Python Counter - Python Collections Counter

Python Counter class is part of Collections module. Counter is a subclass of Dictionary and used to keep track of elements and their count.

Python Counter

Counter is an unordered collection where elements are stored as Dict keys and their count as dict value. Counter elements count can be positive, zero or negative integers. However there is no restriction on it’s keys and values. Although values are intended to be numbers but we can store other objects too.

Creating Python Counter Object

We can create an empty Counter or start with some initial values too.

from collections import Counter

# empty Counter
counter = Counter()
print(counter)  # Counter()

# Counter with initial values
counter = Counter(['a', 'a', 'b'])
print(counter)  # Counter({'a': 2, 'b': 1})

counter = Counter(a=2, b=3, c=1)
print(counter)  # Counter({'b': 3, 'a': 2, 'c': 1})

We can also use any Iterable as argument for creating Counter object. So string literal and List can be used too for creating Counter object.

# Iterable as argument for Counter
counter = Counter('abc')
print(counter)  # Counter({'a': 1, 'b': 1, 'c': 1})

# List as argument to Counter
words_list = ['Cat', 'Dog', 'Horse', 'Dog']
counter = Counter(words_list)
print(counter)  # Counter({'Dog': 2, 'Cat': 1, 'Horse': 1})

# Dictionary as argument to Counter
word_count_dict = {'Dog': 2, 'Cat': 1, 'Horse': 1}
counter = Counter(word_count_dict)
print(counter)  # Counter({'Dog': 2, 'Cat': 1, 'Horse': 1})

As I mentioned above, we can use non-numeric data for count values too, but that will defect the purpose of Counter class.

# Counter works with non-numbers too
special_counter = Counter(name='Pankaj', age=20)
print(special_counter)  # Counter({'name': 'Pankaj', 'age': 20})

Python Counter Methods

Let’s look into Counter class methods and some other operations we can perform on it.

Getting Count of Elements

# getting count
counter = Counter({'Dog': 2, 'Cat': 1, 'Horse': 1})
countDog = counter['Dog']
print(countDog)  # 2

If we try to get the count of non-existing key, it will return 0 and not throw KeyError.

# getting count for non existing key, don't cause KeyError
print(counter['Unicorn'])  # 0

Setting Count of Elements

We can also set the count of existing element in the counter. If the element doesn’t exist, then it gets added to the counter.

counter = Counter({'Dog': 2, 'Cat': 1, 'Horse': 1})
# setting count
counter['Horse'] = 0
print(counter)  # Counter({'Dog': 2, 'Cat': 1, 'Horse': 0})

# setting count for non-existing key, adds to Counter
counter['Unicorn'] = 1
print(counter)  # Counter({'Dog': 2, 'Cat': 1, 'Unicorn': 1, 'Horse': 0})

Deleting an element from Counter

We can use del to delete an element from the counter object.

# Delete element from Counter
del counter['Unicorn']
print(counter)  # Counter({'Dog': 2, 'Cat': 1, 'Horse': 0})

elements()

This method returns the list of elements in the counter. Only elements with positive counts are returned.

counter = Counter({'Dog': 2, 'Cat': -1, 'Horse': 0})

# elements()
elements = counter.elements()  # doesn't return elements with count 0 or less
for value in elements:
    print(value)

Above code will print “Dog” two times because it’s count is 2. Other elements will be ignored because they don’t have positive count. Counter is an unordered collection, so elements are returned in no particular order.

most_common(n)

This method returns the most common elements from the counter. If we don’t provide value of ‘n’ then sorted dictionary is returned from most common the least common elements. We can use slicing to get the least common elements on this sorted dictionary.

counter = Counter({'Dog': 2, 'Cat': -1, 'Horse': 0})

# most_common()
most_common_element = counter.most_common(1)
print(most_common_element)  # [('Dog', 2)]

least_common_element = counter.most_common()[:-2:-1]
print(least_common_element)  # [('Cat', -1)]

subtract() and update()

Counter subtract() method is used to subtract element counts from another counter. update() method is used to add counts from another counter.

counter = Counter('ababab')
print(counter)  # Counter({'a': 3, 'b': 3})
c = Counter('abc')
print(c)  # Counter({'a': 1, 'b': 1, 'c': 1})

# subtract
counter.subtract(c)
print(counter)  # Counter({'a': 2, 'b': 2, 'c': -1})

# update
counter.update(c)
print(counter)  # Counter({'a': 3, 'b': 3, 'c': 0})

Python Counter Arithmetic Operations

We can perform some arithmetic operations on Counters too, just like numbers. However only elements with positive count are returned with these operations.

# arithmetic operations
c1 = Counter(a=2, b=0, c=-1)
c2 = Counter(a=1, b=-1, c=2)

c = c1 + c2  # return items having +ve count only
print(c)  # Counter({'a': 3, 'c': 1})

c = c1 - c2  # keeps only +ve count elements
print(c)  # Counter({'a': 1, 'b': 1})

c = c1 & c2  # intersection min(c1[x], c2[x])
print(c)  # Counter({'a': 1})

c = c1 | c2  # union max(c1[x], c2[x])
print(c)  # Counter({'a': 2, 'c': 2})

Miscellaneous Operations on Python Counter

Let’s look at some code snippets for miscellaneous operations we can perform on Counter objects.

counter = Counter({'a': 3, 'b': 3, 'c': 0})
# miscellaneous examples
print(sum(counter.values()))  # 6

print(list(counter))  # ['a', 'b', 'c']
print(set(counter))  # {'a', 'b', 'c'}
print(dict(counter))  # {'a': 3, 'b': 3, 'c': 0}
print(counter.items())  # dict_items([('a', 3), ('b', 3), ('c', 0)])

# remove 0 or negative count elements
counter = Counter(a=2, b=3, c=-1, d=0)
counter = +counter
print(counter)  # Counter({'b': 3, 'a': 2})

# clear all elements
counter.clear()
print(counter)  # Counter()

That’s all for Python Counter class.

You can download the complete example code from my GitHub Repository.

Reference: Python Docs