Writing a Function

As an example, we’ll write a function to determine the factorial of a given number. The factorial of a number n is the product of the numbers from 1 through n. The factorial of 5, for example, is 120.

1 * 2 * 3 * 4 * 5 = 120

We might define this function as follows:

// factorial of val is val * (val - 1) * (val - 2) . . . * ((val - (val - 1)) * 1)
int fact(int val)
{
    int ret = 1; // local variable to hold the result as we calculate it
    while (val > 1)
        ret *= val--;  // assign ret * val to ret and decrement val
    return ret;        // return the result
}

Our function is named fact. It takes one int parameter and returns an int value. Inside the while loop, we compute the factorial using the postfix decrement operator (§ 4.5, p. 147) to reduce the value of val by 1 on each iteration. The return statement ends execution of fact and returns the value of ret.