Find max integer in a list in prolog

Question

I am trying to find the max number in a list. I know there are several solutions available online but I feel the best way to learn is to implement on my own.

I wrote

Answer 1

You aren't ensuring that Res is instantiated. You don't neccesary need a helper predicate to do that. You could make the recursive call before the check if Res is bigger than H where Res is the biggest integer of T.

You can use ->, but you don't have to. But if you don't, a little bit more backtracking would be involved.

If you try to stay more on your route with recursion after the check, you'll need a helper predicate, as lurker has suggested.

Edit: Since the answer is accepted now, here are the three suggestions implemented:

max1([H|T], Y):-  % with the -> operator, call first
    max1(T,X),
    (H > X ->
     H = Y;
     Y = X).
max1([X],X).


max2([H|T], Y):-  % without the -> operator, call first (this might be very inefficient)
    max2(T,Y),
    H < Y.
max2([H|T], H):-
    max2(T,Y),
    H >= Y.
max2([X],X).


max3([H|T], Y) :- max_(T,H,Y).            % with helper predicate
max_([H|T],HighestNow,Highest):-          % call after the test
    (H > HighestNow -> max_(T,H, Highest)
     ;
     max_(T,HighestNow,Highest)).
max_([],X,X).