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


for i = 0 to 100
   print " fctrI(";right$("00";str$(i),2); ") = "; fctrI(i)
   print " fctrR(";right$("00";str$(i),2); ") = "; fctrR(i)
next i
end
 
function fctrI(n)
fctrI = 1
 if n >1 then
  for i = 2 To n
    fctrI = fctrI * i
  next i
 end if
end function
 
function fctrR(n)
fctrR = 1
if n > 1 then fctrR = n * fctrR(n -1)
end function