In SWI-Prolog there is the predicate sort/2 to sort lists. Is there a good way to sort a list of list with fixed length by some index. I mean in example, if I have the following list of elements
[[1,2], [3,1], [2, 5]]Is there a function in SWI-Prolog to sort it by first or second index. By first index the result would be:
[[1,2], [2,5], [3, 1]]By second index:
[[3,1], [1,2], [2, 5]]Can use https://www.swi-prolog.org/pldoc/doc_for?object=predsort/3 if removing duplicates is OK:
test(Nth1, S) :-
L = [[1,2], [3,1], [2, 5], [1,2]],
predsort(nth1_compare(Nth1), L, S).
nth1_compare(Nth1, Comp, L1, L2) :-
nth1(Nth1, L1, V1),
nth1(Nth1, L2, V2),
compare(Comp, V1, V2).
Results:
?- test(1, S).
S = [[1,2],[2,5],[3,1]].
?- test(2, S).
S = [[3,1],[1,2],[2,5]].