Count occurrences of an element in a list in Python

A common task when working with lists is to count how many times a specific element appears. In Python, we have several ways to count occurrences of an element using both built-in and custom methods.

The simplest and most straightforward way to count occurrences of an element in a list is by using the count() method, which is a built-in method specifically designed for this task.

Using count()

In this example, we have a list and we are counting the occurrences of 2 and 3 using the count() method.

a = [1, 3, 2, 6, 3, 2, 8, 2, 9, 2, 7, 3]

# Count occurrences of 2
print(a.count(2))

# Count occurrences of 3
print(a.count(3))

4
3

Below are the other methods by which we can use to count the occurrences of an element in a list.

Using a Loop

In this method, iterate over the list using loop (for loop) and keep a counter variable to count the occurrences. Each time we find the target element, increase the counter by one.

a = [1, 3, 2, 6, 3, 2, 8, 2, 9, 2, 7, 3]

# Initial count is zero
count = 0

# Iterate over the list
for val in a:
  
  	# If num is equal to 3 
	if val == 3:
      
        # Increase the counter
		count += 1

print(count)

3

Using countOf()

The countOf() function is equivalent to using the count() method of a list, but it comes from the operator module.

It takes two arguments: the sequence in which we want to count and the value that we want to count. Let's look at the syntax given below:

import operator

operator.countOf(sequence, value)

Using Counter from collections

The Counter class from the collections module can count occurrences for all elements and returns the results as a dictionary. Let's see how can we use this to count occurrence of a single element.

from collections import Counter

a = [1, 3, 2, 6, 3, 2, 8, 2, 9, 2, 7, 3]

# Create a counter objects
res = Counter(a)

# Get count of 3
print(res[3])

3

Note: This method is not efficient for finding occurrence of single element because it requires O(n) extra space to create a new dictionary. But this method is very efficient when finding all occurrences of elements.

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