Counters in Python | Set 1 (Initialization and Updation)

Counter is a container included in the collections module. Now you all must be wondering what is a container. Don’t worry first let’s discuss about the container. 

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 a dictionary. This is equivalent to a 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 : 

• With sequence of items

• With dictionary containing keys and counts

• With keyword arguments mapping string names to counts

Example of each type of initialization :

Python3

# 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)

Python3

# 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})

• Data can be provided in any of the three ways as mentioned in initialization and the counter’s data will be increased not replaced.
• Counts can be zero and negative also. 
  

Python3

# Python program to demonstrate that counts in
# Counter can be 0 and negative
from collections import Counter

c1 = Counter(A=4,  B=3, C=10)
c2 = Counter(A=10, B=3, C=4)

c1.subtract(c2)
print(c1)

• Output :

 Counter({'c': 6, 'B': 0, 'A': -6})

• We can use Counter to count distinct elements of a list or other collections. 
  

Python3

# An example program where different list items are
# counted using counter
from collections import Counter

# Create a list
z = ['blue', 'red', 'blue', 'yellow', 'blue', 'red']

# Count distinct elements and print Counter aobject
print(Counter(z))

• Output:
  

Counter({'blue': 3, 'red': 2, 'yellow': 1})

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