I am trying to get the longest sublist within a list. I need a rule that recursively searches a list of lists and determines which list has the longest length.
For example:
input: [[1],[1,2],[],[1,2,3,4],[5,6]]
output: [1,2,3,4]
This is what I have so far:
max([H|T], Path, Length) :-
length(H, L),
(L @> Length ->
max(T, H, L) ;
max(T, Path, Length) ).
I would like max() to work like this:
? max([[1],[1,2],[],[1,2,3,4],[5,6]], Path, Distance).
Path = [1,2,3,4]
Distance = 4
When I run a trace, this is the output:
{trace}
| ?- max([[1],[1,2],[],[1,2,3,4],[5,6]], Path, Distance).
1 1 Call: max([[1],[1,2],[],[1,2,3,4],[5,6]],_307,_308) ?
2 2 Call: length([1],_387) ?
2 2 Exit: length([1],1) ?
3 2 Call: 1@>_308 ?
3 2 Exit: 1@>_308 ?
4 2 Call: max([[1,2],[],[1,2,3,4],[5,6]],[1],1) ?
5 3 Call: length([1,2],_462) ?
5 3 Exit: length([1,2],2) ?
6 3 Call: 2@>1 ?
6 3 Exit: 2@>1 ?
7 3 Call: max([[],[1,2,3,4],[5,6]],[1,2],2) ?
8 4 Call: length([],_537) ?
8 4 Exit: length([],0) ?
9 4 Call: 0@>2 ?
9 4 Fail: 0@>2 ?
9 4 Call: max([[1,2,3,4],[5,6]],[1,2],2) ?
10 5 Call: length([1,2,3,4],_587) ?
10 5 Exit: length([1,2,3,4],4) ?
11 5 Call: 4@>2 ?
11 5 Exit: 4@>2 ?
12 5 Call: max([[5,6]],[1,2,3,4],4) ?
13 6 Call: length([5,6],_662) ?
13 6 Exit: length([5,6],2) ?
14 6 Call: 2@>4 ?
14 6 Fail: 2@>4 ?
14 6 Call: max([],[1,2,3,4],4) ?
14 6 Fail: max([],[1,2,3,4],4) ?
12 5 Fail: max([[5,6]],[1,2,3,4],4) ?
9 4 Fail: max([[1,2,3,4],[5,6]],[1,2],2) ?
7 3 Fail: max([[],[1,2,3,4],[5,6]],[1,2],2) ?
4 2 Fail: max([[1,2],[],[1,2,3,4],[5,6]],[1],1) ?
1 1 Fail: max([[1],[1,2],[],[1,2,3,4],[5,6]],_307,_308) ?
(2 ms) no
I believe the issue is that I am not handling the occurrence of an empty set "[]". However, I have attempted several different methods and am unable to get my desired output.
You should define loop ending clause and need one more parameter for returning.
max([], _, Length, Length).
max([H|T], Path, Length, RetLength) :-
length(H, L),
( L @> Length ->
max(T, H, L,RetLength) ;
max(T, Path, Length,RetLength)
).
Test:
?- max([[1],[1,2],[],[1,2,3,4],[5,6]], Path, Distance,Len).
Len = 4.