1DO NOT REINVENT THE WHEEL.COMMON SAYINGC++ COMES WITH LIBRARIES OF...

3.1

Do not reinvent the wheel.Common saying

C++ comes with libraries of predefined functions that you can use in your

programs. There are two kinds of functions in C++: functions that return

(produce) a value and functions that do not return a value. Functions that do

not return a value are called

void functions

. We first discuss functions that

voidfunction

return a value and then discuss

^void

functions.

■ PREDEFINED FUNCTIONS THAT RETURN A VALUE

We will use the

^sqrt

function to illustrate how you use a predefined function

that returns a value. The

^sqrt

function calculates the square root of a number.

Predefined Functions 93

(The square root of a number is that number which when multiplied by itself will pro-

duce the number you started out with. For example, the square root of 9 is 3 because 3

²

is equal to 9.) The function

sqrt

starts with a number, such as 9.0, and computes its

square root, in this case 3.0. The value the function starts out with is called its

argu-argumentment

. The value it computes is called the

value returned.

Some functions may have

value returned

more than one argument, but no function has more than one value returned.

The syntax for using functions in your program is simple. To set a variable named

theRoot

equal to the square root of

^9.0

, you can use the following assignment statement:

theRoot = sqrt(9.0);

The expression

sqrt(9.0)

is known as a

function call

or

function invocation

. An

function call or

argument in a function call can be a constant, such as

9.0

, a variable, or a more compli-

cated expression. A function call is an expression that can be used like any other expres-

invocation

sion. For example, the value returned by

sqrt

is of type

double

; therefore, the following

is legal (although perhaps stingy):

bonus = sqrt(sales)/10;sales

and

^bonus

are variables that would normally be of type

^double

. The function call

sqrt(sales)

is a single item, just as if it were enclosed in parentheses. Thus, the above

assignment statement is equivalent to

bonus = (sqrt(sales))/10;

You can use a function call wherever it is legal to use an expression of the type specified

for the value returned by the function.

Display 3.1 contains a complete program that uses the predefined function

^sqrt

.

The program computes the size of the largest square doghouse that can be built for the

amount of money the user is willing to spend. The program asks the user for an

amount of money and then determines how many square feet of floor space can be pur-

chased for that amount. That calculation yields an area in square feet for the floor of

the doghouse. The function

^sqrt

yields the length of one side of the doghouse floor.

The

cmath

library contains the definition of the function

sqrt

and a number of

#includedirective

other mathematical functions. If your program uses a predefined function from some

library, then it must contain an

include

directive that names that library. For example,

the program in Display 3.1 uses the

sqrt

function and so it contains

#include <cmath>

This particular program has two

^include

directives. It does not matter in what order

you give these two

^include

directives.

^include

directives were discussed in Chapter 1.

Definitions for predefined functions normally place these functions in the

std

namespace and so also require the following

using

directive, as illustrated in Display 3.1:

using namespace std;

94 Function Basics

Display 3.1 A Predefined Function That Returns a Value