Question

I would like to use linq to group the data using value and to return the corresponding indexes as array.

Example

int[] input = {0,0,0,0,1,1,1,1,0,0,0,0,1,1,1,1,2,2,2,2}

Expected output

Dictionary<int,int[]> ouput = {0->[0,1,2,3,8,9,10,11]; 1 -> [4,5,6,7,12,13,14,15]; 2 -> [16,17,18,19]}

Can anybody guide me?

Answer 1

You can use this:

var output = input.Select((x, i) => new { Value=x, Index=i })
                  .GroupBy(x => x.Value)
                  .ToDictionary(x => x.Key, x => x.Select(y => y.Index)
                                                  .ToArray());

This first selects an anonymous type to save the original index in the array, then groups by the value and afterwards transforms the grouped result into a dictionary with the key of each group as the key of the dictionary and from all elements in the corresponding group the index is selected.

A shorter way would be this:

var output2 = input.Select((x, i) => new { Value=x, Index=i })
                   .ToLookup(x => x.Value, x => x.Index);

This would result in a Lookup<int, int> which is semantically the same as the Dictionary<int, int[]>.