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How to get first N elements of a list in C#?


Question

I would like to use Linq to query a bus schedule in my project, so that at any time I can get the next 5 bus arrival times. How can I limit my query to the first 5 results?

More generally, how can I take a slice of a list in C#? (In Python I would use mylist[:5] to get the first 5 elements.)

2017/07/25
1
392
7/25/2017 10:15:39 AM

Accepted Answer

var firstFiveItems = myList.Take(5);

Or to slice:

var secondFiveItems = myList.Skip(5).Take(5);

And of course often it's convenient to get the first five items according to some kind of order:

var firstFiveArrivals = myList.OrderBy(i => i.ArrivalTime).Take(5);
2008/11/26
730
11/26/2008 7:28:02 AM

In case anyone is interested (even if the question does not ask for this version), in C# 2 would be: (I have edited the answer, following some suggestions)

myList.Sort(CLASS_FOR_COMPARER);
List<string> fiveElements = myList.GetRange(0, 5);
2008/11/26

Like pagination you can use below formule for taking slice of list or elements:

var slice = myList.Skip((pageNumber - 1) * pageSize)
                  .Take(pageSize);

Example 1: first five items

var pageNumber = 1;
var pageSize = 5;

Example 2: second five items

var pageNumber = 2;
var pageSize = 5;

Example 3: third five items

var pageNumber = 3;
var pageSize = 5;

If notice to formule parameters pageSize = 5 and pageNumber is changing, if you want to change number of items in slicing you change pageSize.

2018/06/18

To take first 5 elements better use expression like this one:

var firstFiveArrivals = myList.Where([EXPRESSION]).Take(5);

or

var firstFiveArrivals = myList.Where([EXPRESSION]).Take(5).OrderBy([ORDER EXPR]);

It will be faster than orderBy variant, because LINQ engine will not scan trough all list due to delayed execution, and will not sort all array.

class MyList : IEnumerable<int>
{

    int maxCount = 0;

    public int RequestCount
    {
        get;
        private set;
    }
    public MyList(int maxCount)
    {
        this.maxCount = maxCount;
    }
    public void Reset()
    {
        RequestCount = 0;
    }
    #region IEnumerable<int> Members

    public IEnumerator<int> GetEnumerator()
    {
        int i = 0;
        while (i < maxCount)
        {
            RequestCount++;
            yield return i++;
        }
    }

    #endregion

    #region IEnumerable Members

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        throw new NotImplementedException();
    }

    #endregion
}
class Program
{
    static void Main(string[] args)
    {
        var list = new MyList(15);
        list.Take(5).ToArray();
        Console.WriteLine(list.RequestCount); // 5;

        list.Reset();
        list.OrderBy(q => q).Take(5).ToArray();
        Console.WriteLine(list.RequestCount); // 15;

        list.Reset();
        list.Where(q => (q & 1) == 0).Take(5).ToArray();
        Console.WriteLine(list.RequestCount); // 9; (first 5 odd)

        list.Reset();
        list.Where(q => (q & 1) == 0).Take(5).OrderBy(q => q).ToArray();
        Console.WriteLine(list.RequestCount); // 9; (first 5 odd)
    }
}
2008/11/26

Source: https://stackoverflow.com/questions/319973
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