and Exercises 541 96 void showArrPtr(int *arr[], int size) 97 { 98 for (int count = 0; count < size; count++) 99 cout << *(arr[count]) << " "; 100 cout << endl; 101 } Program Output The donations, sorted in ascending order, are: 5 5 5 5 5 5 10 10 10 10 15 25 25 100 100 The donations, in their original order, are: 5 100 5 25 10 5 25 5 5 100 10 15 10 5 10 Review Questions and Exercises Short Answer 1. What does the indirection operator do? 2. Look at the following code. int x = 7; int *iptr = &x; What will be displayed if you send the expression *iptr to cout? What happens if you send the expression ptr to cout? 3. So far you have learned three different uses for the * operator. What are they? 4. What math operations are allowed on pointers? 5. Assuming that ptr is a pointer to an int, what happens when you add 4 to ptr? 6. Look at the following array definition. int numbers[] = { 2, 4, 6, 8, 10 }; What will the following statement display? cout << *(numbers + 3) << endl; 7. What is the purpose of the new operator? 8. What happens when a program uses the new operator to allocate a block of memory, but the amount of requested memory isn’t available? How do programs written with older compilers handle this? 9. What is the purpose of the delete operator? 10. Under what circumstances can you successfully return a pointer from a function? 11. What is the difference between a pointer to a constant and a constant pointer? 12. What are two advantages of declaring a pointer parameter as a constant pointer? Fill-in-the-Blank 13. Each byte in memory is assigned a unique __________. 14. The __________ operator can be used to determine a variable’s address. 542 Chapter 9 Pointers 15. __________ variables are designed to hold addresses. 16. The __________ operator can be used to work with the variable a pointer points to. 17. Array names can be used as __________, and vice versa. 18. Creating variables while a program is running is called __________. 19. The __________ operator is used to dynamically allocate memory. 20. U