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 :

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

    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)

      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


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

    This article is contributed by Mayank Rawat .If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

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