i have this exercise where i get a list of sublists like [[X1,Y1],[X2,Y2]...] which represent an interval (Xi-Yi), and i want to return a list with the biggest interval (it can be more than one interval).
this is what i've got so far. i can't see what im doing wrong but when try to run biggest_interval([[1,2],[5,7],[6,10],[12,15]],L). i get true, followed by a false instead of [6,10]
biggest_interval([H|T],Answer):-
biggest_interval(H,T,-1,Answer).
biggest_interval([],_,_,_).
biggest_interval(_,[],_,_).
biggest_interval([X,Y],[H|T],Biggest,Answer):-
Z is Y-X,
Z =:= Biggest,
append(Answer,[X,Y],L),
!,
biggest_interval(H,T,Biggest,L).
biggest_interval([X,Y],[H|T],Biggest,Answer):-
Z is Y-X,
(
Z > Biggest -> append([],[X,Y],L),
biggest_interval(H,T,Z,L);
true
),
biggest_interval(H,T,Biggest,Answer).
One of the problems with your code is that your predicate biggest_interval/4 does not collect the Answer in the "base case" (it only stops the recursive process).
One possible solution is:
biggest_interval(ListOfLists, Answer) :-
biggest_interval(ListOfLists, -inf, [], Biggest),
reverse(Biggest, Answer). % if order of the pairs is important!
biggest_interval([], _, Answer, Answer) :- !. % collect Answer!
biggest_interval([[X,Y]|Lists], Max, Acc, Answer) :-
Z is Y-X,
( Z = Max -> biggest_interval(Lists, Max, [[X,Y]|Acc], Answer)
; Z > Max -> biggest_interval(Lists, Z, [[X,Y]], Answer)
; biggest_interval(Lists, Max, Acc, Answer) ).
Here are some examples:
?- biggest_interval([[1,2],[5,7],[6,10],[12,15]],L).
L = [[6, 10]].
?- biggest_interval([[1,20],[5,7],[6,10],[12,15]],L).
L = [[1, 20]].
?- biggest_interval([[1,2],[5,7],[6,10],[12,15],[3,10]],L).
L = [[3, 10]].
?- biggest_interval([[1,2],[5,7],[6,10],[12,15],[8,12]],L).
L = [[6, 10], [8, 12]].