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

I would say both answers are just as good and I would probably have written it the way you did. What they do in the second solution is that they check, before the recursive call, that the second element is not a empty list ([]). If you trace the two different solutions on the following query: last_but_one(X,[b]).

You'll see that both give the same answer (false), but the second solution takes shorter amount of steps since it returns false before the recursive call is made.