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


U64 Factorial(U64 n) {
  U64 i, result = 1;
  for (i = 1; i <= n; ++i)
    result *= i;
  return result;
}
 
Print("1:  %d\n", Factorial(1));
Print("10: %d\n", Factorial(10));