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})
      

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

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