Prolog - return second to last list element

Question

puzzling over a problem of trying to return the second to last element in a list written in Prolog. This language is interesting to use but I'm having trouble getting my head wrapped around it. Here is what I have:

secondLast([X], X).
secondLast(X, [Y], X) :- secondLast(Y, K).    
secondLast(X, [Y|Z], K) :- secondLast(Y, Z, K).   
secondLast([X|Z], Ans) :- secondLast(X, Z, Ans).

so calling secondLast([a, b, c, d], X). X should equal c.

Any ideas?

Thanks!

Answer 1

you should apply pattern matching:

secondLast([X,_], X).
secondLast([_|T], X) :- secondLast(T, X).

Answer 2

Can be just:

secondLast(L, X) :-
    append(_, [X, _], L).