Programming in C

160 Chapter 8 Working with Functions

unsigned long int factorial (unsigned int n)

{

unsigned long int result;

if ( n == 0 )

result = 1;

else

result = n * factorial (n - 1);

return result;

}

Program 8.16 Output

0! = 1

1! = 1

2! = 2

3! = 6

4! = 24

5! = 120

6! = 720

7! = 5040

8! = 40320

9! = 362880

10! = 3628800

The fact that the factorialfunction includes a call to itself makes this function recur-

sive.When the function is called to calculate the factorial of 3, the value of the formal

parameter nis set to 3 .Because this value is not zero, the following program statement

result = n * factorial (n - 1);

is executed, which, given the value of n, is evaluated as

result = 3 * factorial (2);

This expression specifies that the factorialfunction is to be called, this time to calcu-

late the factorial of 2.Therefore, the multiplication of 3 by this value is left pending

while factorial (2)is calculated.

Even though you are again calling the same function, you should conceptualize this as

a call to a separate function. Each time any function is called in C—be it recursive or

not—the function gets its own set of local variables and formal parameters with which

to work.Therefore, the local variable resultand the formal parameter nthat exist when

the factorialfunction is called to calculate the factorial of 3 are distinct from the vari-

able resultand the parameter nwhen the function is called to calculate the factorial

of 2.

Program 8.16 Continued