Get indices of matrix from upper triangle without numpy

Question

I would like to get indices for an upper triangle matrix. I am aware of np.triu_indices, but cannot use it in my application at the moment.

I can get its values from the below code, but not sure how to get its indices.

Please advice

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
for i in range(len(matrix)):
    print(matrix[i][i+1:]) # For excluding the main diagonal elems

In the above example, I would like the indices for 2, 3, and 6

Answer 1

You already have the indices, you're using them to access elements in matrix

Instead of using them to index into matrix, add them to a list:

row_indices = []
col_indices = []
for i in range(len(matrix)):
    row = matrix[i]
    col_ix = list(range(i+1, len(row)))
    row_ix = [i] * len(col_ix)
    row_indices.extend(row_ix)
    col_indices.extend(col_ix)

Now row_indices and col_indices contain the indices you're looking for.

Try it online