Multiplying lists and list of lists in Prolog

Question

I'm currently working on a Prolog program and having a lot of trouble figuring out how to implement it.

I want to put a list such as [1,2,2,1] into a function. I want it to multiply into itself to make a new matrix by doing this example.

1 * [1,2,2,1] which would yield [1,2,2,1]
2 * [1,2,2,1] which would yield [2,4,4,2]
2 * [1,2,2,1] which would yield [2,4,4,2]
1 * [1,2,2,1] which would yield [1,2,2,1]

And I want it to create all those together in a matrix like:

[[1,2,2,1],[2,4,4,2],[2,4,4,2],[1,2,2,1]].

Last part would be I want to zero out when I multiply by itself. So the 2nd spot would zero out the second spot making the final matrix:

[[0,2,2,1],[2,0,4,2],[2,4,0,2],[1,2,2,0]].

I want to have a predicate that calls another which makes each list. So heres my thoughts:

main(A,O):-
    second(A,A,O).

second([],_,[]).
second([A|As],B,[O|Os]):- %creates the list of lists.
    third(A,B,O),
    second(As,B,Os).

third(_,[],[]).
third(A,[B|Bs],[O|Os]):-
    fourth(A,B,O),
    third(A,Bs,Os). %multiplies single digit by list.
fourth(A,B,0):- A == B.
fourth(A,B,O):- O is A * B.

I am getting the correct matrix but can not get the zero diagonal.

I just cant figure out a correct way to get the matrix with zeros down the diagonal. Any thoughts?

Answer 1

You can do the zeroes by introducing indices that indicate row and column you are at and check for a match:

main(A, O) :-
    second(A, A, 0, O).

second([], _, _, []).
second([A|As], B, R, [O|Os]) :- %creates the list of lists.
    third(A, B, 0, R, O),
    R1 is R + 1,
    second(As, B, R1, Os).

third(_, [], _, _, []).
third(A, [B|Bs], C, R, [O|Os]) :-
    fourth(A, B, C, R, O),
    C1 is C + 1,
    third(A, Bs, C1, R, Os). %multiplies single digit by list.

fourth(_, _, X, X, 0).
fourth(A, B, C, R, O) :- C \== R, O is A * B.

Check:

| ?-  main([1,2,2,1], L).

L = [[0,2,2,1],[2,0,4,2],[2,4,0,2],[1,2,2,0]] ? ;

no

---

Another interesting approach would be to create a maplist_with_index predicate which works just like maplist but manages an index and implicitly assumes the given predicate accepts the index as its first argument:

maplist_with_index(Pred, L, M) :-
    maplist_with_index_(Pred, 0, L, M).
maplist_with_index_(Pred, I, [H|T], [M|Ms]) :-
    Pred =.. [P|Pt],
    append([P,I|Pt], [H], NewPred),
    Call =.. NewPred,
    call(Call, M),
    I1 is I + 1,
    maplist_with_index_(Pred, I1, T, Ms).
maplist_with_index_(_, _, [], []).

Then, the matrix program, using this predicate, looks like:

main(A, O) :-
    second(A, A, O).

second(M, A, O) :-
    maplist_with_index(third(A), M, O).

third(R, A, E, O) :-
    maplist_with_index(fourth(R, E), A, O).

fourth(X, X, _, _, 0).
fourth(C, R, A, B, O) :- C \== R, O is A * B.