Question

I’m trying to write a rule in prolog
adjacent(X,Y,Zs), as true if X and Y are adjacent to each other in the list Zs.

I currently have:

append([],L,L).
append([H|T],L,[H|LT]):-append(T,L,LT).
sublist(S,L):-append(_,S,P),append(P,_,L).
adjacent(X,Y,Zs):-sublist([X,Y],Zs).

test:

1 ?- sublist([1,2],[1,2,3,4]).
true .

2 ?- sublist([1,3],[1,2,3,4,5]).
ERROR: Out of global stack
3 ?- 

Do you guys have any ideas? Thanks in advance.

Answer 1

adjacent(X,Y, [X,Y|_]).
adjacent(X,Y, [Y,X|_]). % remove this if you want just Y after X
adjacent(X,Y, [_|T]) :- adjacent(X,Y,T).

that should work.

also, you have a predicate in the lists library called nextto(?X, ?Y, ?List) that will do the same (but keep in mind that the semantics of this predicate is that Y follows X in the list, not just plain adjacent in any order).

http://www.swi-prolog.org/pldoc/doc_for?object=section%282,%27A.12%27,swi%28%27/doc/Manual/lists.html%27%29%29