How to find duplicate items in list<>?

Question

I have:

List<string> list = new List<string>() { "a", "a", "b", "b", "r", "t" };

How can I get only "a","b"?

I tried to do like this:

List<string> list = new List<string>() { "a", "a", "b", "b", "r", "t" };
List<string> test_list = new List<string>(); 

test_list = list.Distinct().ToList();

Now test_list has {"a", "b", "r", "t"}
And then:

test_list = test_list.Except(list).ToList();

So that was my fail point, cause Except() deleted all elements.

Could you help me with solution?

Totally expected behavior. Except() excepted from test_list all entries found in list. What are you trying to do? — Tommi, Apr 07 '13 at 19:30
It's a final bachelor work =) I'm doing a system with word search from data base by chosen filter — Monopompom, Apr 07 '13 at 19:36
Alex, since [the @Sachin's answer](http://stackoverflow.com/a/15866810/200449) is the best, as you noted in comment to it, you should accept it as answer by clicking on the left of that answer the shallow check mark helping others to understand that the question was answered — Gennady Vanin Геннадий Ванин, Apr 13 '13 at 05:25

score 16 · Answer 1 · answered Apr 07 '13 at 19:30

16

List<string> list = new List<string>() { "a", "a", "b", "b", "r", "t" };

var dups = list.GroupBy(x => x)
    .Where(x => x.Count() > 1)
    .Select(x => x.Key)
    .ToList();

answered Apr 07 '13 at 19:30

I4V

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This is more readable and understandable, Thanks – Wahid Bitar May 16 '14 at 13:56

dtb · Answer 2 · 2013-04-07T23:12:48.727

10

var list = new List<string> { "a", "a", "b", "b", "r", "t" };

var distinct = new HashSet<string>();    
var duplicates = new HashSet<string>();

foreach (var s in list)
    if (!distinct.Add(s))
        duplicates.Add(s);

// distinct == { "a", "b", "r", "t" }
// duplicates == { "a", "b" }

edited Apr 07 '13 at 23:12

answered Apr 07 '13 at 19:44

dtb

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+1 probably the most efficient approach. – Tim Schmelter Apr 07 '13 at 20:23
@TimSchmelter This is very good if you always want all the duplicate entries.. I have provided a bit more efficient (and lazy as well) solution here: https://stackoverflow.com/a/72647392/661933 – nawfal Jun 16 '22 at 14:22

Sachin · Accepted Answer · 2013-04-07T19:48:11.517

8

Try this

var duplicates = list.GroupBy(a => a).SelectMany(ab => ab.Skip(1).Take(1)).ToList();

edited Apr 07 '13 at 19:48

answered Apr 07 '13 at 19:29

Sachin

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I don't understand why this answer is upvoted. Any one tried it with `{ "a", "a", "b", "b", "a", "r", "t" }` ? – I4V Apr 07 '13 at 19:39
You _could_ do that. Honestly, I had to think too long and hard about _why_ this method actually works. I much prefer l4V's; much more readable and to-the-point. EDIT: Maybe it's faster, but I doubt performance is any issue at all in this case. – Chris Sinclair Apr 07 '13 at 19:43
1

I4V, you're right, it does not work correctly – Monopompom Apr 07 '13 at 19:44
2

@ChrisSinclair Well I think the point is that this *doesn't* work. If you put in `{ "a", "a", "b", "b", "a", "r", "t" }` you get out `"a" "a" "b"` – Matthew Watson Apr 07 '13 at 19:44
@MatthewWatson I was commenting to another poster (forgot the name) who suggested a correction that worked (see MarcinJuraszek's answer) but they subsequently deleted their comment. – Chris Sinclair Apr 07 '13 at 19:46
@ChrisSinclair: Ah, ok. That makes more sense. :) – Matthew Watson Apr 07 '13 at 19:48
((It's fixed with the latest edit)) – Matthew Watson Apr 07 '13 at 19:49
Sorry guy's, I accepted my mistake. I just test with above given problem. But now I have updated my answer – Sachin Apr 07 '13 at 19:49
.GroupBy(a => a).SelectMany(ab => ab.Skip(1)) :D I just found that this one was the best solution for me – Monopompom Apr 07 '13 at 19:51
@Alex, if it is eventually the best solution to you, you should accept it as answer by clicking on the left of answer a shallow check mark – Gennady Vanin Геннадий Ванин Apr 13 '13 at 05:19

score 3 · Answer 4 · answered Apr 07 '13 at 19:30

3

A simple approach is using Enumerable.GroupBy:

var dups = list.GroupBy(s => s)
               .Where(g => g.Count() > 1)
               .Select(g => g.Key);

answered Apr 07 '13 at 19:30

Tim Schmelter

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Sergei G · Answer 5 · 2013-04-16T10:53:50.203

0

var duplicates = list.GroupBy(s => s).SelectMany(g => g.Skip(1).Take(1)).ToList();

edited Apr 16 '13 at 10:53

answered Apr 07 '13 at 19:29

Sergei G

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This is incorrect 1st version of [@Sachin's answer](http://stackoverflow.com/a/15866810/200449), see comments to it and its edits – Gennady Vanin Геннадий Ванин Apr 13 '13 at 11:13
That was the best option for me. I miss one point in my question: if I have {a,a,a,b,b,r}, so at the output I should get {a,a,b} – Monopompom Apr 15 '13 at 09:24

score 0 · Answer 6 · answered Apr 07 '13 at 19:41

0

var duplicates = list.GroupBy(a => a).SelectMany(ab => ab.Skip(1).Take(1)).ToList();

It will be more efficient then the one using Where(g => g.Count() > 1) and will return only one element from every group.

answered Apr 07 '13 at 19:41

MarcinJuraszek

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How to find duplicate items in list<>?

6 Answers6

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