Remove array elements that appear more than once

Question

Array.Distinct() leaves distinct elements. How can I remove all duplicate elements from Array including original one?

For example, given this input array:

{ Dog, Cat, Mouse, Dog, Dog, Parrot, Mouse, Hound }

I'd like to get this output:

{ Cat, Parrot, Hound }

I couldn't find answer to this question on Stackoverflow & google, so why so many downvotes? — VixinG, Jul 09 '14 at 22:35
You seem to have three answers, so it's probably me, but I don't understand what you mean. You want Dog removed? (I did not downvote btw.) — Gerben Rampaart, Jul 09 '14 at 22:37

score 10 · Accepted Answer · edited Jul 09 '14 at 23:23

You can use a bit of Linq:

var singles = myArray
    .GroupBy(x => x)
    .Where(g => g.Count() == 1)
    .SelectMany(g => g);

Or like this:

var singles = myArray
    .GroupBy(x => x)
    .Where(g => !g.Skip(1).Any())
    .SelectMany(g => g);

The GroupBy method will collect all similar elements together.
The Where method will filter out groups that do not meet a certain condition. I've provided two alternatives conditions, but they both do the same thing—they return true only if the group contains exactly one element:
- g.Count() == 1 counts the number of elements in each group and returns true if the result is equal to 1.
- !g.Skip(1).Any() returns true if there are no elements after the first. This is actually marginally more efficient than g.Count() == 1 because it doesn't require enumerating all elements in the group, but it's a bit confusing to some readers, so think twice before introducing this in production code.
Finally, SelectMany will return all elements from in the result collection. Since there is only one element in each group, you're left with only those elements which are not duplicated.

@Selman22 OP doesn't want just the distinct elements, but the elements which do not appear more than once in the input array. — p.s.w.g, Jul 09 '14 at 22:43

Christos · Answer 2 · 2014-07-09T22:37:07.643

3

You could try something like this:

Array.GroupBy(x => x).Where(x => x.Count() == 1).SelectMany(x => x);

edited Jul 09 '14 at 22:37

answered Jul 09 '14 at 22:30

Christos

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Looks simple enough to do it's job. Thanks – VixinG Jul 09 '14 at 22:37

score 2 · Answer 3 · answered Jul 09 '14 at 22:53

Using Group by clause you can get the result.

var list = new List<string>  { "Dog", "Cat", "Mouse", "Dog", "Dog", "Parrot", "Mouse", "Hound" };


        //var list = new List<string> { "a", "b", "a", "c", "a", "b" };
        var q = (from x in list
                 group x by x into g
                 let count = g.Count()
                 where count == 1
                 orderby count descending
                 select g.Key); 
        foreach (var x in q)
        {
            Console.WriteLine(x);
        }

score 1 · Answer 4 · answered Jul 09 '14 at 22:30

1

myList.Where(i => myList.Count(j => j == i) < 2);

Note that this relies on the == operator being a good enough comparison (though you could replace it with whatever comparison you want). It also isn't very efficient, though I doubt any of the other answers will be either.

This works by restricting the result to elements that do not appear more than once in the source list.

answered Jul 09 '14 at 22:30

BradleyDotNET

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Whereas this will work, it will take a very long time on larger lists because it has to go through the entire list for every item. – Jim Mischel Jul 09 '14 at 22:55
@JimMischel, Yes it absolutely has that problem. The group by approach is much more efficient, but I didn't come up with a working one in time :) – BradleyDotNET Jul 09 '14 at 23:04

Remove array elements that appear more than once

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