Definitions

   The factorial of   0   (zero)   is defined as being   1   (unity).
   The   Factorial Function   of a positive integer,    n,    is defined as the product of the sequence:
                 n,   n-1,   n-2,   ...   1 



Task

Write a function to return the factorial of a number.

Solutions can be iterative or recursive.

Support for trapping negative    n    errors is optional.



Related task

   Primorial numbers


begin
    integer procedure factorial(n);
    integer n;
    begin
        integer fact, i;
        fact := 1;
        for i := 2 step 1 until n do
            fact := fact * i;
        factorial := fact
    end;
    integer f; outtext("factorials:"); outimage;
    for f := 0, 1, 2, 6, 9 do begin
        outint(f, 2); outint(factorial(f), 8); outimage
    end
end