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],
         


        

Answer 1

collections.Counter can do this conveniently, and almost like the example given.

>>> from collections import Counter
>>> c = Counter()
>>> for x in a:
...   c[tuple(x)] += 1
...
>>> c
Counter({(2, 2, 2): 3, (1, 1, 1): 3, (3, 3, 0): 3})

This converts each sub-list to a tuple, which can be keys in a dictionary since they are immutable. Lists are mutable so can't be used as dict keys.

Why do you want to avoid using for loops?

And similar to @padraic-cunningham's much cooler answer:

>>> Counter(tuple(x) for x in a)
Counter({(2, 2, 2): 3, (1, 1, 1): 3, (3, 3, 0): 3})
>>> Counter(map(tuple, a))
Counter({(2, 2, 2): 3, (1, 1, 1): 3, (3, 3, 0): 3})