Counter in Python

What is counter?

Counter is a container included in the collections module.

What is container?

Containers are objects that hold objects. They provide a way to access the contained objects and iterate over them. Examples of built in containers are Tuple, list and dictionary. Others are included in Collections module.

A Counter is a subclass of dict. Therefore it is an unordered collection where elements and their respective count are stored as dictionary. This is equivalent to bag or multiset of other languages.

Syntax:

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

initialization

The constructor of counter can be called in any one of the following ways :
1.With sequence of items
2.With dictionary containing keys and counts
3.With keyword arguments mapping string names to counts
Example of each type of initialization :

# A Python program to show different ways to create

# Counter

from collections import Counter

# With sequence of items

print Counter(['B','B','A','B','C','A','B','B','A','C'])

# with dictionary

print Counter({'A':3, 'B':5, 'C':2})

# with keyword arguments

print Counter(A=3, B=5, C=2)

Output of all the three lines is same :

Counter({'B': 5, 'A': 3, 'C': 2})

Counter({'B': 5, 'A': 3, 'C': 2})

Counter({'B': 5, 'A': 3, 'C': 2})

Updation:

We can also create an empty counter in the following manner :

coun = collections.Counter()

And can be updated via update() method .Syntax for the same :

coun.update(Data)

# A Python program to demonstrate update()
from collections import Counter
coun = Counter()
coun.update([1, 2, 3, 1, 2, 1, 1, 2])
print(coun)
coun.update([1, 2, 4])
print(coun)

Output:

Counter({1: 4, 2: 3, 3: 1})
Counter({1: 5, 2: 4, 3: 1, 4: 1})