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


HAI 1.3
 
HOW IZ I Faktorial YR Number
  BOTH SAEM 1 AN BIGGR OF Number AN 1 
  O RLY?
   YA RLY
    FOUND YR 1
   NO WAI
    FOUND YR PRODUKT OF Number AN I IZ Faktorial YR DIFFRENCE OF Number AN 1 MKAY
  OIC
IF U SAY SO
 
IM IN YR LOOP UPPIN YR Index WILE DIFFRINT Index AN 13
  VISIBLE Index "! = " I IZ Faktorial YR Index MKAY
IM OUTTA YR LOOP
KTHXBYE