C# Sort list while also returning the original index positions?

user_190222 0
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I'm interested in sorting a collection, but also returning an index which can be used to map to the original position in the collection (before the sort).

Let me give an example to be more clear:

List<int> A = new List<int>(){3,2,1};
List<int> B;
List<int> idx;

Sort(A,out B,out idx);

After which: 

A = [3,2,1] 
B = [1,2,3]
idx = [2,1,0]

So that the relationship between A,B,idx is:

A[i] == B[ idx[i] ] , for i = 0...2

Does C#/.Net have any built in mechanism to make this easy to implement?

Thanks.

collections sorting .net c#
4 Answers

As for now, you can also utilize anonymous types or value tuples instead of KeyValuePair. It will provide more precise naming and make your code more readable:

Anonymous types (C# 3.0):

List<int> arr = new List<int>() { 3, 2, 1 };

var sorted = arr
    .Select((x, i) => new { Value = x, OriginalIndex = i }))
    .OrderBy(x => x.Value)
    .ToList();

int originalIndexOfTheSmallestItem = sorted[0].OriginalIndex;

List<int> B = sorted.Select(x => x.Value).ToList();
List<int> idx = sorted.Select(x => x.OriginalIndex).ToList();

Value tuples (C# 7.0):

List<int> arr = new List<int>() { 3, 2, 1 };

var sorted = arr
    .Select((x, i) => (Value: x, OriginalIndex: i))
    .OrderBy(x => x.Value)
    .ToList();

int originalIndexOfTheSmallestItem = sorted[0].OriginalIndex;

List<int> B = sorted.Select(x => x.Value).ToList();
List<int> idx = sorted.Select(x => x.OriginalIndex).ToList();

The difference is that you can return value tuple from your method and use it, but anonymous type can only be used within this method.

user_32186920
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