Group and average NumPy matrix

Question

Say I have an arbitrary numpy matrix that looks like this:

arr = [[  6.0   12.0   1.0]
       [  7.0   9.0   1.0]
       [  8.0   7.0   1.0]
       [  4.0   3.0   2.0]
       [  6.0   1.0   2.0]
       [  2.0   5.0   2.0]
       [  9.0   4.0   3.0]
       [  2.0   1.0   4.0]
       [  8.0   4.0   4.0]
       [  3.0   5.0   4.0]]

What would be an efficient way of averaging rows that are grouped by their third column number?

The expected output would be:

result = [[  7.0  9.33  1.0]
          [  4.0  3.0  2.0]
          [  9.0  4.0  3.0]
          [  4.33  3.33  4.0]]

Answer 1

A compact solution is to use numpy_indexed (disclaimer: I am its author), which implements a fully vectorized solution:

import numpy_indexed as npi
npi.group_by(arr[:, 2]).mean(arr)