Question

sort([ [30,100], [10,11] ], X).

gets

X = [[10,11],[30,100]]

How can I sort only by the first index of each sublist?

i.e

X = [[10,100], [30, 11]]

Thanks

Answer 1

The below is my untested code.. there may be one/two cosmetic errors… The input list is split into two based on the head value on the list and the resulted two lists are recursively processed to finally result the sorted output.

sort(Input,Output):-sort(Input,[],Output).

sort([],SortedOut,SortedOut).
sort([[Index1,Index2]|Tail],SortedBig,Out):-
    split(Tail,[Index1,Index2],LessList,BigList),
    !,sort(BigList,SortedBig,NewSort),
    sort(LessList,[[Index1,Index2]|NewSort],Out).

split([],[_D],[],[]).
split([[Index1,Index2]|Rem],[Index21,Index22],[[Index1,Index1]|L1],L2):-
    Index1<Index21,
    !,split(Rem,[Index21,Index22],L1,L2).
split([[Index1,Index2]|Rem],[Index21,Index22],L1,[[Index1,Index1]|L2]):-
    !,split(Rem,[Index21,Index22],L1,L2).

Try this and let me know…