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 :
# 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})