Remove unique elements only

Question

There are many resources on how to remove duplicates and similar issues but I can\'t seem to be able to find any on removing unique elements. I\'m using SWI-Prolog but I don

Answer 1

Using tpartition/4 in tandem with if_/3 and (=)/3, we define remove_unique/2 like this:

remove_unique([], []).
remove_unique([E|Xs0], Ys0) :-
   tpartition(=(E), Xs0, Es, Xs),
   if_(Es = [], Ys0 = Ys, append([E|Es], Ys, Ys0)),
   remove_unique(Xs, Ys).

Here's the sample query, as given by the OP:

?- remove_unique([1,2,2,3,4,5,7,6,7], Xs). 
Xs = [2,2,7,7].                       % succeeds deterministically

Answer 2

As long as you don't know that the list is sorted in any way, and you want to keep the sequence of the non-unique elements, it seems to me you can't avoid making two passes: first count occurrences, then pick only repeating elements.

What if you use a (self-balancing?) binary tree for counting occurrences and look-up during the second pass? Definitely not O(n²), at least...