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 #/
def factorial
    dup 1 > if
        dup 1 - factorial *
    else
        drop 1
    endif
enddef
 
/# iterative #/
def factorial2
    1 swap for * endfor
enddef
 
0 22 2 tolist for
    "Factorial(" print dup print ") = " print factorial2 print nl
endfor