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


{ recursive }
:factorial
    dup 1 great (
    [dup 1 - factorial *]
    [drop 1]
    ) if
;
 
{ iterative }
:factorial2
    1 swap [*] for
;
 
( 0 22 ) [
    "Factorial(" print dup print ") = " print factorial2 print nl
] for
 
" " input