Removing one lists contents from another...

[Cetra]

I've looked through the forum but this problem seems unique. I'm trying to remove the contents of one list from another. For instance:

removed([1,2,3],[2],X).
should give the result X = [1,3]?

So far the code I have come up with is as follows:
removed(L,[],L).
removed([H|T],[H|S],T) :- removed(T,S,T).

This works only for the first element in the list, and I think I need another remove definition to traverse through elements whose heads are not the same, but that part I can't figure out.

 

[Cetra]

I got a little bit further in this program. My source code is as follows:

remove(L,[],L).
remove([H|T],[H|S],Z) :- remove(T,S,Z).
remove([R|T],[Q|S],[R|Z]) :- remove(T,[Q|S],Z).

This works in a few cases such as:
-? remove([1,2,3,1],[1,1],X).

giving the result of X = [2,3]

However the result I'm looking for is to be able to take out multiple like terms in one list, if they are listed in the other. Anyone have any input?

 

[joel76]

I think it should be like that

% remove(L1, L2, LR)
% L1 list to work
% L2 list of elements to remove
% LR list result

% when L2 if empty, it's done
remove(L, [], L).

% you work element by element
remove(L, [H | T], LR) :-
  remove_one(L, H, L1),
  remove(L1, T, LR).

You just have to write remove_one.