How can I access an element in a list for prolog?

Question

So I'm having trouble trying to obtain a single element from a list using Prolog without using recursion, only relying on loops, such as foreach, foreachelem, etc....

So I have a list Xs = [1,2,3], and I want to obtain each element individually. The best I've come up so far is this...

sumOfList(Xs, Max) :-
    ( foreach(List, Xs), count(I, 1, _), param(Xs)
         do
            ( foreach(List2, Xs), count(J, 2, _), param(Xs, I, List)
                do
                   ( List =< List2
                     -> Max is List2;
                     Max is List1
                   )
            )
    ).

This is the best I've got so far... So I'm wondering, how do I create some kind of placeholder for Max that can be used for the final calculation?

I suppose you want to learn the loop construct, but if you really just want the max of a list, simply write `Max is max(Xs)`. — jschimpf, Apr 18 '14 at 01:12

Sergii Dymchenko · Answer 1 · 2014-04-17T19:28:55.910

4

Your question is not very clear for me. That's how you can find the max in the list of non-negative elements with ECLiPSe declarative loops:

max_with_loop(Xs, Max) :-
    ( foreach(Xi, Xs), fromto(0, MaxPrev, MaxCurr, Max) do
        MaxCurr is max(MaxPrev, Xi) ).

The interesting part is fromto(0, MaxPrev, MaxCurr, Max). Using this part you can transmit information between iterations. Initially MaxPrev becomes 0, after each iteration MaxCurr becomes MaxPrev, after the loop MaxCurr becomes Max.

BTW, declarative loops in ECLiPSe are kind of syntactic sugar, they are translated to recursive calls by the system.

Update: variant, suggested by @false in the comments, that also works with negative numbers:

max_with_loop([First | Xs], Max) :-
    ( foreach(Xi, Xs), fromto(First, MaxPrev, MaxCurr, Max) do
        MaxCurr is max(MaxPrev, Xi) ).

edited Apr 17 '14 at 19:28

answered Apr 17 '14 at 17:30

Sergii Dymchenko

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`max_with_loop([-2,-1],-1).` fails – false Apr 17 '14 at 18:54
@false Yes, as I wrote, it will work for non-negative elements only. – Sergii Dymchenko Apr 17 '14 at 19:06
Why don't you define it correctly? It isn't even more complex – false Apr 17 '14 at 19:12
@false To work with negative numbers it should be initialized to -Infinity or to the value of the first element. I thinks it adds unnecessary details, my goal was to show how to transmit information between iterations. – Sergii Dymchenko Apr 17 '14 at 19:21
1

HET! `max_with_loop([I|Xs],Max) :- ... fromto(I, ...) ...` – false Apr 17 '14 at 19:23
@false Yes, like I said, initialize with first element. In elegant way, though. I've updated my answer with your variant. – Sergii Dymchenko Apr 17 '14 at 19:30

How can I access an element in a list for prolog?

1 Answers1