Prolog not returning asked value

Question

I have this program in Prolog, it removes elements at each nth element from a list, like: removenth([1,2,3,4,5,6], 2, R). it should return : R = [1,3,5].

I have this:

removeallNth(F, N, R):- removeallNth(F, N, 1, R).
removeallNth([], _, _, R).
removeallNth([H|T], N, C, R):- N \== C, Nc is C + 1, concat(R,H,S),  
removeallNth(T, N, Nc, S).
removeallNth([_|T], N, C, R):- N == C, removeallNth(T, N, 1, R).

The problem is that it returns true instead of R = [1,3,5]. I checked in SWI-Prolog debugger and it arrives to the correct result but then it keeps checking stuff. I understand it has to do with unification but I don't know how to apply it.

gusbro gusbro · Accepted Answer · 2011-11-16T14:39:15

Consider this modification to your program:

removeallNth(F, N, R):- removeallNth(F, N, 1, R).

removeallNth([], _, _, []).
removeallNth([H|T], N, C, [H|R]):-
  N \== C, Nc is C + 1,
  removeallNth(T, N, Nc, R).
removeallNth([_|T], N, C, R):-
  N == C,
  removeallNth(T, N, 1, R).

In the first clause of removeallNth/4 you have to return an empty list.

In the second clause of removeallNth/4 you don't need to do the concat, you just have to return add the item in the list returned in the 4th argument of the head of that clause.

Prolog not returning asked value

1 Answers