How to split list in Prolog?

Question

I want to write a predicate split(List, Pivot, Result) holds when Result is a list of sublists that List divided by Pivot. For example split(['_', '_', '#', '_', '#', '_'], '#', [['_','_'], ['_'], ['_']]) is true.

My code is like this and it doesn't work:

split(List, Pivot, Result) :-
split(List, Pivot, _, _, Result).

split(List, Pivot, Left, Right, X|Xs) :-
    append(Left, [Pivot|Right], List),
    !,
    member(Pivot, Right)
    ->  X = [Left],
        split(Right, Pivot, _, _, Xs)
    ;   X = [[Left]|[Right]].

I don't think my approach is clever either. Can someone give me some advice? Thank you.

score 3 · Answer 1 · edited May 23 '17 at 10:26

3

We can preserve logical-purity and get the job done in no time, simply by using the right tools!

Let's use the meta-predicate splitlistIf/3 and the reified equality predicate (=)/3 in a query:

?- Xs = ['_','_',#,'_',#,'_'], splitlistIf(=(#),Xs,Ys).
Xs = [ '_','_' ,#,'_',#,'_' ]
Ys = [['_','_'], ['_'],['_']].      % succeeds deterministically

edited May 23 '17 at 10:26

Community

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answered May 15 '15 at 00:39

repeat

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score 1 · Accepted Answer · answered Oct 23 '14 at 03:35

Here is one way of doing it:

split(L,P,R):-split(L,P,[],R).
split([],_,[],[]).
split([],_,S,[S]) :- S \= [].
split([P|T],P,[],R) :- split(T,P,[],R).
split([P|T],P,L,[L|R]) :- L \= [], split(T,P,[],R).
split([H|T],P,S,R) :- H \= P, append(S, [H], S2), split(T,P,S2,R).

Demo.

split/4 predicate adds a parameter at position 3 which means "list constructed so far". split/3 is a simple redirection with the "list so far" set to [].

Clauses on lines 2 and 4 handle situations when two Pivots are in a row, and when the pivot is detected at the end of the sequence. These clauses prevent insertion of empty lists.

I'm still trying to understand your answer, but it works! Thanks! — user3928256, Oct 23 '14 at 04:15

How to split list in Prolog?

2 Answers2