multidimensional array - Print a List inside a nested List that contains an element

Question

I have the following problem.

I'm given a listOfLists, a value (row,col) and I need to get the list inside a list that contains that certain value, up to my value's index inside that list.

For example

?- find_list([[(1,2),(1,3),(1,4)], [(2,2),(2,3),(2,4)]], (1,3), List2).
List2 = [(1,2),(1,3)].

My problem is that if I use member/2 I will only get true or false for if my value is inside listOfList or not, and not the list that I will need to be working with.

How can I get that list that has my value inside it?

Answer 1

Does it matter that the values are two-dimensional coordinates? Is there an ordering on them that you must respect, or is it simply the ordering of the elements in the list? I will assume the latter.

If you want to split a list at some point, the standard append/3 predicate is usually the way to go. For example, assume we want to cut the list [a, b, c, d, e] into a prefix containing the elements before c and a suffix containing the elements after c. Here is how that is done:

?- append(Prefix, [c | Suffix], [a, b, c, d, e]).
Prefix = [a, b],
Suffix = [d, e] ;
false.

Here c is excluded from the prefix, but that's easy to fix:

?- append(Prefix, [c | Suffix], [a, b, c, d, e]), append(Prefix, [c], UpToAndIncludingC).
Prefix = [a, b],
Suffix = [d, e],
UpToAndIncludingC = [a, b, c] ;
false.

We can give this predicate a nice name:

list_pivot_prefix(List, Pivot, Prefix) :-
    append(Prefix0, [Pivot | _Suffix], List),
    append(Prefix0, [Pivot], Prefix).

And your find_list/3 predicate then simply finds all the lists in the given list of lists for which this relation holds:

find_list(Lists, Element, Prefix) :-
    member(List, Lists),
    list_pivot_prefix(List, Element, Prefix).

Here is your test case:

?- find_list([[(1,2),(1,3),(1,4)],[(2,2),(2,3),(2,4)]],(1,3),List2).
List2 = [ (1, 2), (1, 3)] ;
false.