CODEWITHSUNDEEP
prolog
Pravel
Pravel

Reputation: 295

Prolog not returning asked value

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.

Upvotes: 2

Views: 701

Answers (1)

gusbro
gusbro

Reputation: 22585

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.

Upvotes: 4

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