CODEWITHSUNDEEP
recursionprologfactorial
ajuc
ajuc

Reputation: 592

factorial (recursion in general) in prolog - why order of terms matters

Why this factorial implementation doesn't work:

factorial(0, B) :- B is 1.
factorial(A, B) :-
                   A > 0,
                   Ax is A-1,
                   B is A*Bx,
                   factorial(Ax, Bx).

And this works:

factorial2(0, B) :- B is 1.
factorial2(A, B) :-
                   A > 0,
                   Ax is A-1,
                   factorial2(Ax, Bx),
                   B is A*Bx.

Upvotes: 0

Views: 596

Answers (1)

gusbro
gusbro

Reputation: 22585

Because is/2 needs the right hand side to be fully instantiated.

In your first example Bx is not instantiated and is used in the right hand side whereas in your second example it is used after it gets instantiated.

Upvotes: 2

Related Questions