C# sort object list with start position and loop

Question

I have a strange question :)

I have a object list looking like this:

var list = new []
{
    new { Id = 1, Name = "Marcus" },
    new { Id = 2, Name = "Mattias" },
    new { Id = 3, Name = "Patric" },
    new { Id = 4, Name = "Theodor" },
};

I would like to sort the list providing a "start id"

For example, if I provide "start id" 3, the result should look like this:

Id	Name
3	Patric
4	Theodor
1	Marcus
2	Mattias

I have no idea where to start, so I really need some help from you coding gods

The list is from a sql table, but it does not matter for me where the sort take place (in sql query or in c# code)

Are you expecting a Linq or SQL answer? – jarlh May 27 '21 at 13:06 — jarlh, May 27 '21 at 13:06
Is the first list always sorted by Id? – DekuDesu May 27 '21 at 13:07 — DekuDesu, May 27 '21 at 13:07

Enigmativity · Accepted Answer · 2021-05-28T03:04:38.023

5

Try this:

var list = new []
{
    new { Id = 1, Name = "Marcus" },
    new { Id = 2, Name = "Mattias" },
    new { Id = 3, Name = "Patric" },
    new { Id = 4, Name = "Theodor" },
};

var start_id = 3;
var max_id = list.Max(y => y.Id);

var result =
    from x in list
    orderby (x.Id + max_id - start_id) % max_id
    select x;

I get:

edited May 28 '21 at 03:04

answered May 27 '21 at 13:12

Enigmativity

113,464
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172

`list` is known to be an array: use `Length` property instead of `Count` extension method. by the way, relying on number of elements will probably yield wrong results if gaps exist in the `Id` values. – Rubidium 37 May 27 '21 at 13:18
1

@Rubidium37 - `.Count()` on an array is `O(1)` - LINQ is smart. – Enigmativity May 27 '21 at 13:21
@Rubidium37 - Good point, though, on the gaps in ID values issue. – Enigmativity May 27 '21 at 13:21
1

@Rubidium37 - Fixed, but not as efficient. – Enigmativity May 27 '21 at 13:23
1

@Enigmativity you should move `list.Max(y => y.Id)` outside the LINQ code I believe otherwise it will find max for every iteration. – Guru Stron May 27 '21 at 13:33
@GuruStron - Done. Now it's back to being efficient. – Enigmativity May 28 '21 at 03:04

Guru Stron · Answer 2 · 2021-05-27T13:24:35.667

3

With LINQ to objects you can do something like that:

var list = new []
{
    new { Id = 1, Name = "Marcus" },
    new { Id = 2, Name = "Mattias" },
    new { Id = 3, Name = "Patric" },
    new { Id = 4, Name = "Theodor" },
};
var startId = 3;

var result = list
    .GroupBy(i => i.Id >= startId ? 1 : 0) // split in two groups 
    .OrderByDescending(g => g.Key) // sort to have the group with startId first
    .Select(g => g.OrderBy(i => i.Id)) // sort each group
    .SelectMany(i => i) // combine result
    .ToList();

Console.WriteLine(string.Join(", ", result.Select(i => i.Id))); // prints "3, 4, 1, 2"

edited May 27 '21 at 13:24

answered May 27 '21 at 13:20

Guru Stron

102,774
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`g.OrderBy(g => g.Id))` will throw up and error because you've already declared `g` in the `Select` so I suggest renaming it – Izzy May 27 '21 at 13:23
1

@Izzy actually it is not throwing in the latest .NET version. But still I should fix it. – Guru Stron May 27 '21 at 13:24
@Enigmativity falling over for me on .net 4.5 – Izzy May 27 '21 at 13:26
@Izzy I believe it is not related to .NET version, but language version, see [this](https://stackoverflow.com/a/59973360/2501279) and [this](https://github.com/dotnet/csharplang/blob/main/meetings/2019/LDM-2019-01-16.md) – Guru Stron May 27 '21 at 13:27
@GuruStron Thanks for that, I will have a read through it – Izzy May 27 '21 at 13:28

score 1 · Answer 3 · answered May 27 '21 at 13:45

You require 2 criteria to apply:

Order ascending by Id.
Return the Ids greater than threshold before the Ids less than threshold.

You can try:

var offset = 3;
var sorted1 = list
    .OrderBy(item => item.Id < offset)
    .ThenBy(item => item.Id);

The OrderBy condition yields true if Id is less than offset and false otherwise. true is greater than false and therefore is returned later

A dirty way could also be:

var offset = 3;
var sorted2 = list
    .OrderBy(item => unchecked((uint)(item.Id - offset)));

Here the offset is subtracted from Id and the result converted to unsigned int to make the negative values become very large positive ones. A little hacky. Might not work with queries against SQL providers.

score 0 · Answer 4 · answered May 27 '21 at 13:28

Here's a toy Non-Linq Version

object[] ShiftList(int id)
{
    var list = new dynamic[]
    {
        new { Id = 1, Name = "Marcus" },
        new { Id = 2, Name = "Mattias" },
        new { Id = 3, Name = "Patric" },
        new { Id = 4, Name = "Theodor" },
    };

    Span<dynamic> listSpan = list;

    int indexFound = -1;

    for (int i = 0; i < list.Length; i++)
    {
        if (listSpan[i].Id == id)
        {
            indexFound = i;
        }
    }

    if (indexFound is -1)
    {
        return list;
    }

    var left = listSpan.Slice(0, indexFound);

    var right = listSpan[indexFound..];

    object[] objs = new object[list.Length];

    Span<object> objSpan = objs;

    right.CopyTo(objSpan);

    left.CopyTo(objSpan[right.Length..]);

    return objs;
}

score -2 · Answer 5 · answered May 27 '21 at 13:40

-2

Try using foreach and iterate over each object in your list:

            foreach (var item in list)
        {

        }

from here you should be able to use some of the collection methods for a list to reorder your list.

answered May 27 '21 at 13:40

itz4blitz

53
7

Given that they tagged the question with LINQ, I’d assume they’re expecting any C# based solution to be in LINQ. – Jeremy Caney May 27 '21 at 17:43

C# sort object list with start position and loop

5 Answers5