How do I truncate a list in C#?

Question

I know in python you can do something like myList[1:20] but is there anything similar in C#?

score 42 · Accepted Answer · answered Aug 06 '10 at 22:31

42

var itemsOneThroughTwenty = myList.Take(20);
var itemsFiveThroughTwenty = myList.Skip(5).Take(15);

answered Aug 06 '10 at 22:31

Tim Robinson

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Note that these Linq extensions actually create an IEnumerable<> that stops after N items, rather than creating a new array/list. (This is kind of hidden by using 'var' for the variable type. If the code following the truncation iterates through the new list lots of times, you'll actually be re-evaluating the expression tree *each time*. In this case, you might want to tack a `.ToList()` onto the end to force the items to be enumerated and a new list created. – JaredReisinger Aug 06 '10 at 23:24

score 29 · Answer 2 · edited Oct 18 '16 at 07:53

29

You can use List<T>.GetRange():

var subList = myList.GetRange(0, 20);

From MSDN:

Creates a shallow copy of a range of elements in the source List<T>.

public List<T> GetRange(int index, int count)

edited Oct 18 '16 at 07:53

Sнаđошƒаӽ

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answered Aug 06 '10 at 22:31

Dean Harding

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score 20 · Answer 3 · answered Oct 01 '17 at 23:05

20

This might be helpful for efficiency, if you really want to truncate the list, not make a copy. While the python example makes a copy, the original question really was about truncating the list.

Given a List<> object "list" and you want the 1st through 20th elements

list.RemoveRange( 20, list.Count-20 );

This does it in place. This is still O(n) as the references to each object must be removed, but should be a little faster than any other method.

answered Oct 01 '17 at 23:05

Garr Godfrey

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Unless the `list` contains value types, in which case removal of references is skipped in .Net Core. – NetMage Aug 19 '20 at 22:05
@NetMage Good point. You are referring to the O(n) and rightly point out this can be O(1) in those cases – Garr Godfrey Oct 13 '20 at 21:26

score 4 · Answer 4 · answered Dec 12 '12 at 16:17

4

sans LINQ quicky...

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

answered Dec 12 '12 at 16:17

Tim M. Hoefer

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score 0 · Answer 5 · answered Mar 06 '13 at 14:39

0

    public static IEnumerable<TSource> MaxOf<TSource>(this IEnumerable<TSource> source, int maxItems)
    {
        var enumerator = source.GetEnumerator();            
        for (int count = 0; count <= maxItems && enumerator.MoveNext(); count++)
        {
            yield return enumerator.Current;
        }
    }

answered Mar 06 '13 at 14:39

Luiz Carlos

1

As it stands you might consider adding some explanatory text, because less experienced users might struggle to understand your answer. – Raad Mar 06 '13 at 15:01

score 0 · Answer 6 · edited Aug 02 '20 at 10:04

0

The list can be truncated using the RemoveRange keyword . The function is as follows:

  void List<type>.RemoveRange(int index, int count);

This removes the elements from index until count. Using this for type int, to remove from 0 to the not required points:

Code:

int maxlimit = 100;
List<int> list_1 = new List<int>();
if (list_1.Count > maxLimit){
    list_1.RemoveRange(0, (list_1.Count - maxlimit));
}

edited Aug 02 '20 at 10:04

David Buck

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answered Aug 02 '20 at 09:19

aorta

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not sure what this adds to other answers and the example uses an empty list. – Garr Godfrey Oct 13 '20 at 21:28

How do I truncate a list in C#?

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