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I need to find the combinations in a list of lists. For example give the following list,

List = [[1, 2], [1, 2, 3]]

These should be the output,

Comb = [[1,1],[1,2],[1,3],[2,1],[2,2],[2,3]]

Another example:

List = [[1,2],[1,2],[1,2,3]]

Comb = [[1,1,1],[1,1,2],[1,1,3],[1,2,1],[1,2,2],[1,2,3]....etc]

I know how to do it for a list with two sublists but it needs to work for any number of sublists.

I'm new to prolog, please help.

xKalelX
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2 Answers2

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try([],[]).
try([L|Ls],[M|Ms]):-
    member(M,L),
    try(Ls,Ms).

all(L,All) :- findall(M, try(L,M), All).

try returns one list composed of elements of the sublists of the first argument. all finds all such lists.

Alexander Serebrenik
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Here is your answer, cut and dry, as a fellow new prolog programmer myself.

%elementInList(input list, output answer)
answer(ListOfList, AnswerInList) :-
    findall(Ans, combList(ListOfList, Ans), AnswerInList).

%combList(ListOfList, Comb) :-
combList([], []).

combList([Head|Tail], Comb) :-
    combList(Tail, [Element|Tempcomb]),
    elementInList(Head, Element).

%elementInList(List, Element)
elementInList([Head|_], Head).

elementInList([Head|Tail], Element) :-
    elementInList(Tail, Element).

Using the definition answer(InputList, OutputResult), where e.g.

answer([[1,2],[7,8]],Comb).
Comb = [[1, 7], [2, 7], [1, 8], [2, 8]].

and e.g.

answer([[1,2],[2,8],[3,6,9]],Comb).
Comb = [[1, 2, 3], [2, 2, 3], [1, 8, 3], [2, 8, 3], [1, 2, 6], [2, 2, 6], [1, 8|...], [2|...], [...|...]|...].

and so on [X,Y,Z,..] for any number of sublists. You're welcome.

noted
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