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)

Answer 2

You can do:

for x in sorted(np.unique(arr[...,2])):
    results.append([np.average(arr[np.where(arr[...,2]==x)][...,0]), 
                    np.average(arr[np.where(arr[...,2]==x)][...,1]),
                    x])

Testing:

>>> arr
array([[  6.,  12.,   1.],
       [  7.,   9.,   1.],
       [  8.,   7.,   1.],
       [  4.,   3.,   2.],
       [  6.,   1.,   2.],
       [  2.,   5.,   2.],
       [  9.,   4.,   3.],
       [  2.,   1.,   4.],
       [  8.,   4.,   4.],
       [  3.,   5.,   4.]])
>>> results=[]
>>> for x in sorted(np.unique(arr[...,2])):
...     results.append([np.average(arr[np.where(arr[...,2]==x)][...,0]), 
...                     np.average(arr[np.where(arr[...,2]==x)][...,1]),
...                     x])
... 
>>> results
[[7.0, 9.3333333333333339, 1.0], [4.0, 3.0, 2.0], [9.0, 4.0, 3.0], [4.333333333333333, 3.3333333333333335, 4.0]]

The array arr does not need to be sorted, and all the intermediate arrays are views (ie, not new arrays of data). The average is calculated efficiently directly from those views.