Factorial

Here is the definition of N factorial:

N Factorial == N! == N * (N-1) * (N-2) * (N-3) * . . . 4 * 3 * 2 * 1

N must be a positive integer or zero, and 0! is defined to be 1. For example,

 
6! == 6 * 5 * 4 * 3 * 2 * 1 == 720

Let us write a program that computes N! for a value of N. The program checks that N is positive or zero, then computes the factorial.