How many times a number appears in a numpy array

Question

I need to find a way to count how many times each number from 0 to 9 appears in a random matrix created using np.random.randint()

import numpy as np
p = int(input("Length of matrix: "))
m = np.random.randint(0,9,(p,p))
print(m)

For example if length of matrix = 4

• [[3 4 6 5] [3 4 4 3] [4 2 4 8] [6 8 2 7]]

How many times does the number 4 appear? It should return 5.

Answer 1

You should be able to get this pretty simply:

list(m.flatten()).count(x)

Another option which is probably faster, is to use the numpy builtin count_nonzero():

np.count_nonzero(m == x)

Hooray builtin functions.

Answer 2

You can use sum function:

In [52]: m = np.random.randint(0,9,(4,4))
In [53]: m
Out[53]: 
array([[8, 8, 2, 1],
       [2, 7, 1, 2],
       [8, 6, 8, 7],
       [5, 2, 5, 2]])

In [56]: np.sum(m == 8)
Out[56]: 4

m == 8 will return a boolean array contains True for each 8 then since python evaluates the True as 1 you can sum up the array items in order to get the number of intended items.

Answer 3

If you want to get the frequency from all matrix elements, here's a simple solution using numpy.ndarray.flatten and collections.Counter:

import numpy as np
import collections

p = int(input("Length of matrix: "))
m = np.random.randint(0, 9, (p, p))
print(m)
print(collections.Counter(m.flatten()))

For example, when p=3 you'd get something like this:

[[8 4 8]
 [5 1 1]
 [1 1 1]]
Counter({1: 5, 8: 2, 4: 1, 5: 1})