Counters in Python | Set 1 (Initialization and Updation)

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 :

With sequence of items

With dictionary containing keys and counts

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})

• 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.

  # 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.

  # 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 aboject
  print(Counter(z))
  

  Output:

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

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