count occurrences of arrays in multidimensional arrays in python

Question

I have the following type of arrays:

a = array([[1,1,1],
           [1,1,1],
           [1,1,1],
           [2,2,2],
           [2,2,2],
           [2,2,2],
           [3,3,0],
           [3,3,0],
           [3,3,0]])

I would like to count the number of occurrences of each type of array such as

[1,1,1]:3, [2,2,2]:3, and [3,3,0]: 3 

How could I achieve this in python? Is it possible without using a for loop and counting into a dictionary? It has to be fast and should take less than 0.1 seconds or so. I looked into Counter, numpy bincount, etc. But, those are for individual element not for an array.

Thanks.

Answer 1

If you don't mind mapping to tuples just to get the count you can use a Counter dict which runs in 28.5 µs on my machine using python3 which is well below your threshold:

In [5]: timeit Counter(map(tuple, a))
10000 loops, best of 3: 28.5 µs per loop

In [6]: c = Counter(map(tuple, a))

In [7]: c
Out[7]: Counter({(2, 2, 2): 3, (1, 1, 1): 3, (3, 3, 0): 3})