How can I generate combinations of 5 numbers taken 3 at a time without repetition using Linq?

Question

So I have this array: int[] M = { 1, 2, 3, 4, 5 } , and from this array I want to generate combinations of 3 numbers at a time without repetition using Linq.

I have already succeeded to generate these combinations using 3 for loops:

        List<(int,int,int)> combinations = new List<(int,int,int)>();

        for(int i = 0; i < M.Length / 2 + 1; i++)
        {
            for(int j = i+1; j < M.Length - 1; j++)
            {
                for(int k = j+1; k < M.Length; k++)
                {
                    combinations.Add((M[i], M[j], M[k]));
                }
            }
        }

This is what I tried using Linq:

        var combinationsL = M.SelectMany(i => Enumerable.Range(0, M.Length / 2 + 1)
                                .SelectMany(j => Enumerable.Range(1, M.Length - 1 - i)
                                    .SelectMany(k => Enumerable.Range(2, M.Length- j)
                                        .Select(item => new int[] { M.ElementAt(i), M.ElementAt(j), 
                                                                               M.ElementAt(k) }))));

and it doesn't give me the expected result.

This is the expected result:

var combo = new[] { (1, 2, 3), (1, 2, 4), (1, 2, 5), (1, 3, 4), (1, 3, 5), (1, 4, 5), (2, 3, 4), (2, 3, 5), (2, 4, 5), (3, 4, 5) };

Can anyone tell me what I did wrong?

Answer 1

No need to install libraries, this is the formula you're looking for:

var result = M.SelectMany((x, i) => 
                 M.Skip(i + 1).SelectMany((y, j) => 
                     M.Skip(i + j + 2).Select(z => new int[] { x, y, z })));

Answer 2

The MoreLINQ library , available as a NuGet package , has a Subsets method overload that:

Returns a sequence of IList<T> representing all subsets of a given size that are part of the original sequence.

It should match your need.