getting main diagonal of square matrix in linear representation

Question

I have int array representing elements of square matrix. I need to get rows,columns, 2 main diagonals of it. I have trouble with getting second diagonal (for 3*3 matrix it is elements with indexes 2,4,6 for 6*6 - 5,10,15,20,25,30). I'm wondering is there a neat way to do it without for loops.

        var matrix = new int[] { 6, 7, 2, 1, 5, 9, 8, 3, 4 };

        int size = 3;

        int i = 0;
        var rows = matrix.GroupBy(x => i++ / size);

        i = 0;
        var columns = matrix.GroupBy(x => i++ % size);

        var diag1 = matrix.Where((x, index)=>index%(size+1)==0);

        var diag2 = matrix.Where((x, index) => index % (size - 1) == 0);//PROBLEM - takes 0,8 indexes also.

Answer 1

Try this

            var matrix = new int[] { 6, 7, 2, 1, 5, 9, 8, 3, 4 };

            int size = 3;

            var rows = matrix.Select((x,i) => new {x = x, i = i}).GroupBy(x => x.i / size).Select(x => x.Select(y => y.x).ToArray()).ToArray();

            var columns = matrix.Select((x, i) => new { x = x, i = i }).GroupBy(x => x.i % size).Select(x => x.Select(y => y.x).ToArray()).ToArray();

            var diag1 = matrix.Where((x, index) => (index /size) == (index % size)).Select(x => x).ToArray();

            var diag2 = matrix.Where((x, index) => (index / size) == (size - 1) - (index % size)).Select(x => x).ToArray();