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


def factorial( n ) =
  if n < 0
    error( 'factorial: n should be non-negative' )
  else
    res = 1
 
    for i <- 2..n
      res *= i
 
    res