Python - Counter

Counter in Python 

A counter is a subclass of 'dict' to count hashable items. This collection uses dictionary keys to store items and dictionary values to keep counts of those elements. It is particularly useful for counting occurrences in a list or string.

Create a Counter

We can create a counter in the following way:

• From an iterable, such as a list, or string.
• From a dictionary (mapping keys to counts).
• Using keyword arguments.

Let's see a code example of how you can create a counter:

# import the counter from the collection module
from collections import Counter

# From an iterable
counter1 = Counter([1, 2, 2, 3, 4, 4, 4])
print(counter1)

# From a string
counter2 = Counter("hello world")
print(counter2)

# From a dictionary
counter3 = Counter({'a': 2, 'b': 3})
print(counter3)

# Using keyword arguments
counter4 = Counter(a=1, b=2, c=3)
print(counter4)

Accessing and Updating Counts

You can access and update the counters using keys like a dictionary. You can also add or subtract from counts by updating the Counter.

Let's see a code example:

# Accessing counts
print(counter1[4]) 
# Output: 3

# Updating counts
counter1[4] += 2
print(counter1)
# Output: Counter({4: 5, 2: 2, 1: 1, 3: 1})

Methods of Counter

you can use methods in Counter to efficiently perform common operations related to counting and manipulating frequencies. These methods simplify tasks like updating counts, identifying the most frequent elements, or combining multiple counts. Following are the common methods:

• elements(): Returns an iterator over elements, repeating each as many times as its count.
• most_common(n): Returns the n most common elements as a list of tuples (element, count).
• subtract(iterable_or_mapping): Subtracts counts from another iterable or mapping. Counts can become negative.
• update(iterable_or_mapping): Adds counts from another iterable or mapping.
• clear(): Removes all elements from the Counter.

Example

In this example, we are performing all the arithmetic operations such as: Addition, Subtraction, Intersection, and Union −

# import the counter from the collection module
from collections import Counter

counterA = Counter(a=3, b=2)
counterB = Counter(a=1, b=4, c=2)

# Addition
print(counterA + counterB)

# Subtraction
print(counterA - counterB)

# Intersection
print(counterA & counterB)

# Union
print(counterA | counterB)

Key Features of Counter

The following are the key features of the counter:

• Counts Elements: Counter tally up the occurrences of each unique element in an iterable (e.g., list, string).
• Dictionary-Like: Counter stores elements as keys and their counts as values.
• Supports Arithmetic: Counter can perform addition, subtraction, intersection, and union of counts.

Advantages of Counter

The following are the advantages of counter:

• Simplifies frequency counting tasks.
• Provides convenient arithmetic and set-like operations for counts.
• Easily integrates with other data structures.