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Asked: 2026-05-26T21:27:01+00:00

I have this program in Prolog, it removes elements at each nth element from

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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.

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1 Answer

  1. Editorial Team

    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.

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