How to truncate a list?

Question

What's the easiest way to remove every element after and including the nth element in a System.Collections.Generic.List<T>?

btw this is a duplicate of http://stackoverflow.com/questions/3428242/how-do-i-truncate-a-list-in-c — PandaWood, Jan 25 '11 at 00:12
@PandaWood: Yep. But too late now :) Two different solutions too. — mpen, Jan 25 '11 at 02:36
Question header not so adequate to a question. Truncate in SQL means delete all. So the answer would be `list.Clear();` :) — Pawel Cioch, Mar 17 '16 at 15:23
@PawelCioch This isn't SQL. And that's exactly why you're supposed to read the question body. Truncate [means](http://www.dictionary.com/browse/truncate) "to shorten". — mpen, Mar 17 '16 at 19:04
I'm not native English speaker so some words sticks to one technology or one meaning. Never checked Truncate in the dictionary, so the header was a trigger for SQL :) Thanks for pointing it out. — Pawel Cioch, Mar 30 '16 at 01:33

score 60 · Accepted Answer · edited Sep 23 '10 at 13:35

60

If you can use RemoveRange method, simply do:

list.RemoveRange(index, count);

Where index is where to start from and count is how much to remove. So to remove everything from a certain index to the end, the code will be:

list.RemoveRange(index, list.Count - index);

Conversely, you can use:

list.GetRange(index, count);

But that will create a new list, which may not be what you want.

edited Sep 23 '10 at 13:35

BrunoLM

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answered Sep 23 '10 at 02:46

Daniel T.

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you mean `list.RemoveRange(index, list.Count - index)`? – mpen Sep 23 '10 at 02:50
Heh, I edited my answer before I saw your comment, but yes, that's what you would do. – Daniel T. Sep 23 '10 at 02:52
4

`RemoveRange` is not a LINQ method, it is an instance method of `List<>`. – Anthony Pegram Sep 23 '10 at 03:15
2

In my mind it's more length that index. `list.RemoveRange(len, list.Count - len); // save first len elems in list` – CoR Jun 01 '19 at 18:26
@CoR - thanks, your comment might have saved 10 minutes of my life! – Vasily Hall Feb 28 '20 at 04:33
This is what I was looking for - if you're only removing elements from the end then no data is copied, only internal 'size' field of the list is updated. – Dmitry Avtonomov Oct 18 '22 at 20:41

score 11 · Answer 2 · answered Dec 12 '12 at 16:13

11

sans LINQ quicky...

    while (myList.Count>countIWant) 
       myList.RemoveAt(myList.Count-1);

answered Dec 12 '12 at 16:13

Tim M. Hoefer

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RemoveRange is probably more efficient and less verbose. – Zar Shardan Dec 03 '16 at 15:29
@ZarShardan How is it less verbose? You have to check if the index exists and calculate the count. If it would allow giving an index that doesn't exist and a count over what actually exists without throwing an exception it would be easier, but as it is, it is actually more verbose. – selalerer Jan 24 '17 at 07:25
@selalerer even with the index check it is only marginally more verbose, but still more efficient.Besides, it communicates the intent better, so easier to read. if(idx < list.Count) list.RemoveRange(idx, list.Count - idx); – Zar Shardan Jan 25 '17 at 14:18
this works also on ObservableCollection, while RemoveRange doesn't. – Maverick Meerkat Jul 23 '17 at 11:36
This worked very well to truncate a list to match the length of another list. It prevents issues with `RemoveRange` at an index of `.Count - 1` when the length of the leading list is `0` (and thus avoids a condition on the value of `.Count`). – Superman.Lopez May 31 '21 at 13:09

score 3 · Answer 3 · edited Sep 23 '10 at 04:31

3

list.Take(n);

edited Sep 23 '10 at 04:31

NullUserException

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answered Sep 23 '10 at 02:46

zerkms

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This method will create a new list instead of editing the existing one, which may or may not be what you want. – Daniel T. Sep 23 '10 at 02:53
@Daniel T.: that is why i upvoted your answer just after few seconds you've posted it - because yours is more correct and i did not know that extension method ;-) – zerkms Sep 23 '10 at 03:07
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@Daniet T, this does not create a new list. It simply allows you to enumerate a sequence of the umutated original list. – Anthony Pegram Sep 23 '10 at 03:11
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@Anthony Pegram: but if the OP needs a `List` as result - he will apply `ToList()` anyway. – zerkms Sep 23 '10 at 03:13
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@zerkms, if ToList() were part of your answer, the comment would be correct. But it isn't... so it isn't. – Anthony Pegram Sep 23 '10 at 03:16
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-1 What does this method do? Does it return a new list? Does it modify the list? How performatic is it? Is there another way to do it? Which is faster? – BrunoLM Sep 23 '10 at 09:59
This is great for truncating a list returned from a function – Karan Harsh Wardhan Feb 05 '19 at 06:46

score 2 · Answer 4 · answered Sep 23 '10 at 03:20

2

If LINQ is not an option, you can loop through the list backwards:

for(int i = list.Count - 1; i >= index; i--)
{
    list.RemoveAt(i);
}

answered Sep 23 '10 at 03:20

Adam Lear

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+1 for simple way, though `RemoveRange` is likely to be more efficient. – tia Sep 23 '10 at 04:37
1

RemoveRange is always available when there's List, as it has nothing to do with LINQ. – Jon Hanna Sep 23 '10 at 09:55
Ah, neat. I haven't noticed it before. Thanks! – Adam Lear Sep 23 '10 at 13:02

score 1 · Answer 5 · answered Sep 23 '10 at 05:18

Here is the sample app to do it

    static void Main(string[] args)
    {
        List<int> lint = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        Console.WriteLine("List Elements");
        lint.ForEach(delegate(int i) {  Console.WriteLine(i); });

        lint.RemoveRange(8, lint.Count - 8);

        Console.WriteLine("List Elements after removal");
        lint.ForEach(delegate(int i) { Console.WriteLine(i); });

        Console.Read();

    }

How to truncate a list?

5 Answers5