Python collections.Counter() - A Complete Guide

When we want to count the frequency of elements in an iterable in Python, one of the most commonly used tools is the collections.Counter() class. In this article, we will cover everything you need to know about using Counter() in Python.

Introduction to collections.Counter()

collections.Counter() is a Python class that is used for counting the frequency of elements in an iterable. It is a subclass of the built-in dictionary class and inherits all its methods.

To create a Counter() object, we can pass an iterable (such as a list, tuple, or string) as an argument:

from collections import Counter

my_list = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
my_counter = Counter(my_list)

print(my_counter)

This will output:

Counter({'apple': 3, 'banana': 2, 'orange': 1})

As we can see, the Counter() object has counted the frequency of each element in the my_list input.

Common Uses of collections.Counter()

Finding the Most Common Elements

One of the most common uses of Counter() is finding the most common elements in a list. We can do this using the most_common() method:

from collections import Counter

my_list = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
my_counter = Counter(my_list)

most_common_elements = my_counter.most_common(2)

print(most_common_elements)

This will output:

[('apple', 3), ('banana', 2)]

As we can see, the most_common() method returns a list of the most common elements and their frequencies, in descending order.

Updating Counters

We can also update the counts in a Counter() object using the update() method:

from collections import Counter

my_list = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
my_counter = Counter(my_list)

new_list = ['orange', 'orange', 'orange', 'pear', 'apple']
my_counter.update(new_list)

print(my_counter)

This will output:

Counter({'orange': 4, 'apple': 4, 'banana': 2, 'pear': 1})

As we can see, the update() method has added the counts of the new elements to the Counter() object.

Arithmetic with Counters

We can also perform arithmetic with Counter() objects. For example, we can add two Counter() objects together:

from collections import Counter

list1 = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
counter1 = Counter(list1)

list2 = ['orange', 'orange', 'orange', 'pear', 'apple']
counter2 = Counter(list2)

sum_counters = counter1 + counter2

print(sum_counters)

This will output:

Counter({'apple': 4, 'orange': 4, 'banana': 2, 'pear': 1})

As we can see, the resulting Counter() object contains the element counts from both counter1 and counter2.

Conclusion

collections.Counter() is a powerful tool in Python for counting the frequency of elements in an iterable. It allows us to easily find the most common elements, update counts, and perform arithmetic with Counter() objects.