Library Functions

book : problem solving and program design

 

CHAPTER 3
 Library Functions

·         Predefined Functions and Code Reuse

A primary goal of software engineering is to write error-free code. Code reuse, reusing program fragments that have already been written and tested whenever possible, is one way to accomplish this goal. Stated more simply, “Why reinvent the wheel?”

·         Top-Down Design with Functions

C promotes reuse by providing many predefined functions that can be used to perform mathematical computations. C’s standard math library defines a function named sqrt that performs the square root computation. The function call in the assignment statement

y = sqrt(x);

function call

function

name

argument

activates the code for function sqrt , passing the argument x to the function. You activate a function by writing a function call. After the function executes, the function result is substituted for the function call. If x is 16.0 , the assignment statement

above is evaluated as follows:

1. x is 16.0 , so function sqrt computes the 116.0 or 4.0.

2. The function result, 4.0 , is assigned to y .

A function can be thought of as a “black box” that has passed one or more input values and automatically returns a single output value. Figure 3.6 illustrates this for the call to function sqrt . The value of x ( 16.0 ) is the function input, and the function

result, or output, is 116.0 (result is 4.0 ).

If w is 9.0 , the assignment statement

z = 5.7 + sqrt(w);

is evaluated as follows:

1. w is 9.0 , so function sqrt computes the square root of 9.0 , or 3.0 .

2. The values 5.7 and 3.0 are added together.

3. The sum, 8.7 , is stored in z .