How to get a list of indexes selected by a specific value efficiently with numpy arrays?

Question

I have a numpy array like this:

import numpy as np
arr = np.array([9, 6, 3, 8, 2, 3, 3, 4, 4, 9, 5, 6, 6, 6, 6, 7, 8, 9])

And I want to get

Answer 1

This should not be too slow. The array is iterated only once. The result (ind) is a dictionary value -> list of indexes.

import numpy as np
arr = np.array([2, 3, 3, 4, 4, 9, 5, 6, 6, 6, 6, 7, 8, 9])

ind = dict()
for i, val in enumerate(arr):
  ind.setdefault(val, []).append(i)