Question

I am trying to do what I think is a ‘de-intersect’ (I’m not sure what the proper name is, but that’s what Tim Sweeney of EpicGames called it in the old UnrealEd)

// foo and bar have some identical elements (given a case-insensitive match) List‹string› foo = GetFoo(); List‹string› bar = GetBar();  // remove non matches foo = foo.Where(x => bar.Contains(x, StringComparer.InvariantCultureIgnoreCase)).ToList(); bar = bar.Where(x => foo.Contains(x, StringComparer.InvariantCultureIgnoreCase)).ToList(); 

Then later on, I do another thing where I subtract the result from the original, to see which elements I removed. That’s super-fast using .Except(), so no troubles there.

There must be a faster way to do this, because this one is pretty bad-performing with ~30,000 elements (of string) in either List. Preferably, a method to do this step and the one later on in one fell swoop would be nice. I tried using .Exists() instead of .Contains(), but it’s slightly slower. I feel a bit thick, but I think it should be possible with some combination of .Except() and .Intersect() and/or .Union().

Answer 1

With intersect it would be done like this:

var matches = ((from f in foo                  select f)               .Intersect(                   from b in bar                    select b, StringComparer.InvariantCultureIgnoreCase))