Prolog Recursion in lists, last but one element

Question

The question is to find the last but one character in a list, e.g.

?- last_but_one(X, [a,b,c,d]).
X = c.

My code is:

last         


        

Answer 1

Here is another approach using DCGs. I think that this solution is much more "graphical", but it seems quite slow in SICStus:

last_but_one_dcg(L, Ls) :-
   phrase( ( ..., [L,_] ), Ls).

... --> [].
... --> [_], ... .

So we describe how a list must look like such that it has a last-but-one element. It looks like this: Anything (...) in front, and then two elements at the end.

It gets a bit faster by expanding phrase/2. Note that the expansion itself is no longer a conforming program.

last_but_one_dcgx(L, Ls) :-
   ...(Ls, Ls2),
   Ls2 = [L,_].