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So before I start, here is the problem:

I have an example List Xs = [1,2,3] and List Ys = [2,3,4]. The two common elemental members from these two lists are [2,3]. So I'm trying to create a predicate setIntersection(Xs, Ys, Zs), where the resulting list ( [2,3] ), will be bound to Zs, and I'm trying to do this using the Prolog language that can run on the ECLiPSe program on Windows and it must be using the loop iterators, NO RECURSION.

Here's the code that I have so far:

setIntersection(Xs, Ys, Zs) :-
    ( foreach(Alpha, Xs), fromto([Ys], [Head|Tail], Tail, []), foreach(Bravo, Ys)
        do
             ( (Alpha =:= Head)
                   -> Bravo = Head;
                   fail
             )
    ).

Another version of the code I have is this:

setIntersection(Xs, Ys, Zs) :-
    ( foreach(Alpha, Xs), param(Ys, Zs)
        do
             ( foreach(Bravo, Ys), foreach(Charlie, Zs), param(Alpha)
                   do
                         ( ( Alpha =:= Bravo)
                            -> Charlie is Alpha;
                            !
                         )
             )
    ).

THE RESULT (from the second code):

?- setIntersection([1,2,3], [2,3,4], X).
X = [2, 3, _412]
Yes (0.00s cpu)

The results I get is very strange for the code above.

Please help me out with this problem, I would be very grateful. Thank you!

Grzegorz Adam Kowalski
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user3390252
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1 Answers1

2

I assume that you are required to do this using ECLiPse declarative loops, but just in case: are you aware about intersection/3 already implemented in ECLiPSe?

[eclipse]: intersection([1, 2, 3], [2, 3, 4], X).
X = [2, 3]
Yes (0.00s cpu)

And here is one of many ways to do it with loops:

setIntersection(Xs, Ys, Zs) :-
    ( foreach(Xi, Xs), fromto([], ZsPrev, ZsCurr, Zs), param(Ys) do
        ( memberchk(Xi, Ys)  -> 
            ZsCurr = [Xi | ZsPrev]
        ;
            ZsCurr = ZsPrev
        )
    ).

Test run:

[eclipse]: setIntersection([1,2,3], [2,3,4], X).
X = [3, 2]
Yes (0.00s cpu)

Elements are in reversed order compared to what you gave in example, but with sets it should not matter. You can reverse or sort elements in the end, if you want.

BTW, why is "Perl" in the header of the question?

Sergii Dymchenko
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  • Perl is the filename (.pl) that runs on the ECLiPSe program. – user3390252 Apr 29 '14 at 02:30
  • Also, can you explain to me how the fromto function works for lists? I'm confused and I only know how to do it for just regular numbers without a list. – user3390252 Apr 29 '14 at 02:31
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    ".pl" is an extension that is often used for Prolog and Perl. Giving a Prolog file extension ".pl" doesn't make it Perl program. And if your program uses ECLiPSe-specific features, the recommended extension is ".ecl". – Sergii Dymchenko Apr 29 '14 at 03:32
  • `fromto` is used to transport information between iterations. In `fromto([], ZsPrev, ZsCurr, Zs)` 'ZsPrev' becomes '[]' at the very beginning, after each iterations 'ZsPrev' becomes 'ZsCurr'. After the loop 'Zs' becomes 'ZsCurr'. – Sergii Dymchenko Apr 29 '14 at 03:35
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    To build the result list in the correct order, just swap the arguments of the fromto: `fromto(Zs,ZsCurr,ZsPrev,[])` – jschimpf Apr 29 '14 at 10:53
  • @jschimpf This is cool. I tested it and it's as effective for large lists as building as I did and then reverse, i.e. much faster than using `append(ZsPrev, [Xi], ZsCurr)`. – Sergii Dymchenko Apr 29 '14 at 17:36