% From the book
% PROLOG PROGRAMMING IN DEPTH
% by Michael A. Covington, Donald Nute, and Andre Vellino
% (Prentice Hall, 1997).
% Copyright 1997 Prentice-Hall, Inc.
% For educational use only

% File FACTORI2.PL
% Another way to compute factorials

factorial(N,FactN) :- fact_iter(N,FactN,0,1).

fact_iter(N,FactN,N,FactN) :- !.

fact_iter(N,FactN,I,J) :-
  I<N,
  NewI is I+1,
  NewJ is J*NewI,
  fact_iter(N,FactN,NewI,NewJ).