How to take last records from list

Question

I have list as follows

static List<MessageDetail> CurrentMessage = new List<MessageDetail>();

Dynamically, values assigned to this list for example:

CurrentMessage.Add(new MessageDetail { UserName = 123,GroupName = somegrp, Message = somemsg });

Here, I want to take last 5 or so records.

// this returns first 5 result, dont want to user orderby clause either
CurrentMessagesForGroup = CurrentMessage
                          .Where(c => c.GroupName == groupName)
                          .Take(5).ToList();

Is there a way to implement TakeLast() attribute? Or any kind of help will be appreciated. Thanks for your time.

If you don't want to use order by then how do you fix the last five entries?? By entry time or something else? — Rashik Hasnat, Jul 26 '16 at 10:58
Possible duplicate of [Using Linq to get the last N elements of a collection?](http://stackoverflow.com/questions/3453274/using-linq-to-get-the-last-n-elements-of-a-collection) — Jaimie Knox, Jul 26 '16 at 18:08

Salah Akbari · Accepted Answer · 2016-07-26T18:41:54.860

4

Use skip:

CurrentMessagesForGroup = CurrentMessage
                    .Where(c => c.GroupName == groupName).Skip(Math.Max(0, CurrentMessage.Count() - 5)).ToList();

EDIT: I also find this that I think it is more easier to use (MoreLinq):

using MoreLinq;

var CurrentMessagesForGroup2 = CurrentMessage.TakeLast(5);

edited Jul 26 '16 at 18:41

answered Jul 26 '16 at 10:59

Salah Akbari

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Thanks for answer, exactly what i need.P1 for effort – Suhail Mumtaz Awan Jul 26 '16 at 11:11
1

@SuhailMumtazAwan Check my updated answer. I think it can be more simple if you use moreLINQ for this purpose. – Salah Akbari Jul 26 '16 at 11:16
@S.Akbari I'd personally link to https://morelinq.github.io, rather than a google project page. Also, TakeLast employs a Queue to reverse the order. Don't know how effective that is, I'm not good at big O notation. =) – J. Steen Jul 26 '16 at 11:17
@J.Steen Thanks. Updated. – Salah Akbari Jul 26 '16 at 11:18

Murray Foxcroft · Answer 2 · 2016-07-26T11:00:24.070

3

Use an OrderBy (ASC or DESC) to get the records lined up correctly for your Take operation.

Ascending:

CurrentMessagesForGroup = CurrentMessage
                        .Where(c => c.GroupName == groupName)
                        .OrderBy(c => c.GroupName)
                        .Take(5)
                        .ToList();

or Descending:

CurrentMessagesForGroup = CurrentMessage
                        .Where(c => c.GroupName == groupName)
                        .OrderByDescending(c => c.GroupName)
                        .Take(5)
                        .ToList();

edited Jul 26 '16 at 11:00

answered Jul 26 '16 at 10:57

Murray Foxcroft

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Thanks for answer but as i already stated i dont want to use orderby clause . – Suhail Mumtaz Awan Jul 26 '16 at 11:02
1

I missed that in the comment. If you just want the last 5 added, then go with S.Akbari's answer, covered here as well http://stackoverflow.com/questions/3453274/using-linq-to-get-the-last-n-elements-of-a-collection – Murray Foxcroft Jul 26 '16 at 11:08

score 2 · Answer 3 · answered Aug 06 '20 at 11:03

2

If anyone using DotNet Core 2 or above or DotNet Standard 2.1 or above then you can use Linq's built in .TakeLast()

Reference: Microsoft Documentation here

answered Aug 06 '20 at 11:03

Shuvo Amin

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score 1 · Answer 4 · answered Jul 26 '16 at 11:08

1

You could use Reverse(), which is slightly perverse.

CurrentMessagesForGroup = CurrentMessage
                    .Where(c => c.GroupName == groupName)
                    .Reverse()
                    .Take(5).ToList();

answered Jul 26 '16 at 11:08

J. Steen

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Thanks for answer but this doesn't fullfil needs.P1 for effort – Suhail Mumtaz Awan Jul 26 '16 at 11:11
I'm curious - how doesn't it fulfil your need? I'd personally go with S. Akbari's answer myself, I'm just providing another option. =) – J. Steen Jul 26 '16 at 11:11

score 1 · Answer 5 · edited May 23 '17 at 12:32

1

I use an extension method for this.

public static IEnumerable<T> TakeLast<T>(this IEnumerable<T> source, int numElements)
{
    return source.Skip(Math.Max(0, source.Count() - numElements));
}

And to use it:

CurrentMessagesForGroup = CurrentMessage.Where(c => c.GroupName == groupName).TakeLast(5).ToList();

Edit: Credit to Using Linq to get the last N elements of a collection?

edited May 23 '17 at 12:32

Community

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answered Jul 26 '16 at 17:58

Jaimie Knox

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Antonio Gil Espinosa · Answer 6 · 2020-05-22T11:52:13.600

I like this implementation, it uses a circular buffer.

public static IEnumerable<T> TakeLast<T>(this IEnumerable<T> input, int n)
{
    if (n == 0)
        yield break;

    int tail = 0;
    int head = 0;
    int count = 0;

    T[] buffer = new T[n];

    foreach (T item in input)
    {
        buffer[tail] = item;

        tail = (tail + 1) % n;

        if (count < n)
            count++;
        else
            head = (head + 1) % n;
    }

    for (int i = 0; i < count; i++)
        yield return buffer[(head + i) % n];
}

score 0 · Answer 7 · answered Jul 26 '16 at 11:14

0

If the MessageDetails class has numeric Id or Created date time we can use

var lastRecords= CurrentMessage.OrderByDescending(i=>i.Id).Where(p=>p.GroupName==groupName).Take(5).ToList();

answered Jul 26 '16 at 11:14

Syam Kumar

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How to take last records from list

7 Answers7