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 Countercounter = Counter()print(counter) # Counter()# Counter with initial valuescounter = 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 Countercounter = Counter('abc')print(counter) # Counter({'a': 1, 'b': 1, 'c': 1})# List as argument to Counterwords_list = ['Cat', 'Dog', 'Horse', 'Dog']counter = Counter(words_list)print(counter) # Counter({'Dog': 2, 'Cat': 1, 'Horse': 1})# Dictionary as argument to Counterword_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 toospecial_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 countcounter = Counter({'Dog': 2, 'Cat': 1, 'Horse': 1})countDog = counter['Dog']print(countDog) # 2If 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 KeyErrorprint(counter['Unicorn']) # 0Setting 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 countcounter['Horse'] = 0print(counter) # Counter({'Dog': 2, 'Cat': 1, 'Horse': 0})# setting count for non-existing key, adds to Countercounter['Unicorn'] = 1print(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 Counterdel 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 lessfor 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})# subtractcounter.subtract(c)print(counter) # Counter({'a': 2, 'b': 2, 'c': -1})# updatecounter.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 operationsc1 = Counter(a=2, b=0, c=-1)c2 = Counter(a=1, b=-1, c=2)c = c1 + c2 # return items having +ve count onlyprint(c) # Counter({'a': 3, 'c': 1})c = c1 - c2 # keeps only +ve count elementsprint(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 examplesprint(sum(counter.values())) # 6print(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 elementscounter = Counter(a=2, b=3, c=-1, d=0)counter = +counterprint(counter) # Counter({'b': 3, 'a': 2})# clear all elementscounter.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
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