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          


        

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)