How can i delete every item from list in Prolog

Question

I want to define a relation del_all(X, L, L1) for removing all items X (if any) from list L.

Sample query:

?- del_all(a, [a,b,a,c,d,a], L1).
L1 = [b, c, d].                           % expected answer

You want to delete all *equal* items or all *unifiable* items? Or is your list elements *always* ground, making the distinction mute? — Paulo Moura, May 26 '16 at 13:52

score 2 · Answer 1 · answered May 20 '16 at 07:15

2

Here is another version that combines (=)/2 and dif/2:

list_without([], _, []).
list_without([X|Xs], H, Ys0) :-
   if_(X = H, Ys0 = Ys, Ys0 = [H|Ys]),
   list_without(Xs, H, Ys).

It uses if_/3

answered May 20 '16 at 07:15

false

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score 1 · Answer 2 · edited May 22 '16 at 12:12

1

dele_all(X, [], []).
dele_all(X, [H|T], R) :-
   H = X,
   dele_all(X, T, R).
dele_all(X, [H|T], [H|R]) :-
   dif(H, X),
   dele_all(X, T, R).

edited May 22 '16 at 12:12

repeat

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answered May 19 '16 at 18:35

Mohamed Hani

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score 1 · Answer 3 · edited May 23 '17 at 12:00

1

Partitioning list items according to some given criterion is a common idiom in Prolog.

Using meta-predicate tfilter/3 in combination with dif/3 we can simply query:

?- tfilter(dif(a), [a,b,a,c,d,a], L1).
L1 = [b,c,d].                            % expected result

edited May 23 '17 at 12:00

Community

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answered May 22 '16 at 12:33

repeat

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score 0 · Answer 4 · answered May 26 '16 at 09:42

0

del_all(_,[],[]).
del_all(Item,[Item|Rest],RRest) :-
    !,
    del_all(Item,Rest,RRest).
del_all(Item,[OtherItem|Rest],[OtherItem|RRest]) :-
    del_all(Item,Rest,RRest).

Simple as that.

answered May 26 '16 at 09:42

Zebollo

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How can i delete every item from list in Prolog

4 Answers4