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


PROCEDURE FactIter(n: CARDINAL): CARDINAL =
  VAR
    result := n;
    counter := n - 1;
 
  BEGIN
    FOR i := counter TO 1 BY -1 DO
      result := result * i;
    END;
    RETURN result;
  END FactIter;