Removal operation in List

Question

I have 2 Lists of type string:

List1 has items - item1, item2, item3
List2 has items - item2, item3

Now I need to compare both lists and remove the duplicated items from List1. The modified List1 should have just one item, which is "item1".

foreach loops may work, but what I want to know is there any inbuilt method which does this?

EDIT

Thanks for the answers guys. I was just thinking what would be the case if I wanted to add the missed out items into the List. So just raised another question similar to this.

Add operation in List<string>

TIA!

score 10 · Answer 1 · answered Aug 03 '12 at 13:31

You could use:

list1.RemoveAll(item => list2.Contains(item));

(As Marc says, this is O(N*M) - fine if you expect the lists to be very short, but awful if either of them could get big.)

Or if you don't mind it being a new list (rather than modifying the existing one), you could just use:

list1 = list1.Except(list2).ToList();

score 10 · Accepted Answer · answered Aug 03 '12 at 13:31

I suspect the best approach to use here would be to put the second list into a hash first, i.e.

var hash = new HashSet<TheType>(List2);
List1.RemoveAll(hash.Contains);

This avoids having O(n*m) performance, instead being O(n+m)

Example:

List<int> List1 = new List<int> {1,2,3};
List<int> List2 = new List<int> {2,3};

var hash = new HashSet<int>(List2);
List1.RemoveAll(hash.Contains);
// now List1 just has {1}

Nikhil Agrawal · Answer 3 · 2012-08-03T13:32:12.253

3

Use

List1 = List1.Except(List2).ToList();

edited Aug 03 '12 at 13:32

answered Aug 03 '12 at 13:29

Nikhil Agrawal

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That won't compile - Remove take a single item. – Jon Skeet Aug 03 '12 at 13:31
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It still won't compile, as `Except` returns an `IEnumerable`. – Jon Skeet Aug 03 '12 at 13:32
Now it is *reassigning* `List1`, rather than mutating the original `List1` – Marc Gravell Aug 03 '12 at 13:33
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@NikhilAgrawal: At which point your answer is the same as the second half as mine, but without the extra information :) – Jon Skeet Aug 03 '12 at 13:35

Pierluc SS · Answer 4 · 2012-08-03T13:38:34.633

1

I recommend using:

List1 = List1.Except(List2).ToList();

edited Aug 03 '12 at 13:38

answered Aug 03 '12 at 13:32

Pierluc SS

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Removal operation in List

EDIT

4 Answers4