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Python len() Function

Python collections Counter

Last Updated : 18 Dec, 2024

Counters are a subclass of the dict class in Python collections module. They are used to count the occurrences of elements in an iterable or to count the frequency of items in a mapping. Counters provide a clean and efficient way to tally up elements and perform various operations related to counting.

Example:

Python

from collections import Counter  # Example list of elements val = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]  # Creating a Counter ctr = Counter(val)  print(ctr)

Output

Counter({4: 4, 3: 3, 2: 2, 1: 1})

In this example, the Counter counts the occurrences of each element in the list.

Let’s take a look at counters in python in detail:

Table of Content

Syntax

class collections.Counter([iterable-or-mapping])

Initialization:

The constructor of the counter can be called in any one of the following ways:

With a sequence of items
With a dictionary containing keys and counts
With keyword arguments mapping string names to counts

Creating a Counter

To use a Counter, we first need to import it from the collections module.

Python

from collections import Counter  # Creating a Counter from a list ctr1 = Counter([1, 2, 2, 3, 3, 3])  # Creating a Counter from a dictionary ctr2 = Counter({1: 2, 2: 3, 3: 1})  # Creating a Counter from a string ctr3 = Counter('hello')  print(ctr1) print(ctr2) print(ctr3)

Output

Counter({3: 3, 2: 2, 1: 1}) Counter({2: 3, 1: 2, 3: 1}) Counter({'l': 2, 'h': 1, 'e': 1, 'o': 1})

In these examples, we create Counters from different types of iterables.

Accessing Counter Elements

We can access the count of each element using the element as the key. If an element is not in the Counter, it returns 0.

Example:

Python

from collections import Counter  ctr = Counter([1, 2, 2, 3, 3, 3])  # Accessing count of an element print(ctr[1])   print(ctr[2])   print(ctr[3])   print(ctr[4])  # (element not present)

Output

1 2 3 0

This example shows how to access the count of specific elements in the Counter.

Updating counters

Counters can be updated by adding new elements or by updating the counts of existing elements. We can use the update() method to achieve this.

Example:

Python

from collections import Counter ctr = Counter([1, 2, 2]) # Adding new elements ctr.update([2, 3, 3, 3]) print(ctr)

Output

Counter({2: 3, 3: 3, 1: 1})

Counter Methods

elements(): Returns an iterator over elements repeating each as many times as its count. Elements are returned in arbitrary order.

Python

from collections import Counter ctr = Counter([1, 2, 2, 3, 3, 3]) items = list(ctr.elements()) print(items)

Output

[1, 2, 2, 3, 3, 3]

The elements() method returns an iterator that produces all elements in the Counter.

most_common(): Returns a list of the n most common elements and their counts from the most common to the least. If n is not specified, it returns all elements in the Counter.

Python

from collections import Counter ctr = Counter([1, 2, 2, 3, 3, 3]) common = ctr.most_common(2) print(common)

Output

[(3, 3), (2, 2)]

subtract(): Subtracts element counts from another iterable or mapping. Counts can go negative.

Python

from collections import Counter ctr = Counter([1, 2, 2, 3, 3, 3]) ctr.subtract([2, 3, 3]) print(ctr)

Output

Counter({1: 1, 2: 1, 3: 1})

The subtract() method decreases the counts for elements found in another iterable.

Arithmetic Operations on Counters

Counters support addition, subtraction, intersection union operations, allowing for various arithmetic operations.

Example:

Python

from collections import Counter  ctr1 = Counter([1, 2, 2, 3]) ctr2 = Counter([2, 3, 3, 4])  # Addition print(ctr1 + ctr2)   # Subtraction print(ctr1 - ctr2)    # Intersection print(ctr1 & ctr2)   # Union print(ctr1 | ctr2)

Output

Counter({2: 3, 3: 3, 1: 1, 4: 1}) Counter({1: 1, 2: 1}) Counter({2: 1, 3: 1}) Counter({2: 2, 3: 2, 1: 1, 4: 1})

If you want to learn more about accessing counters in Python, the article (Accessing Counters) is a great resource.

Python len() Function

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