Count the occurrence of a certain item in an ndarray - Numpy

In this article, the task is to find out how to count the occurrence of a certain item in an nd-Array in Python.

Example

Array = [[ 0  1  2  3]
        [ 4  5  2  7]  
        [ 8  2 10 11]]
        
Input: element = 2

Output: 3 times

Count the occurrence of a certain item in an array using a loop

Here we are using a loop to count the occurrence of an element in an array.

import numpy as np

arr = np.array([2, 3, 4, 5, 3, 3, 
                5, 4, 7, 8, 3])

print('Numpy Array:')
print(arr)
c = 0
element = 3

for j in arr:
    if j == element:
        c += 1
        
print("element occurred", c, "times")

Output:

Numpy Array:[2 3 4 5 3 3 5 4 7 8 3]
element occurred 4 times

Count the occurrence of a certain item in an array using count_nonzero() 

Here we are using the count_nonzero() function to count the occurrence of an item in the array if the match the target value.

Example 1:

For 1D array

import numpy as np

a = np.array([2, 3, 4, 5, 3, 3, 5, 4, 7, 8, 3])

print('Numpy Array:')
print(a)

# Count element '3' in numpy array
c = np.count_nonzero(a == 3)
print('Total occurrences of "3" in array: ', c)

Output:

Numpy Array: [2 3 4 5 3 3 5 4 7 8 3] 
Total occurrences of "3" in array: 4

Example 2:

For 2D array

import numpy as np

a = np.array([[1, 3, 6],
              [1, 3, 4],
              [5, 3, 6],
              [4, 7, 8],
              [3, 6, 1]])

print('Numpy Array:')
print(a)

# Count '3' in numpy array
c = np.count_nonzero(a == 3)
print('Occurrences of "3" in array: ', c)

Output:

Numpy Array:
[[1 3 6]
[1 3 4]
[5 3 6]
[4 7 8]
[3 6 1]]
Occurrences of "3" in array:  4

Count the occurrence of a certain item in an array using sum()

A True is equivalent to 1 in Python, so usually add the True or non-zero values in the array to get the sum of values in the array that matches the condition.

import numpy as np

arr = np.array([2, 3, 4, 5, 3, 3, 5, 4, 7, 8, 3])

print('Numpy Array:')
print(arr)

# Count '3' in array
count = (arr == 3).sum()

print('Occurrences of "3" in array: ', count)

Output:

Numpy Array: 
[2 3 4 5 3 3 5 4 7 8 3] 
Occurrences of "3" in array: 4

Counting the matching value to count the occurrence of an item

Here we are using the concept of matching the value present in the array and finding the occurrence of that element in the array. 

import numpy as np

a = np.array([2, 3, 4, 5, 3, 3,
              5, 4, 7, 8, 3])

# Count '3' in numpy array
c = a[a == 3].shape[0]

print('Occurrences of "3" in array is: ', c)

Output:

Occurrences of "3" in array is: 4

Count the occurrence of a certain item in an array using the tolist()

Here we are converting the Numpy array into a list using the tolist() and then counting the number of elements that match the target elements.

import numpy as np

a = np.array([2, 3, 4, 5, 3, 3,
              5, 4, 7, 8, 3])

# Count element '3' in numpy array
c = a.tolist().count(5)
print('Occurrences of "3" in array: ', c)

Output:

Occurrences of "3" in array: 2