C - Program - 2e - Chapter04.pdf - po ang - Januszek66

Functions

Part I: The Basics

CHAPTER OBJECTIVES

Present the concept of a function—which is a modular block of

code—in the C++ language.

KEY TERMS AND CONCEPTS

arguments

automatic variable

C++ libraries

call-by-reference

call-by-value

called function

calling function

call statement

flags

function

function body

function call

function header line

function declaration or prototype

global variable

input arguments

local variable

return statement

return type

static variable

variable scope

Demonstrate the basic format for any C++ function.

Describe how to write functions and how to pass data

between them.

Introduce the notion of variable scope—how long a variable is

“alive” in a program.

Illustrate how the location of the variable declaration deter-

mines the scope of the variable.

Present many simple program examples to illustrate these basic

function concepts.

KEYWORDS AND OPERATORS

return

void

Little Picture, Big Picture

I n the beginning chapters of this text, we saw functions used three different

ways. First, when we wrote our programs in Chapter 2 and 3, we used the

main function to contain our program code. Next, we included various

libraries or the standard namespace so that we could use their functions, such

as the srand() and rand() when working with random numbers or sqrt() when

we needed the square root of a value. Last, we called functions that belonged

to classes—for example, when we added an element to a vector object, we

used the push_back() function.

In this chapter, we learn the basic mechanics of writing functions. Our

functions here will be standalone blocks of code that are designed to do a

specific task (or tasks). For example, the srand() and rand() functions within the

cstdlib library are standalone functions. In the “old days” of C programming, all

C-based software was constructed with these types of functions. But in the

programming world today, the world revolves around classes and objects. We

have used the string and vector classes by creating our own objects and calling

the class functions. We have also used the pre-defined objects in iostream ( cout

and cin ). Don’t lose sight of our ultimate goal—to become experts at writing our

own classes—and classes contain functions that do the tasks defined in the

class “job description.”

Before we can begin writing our own classes, we need to write standalone

functions. It is not a difficult task, learning to write functions, but you must pay

attention to how the functions are set up. Then the jump from writing these

types of functions to class functions is small. Now is the time to concentrate on

the parts and the pieces, the prototypes and the calls, the input lists and the

return values. Spending time here will pay off when we get to classes, I promise.

4.1

Functions in C++

A function is a complete section of C++ code with a definite start point, an end

point, and its own set of variables. Functions can be passed data values and they

can return data. Whether the function is standalone or part of a class, it should be

designed so that it has one primary task to accomplish.

function

a discrete module or

unit of code that per-

forms specific tasks

179

180

Chapter 4

❚

Functions Part 1: The Basics

Life with Just a main Function

At this stage in your study of C++ programming, you are familiar with data vari-

ables and arithmetic operations, if and switch statements, as well as for , do

while , and while loops. We have also seen how to use the cin , cout and

getline objects from iostream, and have used the string and vector classes. But

our entire program has been contained with the main function. Program 4-1

shows a simple program that asks the user her name and age, and writes the in-

formation to the screen.

Program 4-1

1 //How old are you?

2 //Entire program contained in the main function.

3 #include <iostream>

4 #include <string>

6 using namespace std;

8 int main()

9 {

10 int age;

11 string name;

13 //Write a greeting

14 cout << "\n Hello from a C++ program! \n";

16 //Ask for the user's name

17 cout << "\n What is your name? ";

18 getline(cin,name);

20 //Ask for age

21 cout << "\n How old are you? ";

22 cin >> age;

24 //Write information

25 cout << "\n Hi " << name << "! You are "

<< age << " years old. \n";

28 return 0;

29 }

Output

Hello from a C++ program!

What is your name? Mary Jones

How old are you? 39

Hi Mary Jones! You are 39 years old.

181

Section 4.1

❚

Functions in C++

Life with Functions

Nearly all of our C++ programs are more complicated than asking for a person’s

name and age. As a first look at how to write functions in C++, we’ll re-write this

program using functions. There are four separate tasks in Program 4-1, writing a

greeting, asking for the user’s name, age, followed by writing the information to the

screen. In Program 4-2 these four tasks have been broken out into four separate

blocks of code. These are functions. By breaking the different tasks into functions,

we organize the program into regions that accomplish specific tasks. For now, exam-

ine Program 4-2 and its output. We’ll dissect this program in the following section.

Program 4-2

1 //Simple Functions and How Old Are You?

3 #include <iostream>

4 #include <string>

6 using namespace std;

8 //Function prototypes (or declarations)

9 void WriteHello();

10 string AskForName();

11 int AskForAge();

12 void Write(string name, int age);

14 int main()

15 {

16 int age;

17 string name;

19 //Write a greeting

20 WriteHello();

//call, no inputs, no return value

22 //Ask for the user's name

23 name = AskForName();

//call, returns name

25 //Ask for age

26 age = AskForAge();

//call, value assign into age

28 //Write information

29 Write(name, age);

//call, pass name, age to Write

31 return 0;

32 }

34 //Function definitions follow main

36 //Write_Hello writes a greeting message to the screen

182

Chapter 4

❚

Functions Part 1: The Basics

37 void WriteHello()

38 {

39 cout << "\n Hello from a C++ function!\n";

40 }

42 //AskForAge asks the user for age, returns age.

43 int AskForAge()

44 {

45 int age;

46 cout << "\n How old are you? ";

47 cin >> age;

48 cin.ignore(); //remove enter key

49 return age;

50 }

52 //AskForName asks for the user's name

53 string AskForName()

54 {

55 string name;

56 cout << "\n What is your name? ";

57 getline(cin,name);

58 return name;

59 }

62 //Write writes the name and age to the screen

63 void Write(string name, int age)

64 {

65 cout << "\n Hi " << name << "! You are "

<< age << " years old. \n";

67 }

Output

Hello from a C++ function!

What is your name? Mary Jones

How old are you? 39

Hi Mary Jones! You are 39 years old.

Functions are Good

When writing a program, you should organize the code into logical building blocks.

This may involve laying out classes whose job tasks are then described and defined

with functions. (We’ll learn how to write classes in Chapter 7.) A well-written func-

tion can be reused in many programs. Use descriptive names for functions and

variables, write with an easy-to-read style, and trap for errors you might encounter.

You will then be well on your way to writing clearly understood software.

C programmers concentrated on writing functions. The function is the basic pro-

gramming unit in a C program. We are heading toward designing and writing classes

and using objects. In object-oriented software, the basic programming unit for C++ is

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