LINQ OrderBy custom order

Question

Say I have the following character array:

char[] c = new char[] { 'G', 'R', 'D', 'D', 'G', 'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C', 'G', 'R', 'D', 'D', 'G', 'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C' };


c.OrderBy(y => y).ForEach(x => Console.WriteLine(x));
//CCCCDDDDDDGGGGGGRRRRRRRRRR

How do I use LINQ to produce the following order?

//DDDDDDGGGGGGRRRRRRRRRRCCCC

How are we meant to guess the ordering rules if you don't tell us? — spender, Feb 26 '15 at 12:31
https://stackoverflow.com/questions/2013198/linq-to-sql-dynamic-orderby-case-when/56868434#56868434 for classes — Tiju John, Aug 05 '19 at 13:12

score 37 · Accepted Answer · edited May 18 '21 at 12:14

37

Maybe you want to do something like this:

char [] customOrder = { 'D', 'G', 'R', 'C'};
char [] c = new char[] { 'G', 'R', 'D', 'D', 'G', 'R',
                         'R', 'C', 'D', 'G', 'R', 'R',
                         'C', 'G', 'R', 'D', 'D', 'G',
                         'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C' };

foreach (char item in c.OrderBy(ch => Array.IndexOf(customOrder, ch))) {
    Console.Write(item);
}

edited May 18 '21 at 12:14

Peter Mortensen

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answered Feb 26 '15 at 12:33

Perfect28

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@TimSchmelter he is using ForEach LINQ extension method. – Euphoric Feb 26 '15 at 12:45
1

@Euphoric: there is no `ForEach` extension method apart from one that he might have written from the scratch. http://stackoverflow.com/a/200584/284240 – Tim Schmelter Feb 26 '15 at 12:47
@TimSchmelter https://msdn.microsoft.com/query/dev11.query?appId=Dev11IDEF1&l=EN-US&k=k(%22System.Collections.Generic.List%601.ForEach%22);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.5);k(DevLang-csharp)&rd=true – BRAHIM Kamel Feb 26 '15 at 12:48
@KamelBRAHIM: that is not an extension method but an instance method of `List`. – Tim Schmelter Feb 26 '15 at 12:49
@TimSchmelter ok so you have such thing built-in it's not important an extension or an instance – BRAHIM Kamel Feb 26 '15 at 12:51
1

@KamelBRAHIM: no, but you have said that it's an extension method and it is not. It was there since .NET 2(so even before LINQ). It's important because OP uses `OrderBy(...).ForEach` and this answer used it also. Using `ToList().ForEach` is a bad habit because you are looping all items in the sequence to create a new collection from the scratch, just to enable to loop it again in `ForEach`. Use `foreach` if you want to loop(consume) a query. – Tim Schmelter Feb 26 '15 at 12:55
@TimSchmelter me?? :D are you sure ?? – BRAHIM Kamel Feb 26 '15 at 13:00

score 7 · Answer 2 · answered Feb 26 '15 at 12:36

7

You could use another collection which defines the order:

char[] order = {'D','G','R','C'};
var customOrder = c.OrderBy(chr =>
{
    int index = Array.IndexOf(order, chr);
    if (index == -1) return int.MaxValue;
    return index;
});

answered Feb 26 '15 at 12:36

Tim Schmelter

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score 5 · Answer 3 · answered Feb 26 '15 at 12:36

If you want to define the order as relation between the items, then you have to use IComparer with the other OrderBy method.

public class Comparer : IComparer<char>
{
    public int Compare(Char a, Char b)
    {
       //return positive if a should be higher, return negative if b should be higher
    }
}

c.OrderBy(c => c, new Comparer()).ForEach(x => Console.WriteLine(x));

score 4 · Answer 4 · answered Feb 26 '15 at 13:04

My own solution (thank you to those who led me in the right direction)

char[] c = new char[] { 'G', 'R', 'D', 'D', 'G', 'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C', 'G', 'R', 'D', 'D', 'G', 'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C' };
c.OrderBy(x => "CDRG".IndexOf(x)).ForEach(Console.Write);

Produces:

CCCCDDDDDDRRRRRRRRRRGGGGGG

score 2 · Answer 5 · answered Feb 26 '15 at 12:32

You can use a dictionary with the order of the elements:

Dictionary<char, int> order = new Dictionary<char,int> {
    { 'D', 0 },
    { 'G', 1 },
    { 'R', 2 },
    { 'C', 3 },
};

char[] c = new char[] { 'G', 'R', 'D', 'D', 'G', 'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C', 'G', 'R', 'D', 'D', 'G', 'R', 'R', 'C', 'D', 'G', 'R', 'R', 'C' };

// Here we search the dictionary for the "order" to be used
// and we compare the value with the value of the other 
// compared item
Array.Sort(c, (p, q) => order[p].CompareTo(order[q]));

var str = new string(c);
Console.WriteLine(str);

score 2 · Answer 6 · answered Feb 26 '15 at 12:38

2

I'd approach this with a join ensuring that the ordering array is on the left side of the join.

var ordering = "CDGR".ToCharArray();

var orderedOutput = ordering.Join(c, a => a, b => b, (a, b) => b);

answered Feb 26 '15 at 12:38

spender

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LINQ OrderBy custom order

6 Answers6

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