prolog list processing (SIMPLE) 1

[toneydiscount]

Hello,
I am trying to become familiar with prolog and I am trying to do a simple compare of two lists.

For example, let's say that I provide two prolog facts:
list1([a,b,c]).
list2([w,x,y,z]).

Now how would I compare the two, I know there is length function but not sure how to use it.
Is it something like this?

Longer(List1, List2) :- length(List1, N) > length(List2, M).

So in this case it should return no becasue list2 is longer. Am I even close?

 

[joel76]

Prolog use predicates, not functions.
you can write longer (notice the small letter "l") like that

longer(L1, L2) :-
  length(L1, N1),
  length(L2, N2),
  N1 > N2.

But, if you use length to compare these lists, you will travel through these lists two times, but it can be done with only one travel :

% empty lists have the same longer so longer fails.
longer([], []) :- !, fail.

% when the first list is empty and not the second longer fails
longer([], _) :- ! fail.

% when the second list is empty, but not th first, longer succeeds.
longer(_, []) :- !.

% In the general case, we have just to test the lists without theire first element :
longer([_|T1], [_|T2]) :-
  longer(T1, T2).

The cut is necessary to stop backtraking.
You can see that we travel only one time through the lists, the program ends when the shortest list is finished.