Prolog, X before Y in a List

Question

I am having trouble understanding prolog, I have to find if X is before Y in a list.

so I have a base case with an empty list

before(X, Y, [ ]).
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Answer 1

In Prolog, list processing is in most cases done without using indices: you use recursion, and you try to formulate it as a logical expression. If you don't want to use built-ins, you can write:

before(X,Y,[X|T]) :-
    !,
    contains(Y,T).
before(X,Y,[_|T]) :-
    before(X,Y,T).

contains(Y,[Y|_]) :-
    !.
contains(Y,[_|T]) :-
    contains(Y,T).

The code makes use of a defined contains/2 predicate that checks whether the list L contains Y. Now the before/2 predicate contains two clauses. The first clause specifies that the first element of the list is X, in that case, we only need to check whether the remainder of the list contains an Y. In case the first element of the list is not an X, the list is shifted one further, and by using recursion, we try to find a location where there is an X.

Note that this predicate requires that both X and Y are elements of the list. Furthermore there can be multiple X and Ys. So before(a,b,[b,a,a,b]) will succeed, simple because there is an a and a b such that the a is before the b.

EDIT

If you want to use the predicate in a reversed way (query fashion), you should omit the cuts:

before(X,Y,[X|T]) :-
    contains(Y,T).
before(X,Y,[_|T]) :-
    before(X,Y,T).

contains(Y,[Y|_]).
contains(Y,[_|T]) :-
    contains(Y,T).

Then you can query like:

?- before(X,Y,[a,b,c]).
X = a,
Y = b ;
X = a,
Y = c ;
X = b,
Y = c ;
false.