====== Factorial 2 ======
{{tag>math recursion}}
===== Description =====
Factorial 2 way

**Source**:  Adventure in Prolog (on-line tutorial)
===== Download =====
Program source code: {{factorial_2.pl}}
===== Listing =====
<code prolog>
factorial_1(1,1).
factorial_1(N,F):-
  N > 1,
  NN is N - 1,
  factorial_1(NN,FF),
  F is N * FF.

% ?- factorial_1(5,X).
% X = 120

factorial_2(1,F,F).
factorial_2(N,T,F):-
  N > 1,
  TT is N * T,
  NN is N - 1,
  factorial_2(NN,TT,F).

% ?- factorial_2(5,1,X).
% X = 120</code>
===== Comments =====