Prolog remove element from list,weird result

Question

So I have this code to remove any element=X from a list.

remove(X,[],L).
remove(X,[X|T],Result):- remove(X,T,Result), !.
remove(X,[H|T],[H|Result]):- remove(X,T,Result).

But when I run it, in the result I get this weird |_G19. I understand this has to do with the [Head|Tail] thingy but I can't figure out a way to get rid of it from my result.

?- remove(2,[1,2,3,2,5,6,2],R).
R = [1, 3, 5, 6|_G19].

So what's wrong with it? Why does this 6|_G19 appears? How do I get rid of it? Everything else is working as intended.

Guillermo Merino · Accepted Answer · 2015-02-25T20:15:00.803

1

Your mistake is on the first clause:

remove(X,[],L).

For you the second parameter is the output, so... in this base case, what is L? that's why you're getting _G19 (a pointer to something).

If you do directly:

remove(X,[],[]).
remove(X,[X|T],Result):- remove(X,T,Result).
remove(X,[H|T],[H|Result]):- remove(X,T,Result).

You should have it

edited Feb 25 '15 at 20:15

answered Feb 25 '15 at 20:06

Guillermo Merino

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No, that doesn't work,because then it just outputs false and nothing else. And that's because it never succeeds without the first clause. – DieCriminal Feb 25 '15 at 20:15
1

Sorry, I made a mistake writting it, it's edited now, it should work, the problem was still the base case – Guillermo Merino Feb 25 '15 at 20:16
Yes now it works. Yea, I didn't realize that technically L was an empty list there too. Thank you. – DieCriminal Feb 25 '15 at 20:22
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`remove(a,[a],[a]).` succeeds. It should fail. Hint: use `dif/2`! – false Feb 25 '15 at 22:12

score 1 · Answer 2 · edited May 23 '17 at 11:43

Simply use the meta-predicate tfilter/3 in combination with reified term inequality dif/3:

remove(E,Xs,Ys) :- tfilter(dif(E),Xs,Ys).

Let's look at the query the OP gave in the question:

?- remove(2,[1,2,3,2,5,6,2],R).
R = [1,3,5,6].                     % succeeds deterministically

As both tfilter/3 and dif/3 are pure, we get sound answers if we ask more general queries:

?- remove(X,[A,B,C],Ys).
Ys = [     ],     X=A ,     X=B ,     X=C  ;
Ys = [    C],     X=A ,     X=B , dif(X,C) ;
Ys = [  B  ],     X=A , dif(X,B),     X=C  ;
Ys = [  B,C],     X=A , dif(X,B), dif(X,C) ;
Ys = [A    ], dif(X,A),     X=B ,     X=C  ;
Ys = [A,  C], dif(X,A),     X=B , dif(X,C) ;
Ys = [A,B  ], dif(X,A), dif(X,B),     X=C  ;
Ys = [A,B,C], dif(X,A), dif(X,B), dif(X,C).

Prolog remove element from list,weird result

2 Answers2