{"id":1585,"date":"2019-12-27T13:16:30","date_gmt":"2019-12-27T13:16:30","guid":{"rendered":"https:\/\/www.askpython.com\/?p=1585"},"modified":"2022-08-06T13:17:15","modified_gmt":"2022-08-06T13:17:15","slug":"counter-in-python","status":"publish","type":"post","link":"https:\/\/www.askpython.com\/python\/counter-in-python","title":{"rendered":"Counter in Python"},"content":{"rendered":"\n

A Counter is a subclass of dict<\/code> and is part of the Collections<\/strong> module. It is used for counting hashable objects.<\/p>\n\n\n\n

It is an unordered collection where the elements are stored as Dictionary Keys and their counts are the values.<\/p>\n\n\n\n

Counter Object format: {element1: count1, element2: count2}<\/code><\/p>\n\n\n\n

Elements are counted from an\u00a0iterable<\/em>\u00a0or initialized from another mapping\u00a0(or counter)<\/p>\n\n\n\n

Initialization of the Counter<\/code> object is done using the Counter()<\/code> call.<\/p>\n\n\n\n

We can also pass an iterable into the call and get the corresponding mapped object.<\/p>\n\n\n

\n>>> from collections import Counter\n>>> # Empty Counter object\n>>> c = Counter()\n>>> c\nCounter()\n\n>>> # Tuple of values\n>>> d = Counter(a=1, b=2, c=1)\n>>> d\nCounter({'b': 2, 'a': 1, 'c': 1})\n\n>>> # Pass a list, which is an iterable\n>>> e = Counter(['a', 'b', 'a', 'c'])\n>>> e\nCounter({'a': 2, 'b': 1, 'c': 1})\n\n>>> # Pass a string, also an iterable\n>>> f = Counter('red-dish')\n>>> f\nCounter({'d': 2, 'r': 1, 'e': 1, '-': 1, 'i': 1, 's': 1, 'h': 1})\n\n>>> # Pass a Dictionary\n>>> g = Counter({'a': 10, 'b': 11})\n>>> g\nCounter({'b': 11, 'a': 10})\n<\/pre><\/div>\n\n\n
Note that when the Counter object is displayed, the key-value pairs are displayed on the order of descending counts.<\/p>\n\n\n\n
Counter objects have a dictionary interface except that they return a zero count for missing items instead of raising a KeyError<\/a><\/code>.<\/p>\n\n\n\n
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Counter Methods<\/h2>\n\n\n\n
1. Get the count of individual elements<\/h3>\n\n\n\n
Individual element counts are accessed in the same way as that of a Dictionary, meaning that counter_object[key]<\/code> gives the count of key<\/code>.<\/p>\n\n\n
\n>>> c = Counter(a=1, b=2, c=1)\n>>> c\nCounter({'b': 2, 'a': 1, 'c': 1})\n>>> c['b']\n2\n>>> c['d'] # Does not give KeyError, unlike a Dictionary\n0\n<\/pre><\/div>\n\n\n
2. Set the count of elements<\/h3>\n\n\n\n
To set the count of an element, use counter_object[key] = value<\/code>. If key<\/code> does not exist, it gets added to the Counter Dictionary, along with the new count.<\/p>\n\n\n
\n>>> c = Counter(a=1, b=2, c=1)\n>>> c\nCounter({'b': 2, 'a': 1, 'c': 1})\n>>> c['d'] = 4\n>>> c\nCounter({'d': 4, 'b': 2, 'a': 1, 'c': 1})\n<\/pre><\/div>\n\n\n
3. Remove elements from the Counter<\/h3>\n\n\n\n
To remove a key from the Counter object, use del counter_object[key]<\/code>.<\/p>\n\n\n
\n>>> del c['d']\n>>> c\nCounter({'b': 2, 'a': 1, 'c': 1})\n<\/pre><\/div>\n\n\n
4. elements()<\/h3>\n\n\n\n
This method returns an iterator over the elements, values of which are repeated as many times as their counts. This method ignores all<\/em> elements that have their counts less than one.<\/p>\n\n\n
\n>>> c\nCounter({'b': 2, 'a': 1, 'c': 1})\n>>> c['d'] = -1\n>>> c\n>>> c.elements()\n<itertools.chain object at 0x102e2a208>\n>>> type(c.elements())\n<class 'itertools.chain'>\n>>> for i in c.elements():\n...     print(i)\n...\na\nb\nb\nc\n>>> list(c.elements())\n['a', 'b', 'b', 'c']\n\n>>> c['d'] = -1\n>>> c\nCounter({'b': 2, 'a': 1, 'c': 1, 'd': -1})\n>>> # Ignores d since count[d] < 1\n>>> list(c.elements())\n['a', 'b', 'b', 'c']\n<\/pre><\/div>\n\n\n
5. most_common(n)<\/h3>\n\n\n\n
This returns a list of the n<\/em> most common elements and their counts from the most common to the least. If n<\/em> is omitted or None<\/code>, most_common()<\/em><\/code> returns all<\/em> elements in the counter. Elements with equal counts are ordered arbitrarily.<\/p>\n\n\n
\n>>> c\nCounter({'b': 2, 'a': 1, 'c': 1, 'd': -1})\n>>> c.most_common()\n[('b', 2), ('a', 1), ('c', 1), ('d', -1)]\n>>> c.most_common(2)\n[('b', 2), ('a', 1)]\n<\/pre><\/div>\n\n\n
6. subtract(iterable\/mapping)<\/h3>\n\n\n\n
This returns a mapping\/iterable after subtracting the contents of the two iterables\/mappings. Elements are not replaced, and only their counts are subtracted.<\/p>\n\n\n
\n>>> a = Counter('redblue')\n>>> a\nCounter({'e': 2, 'r': 1, 'd': 1, 'b': 1, 'l': 1, 'u': 1})\n>>> b = Counter('blueorange')\n>>> b\nCounter({'e': 2, 'b': 1, 'l': 1, 'u': 1, 'o': 1, 'r': 1, 'a': 1, 'n': 1, 'g': 1})\n\n>>> # Subtracts b from a and updates a accordingly\n>>> a.subtract(b)\n>>> a\nCounter({'d': 1, 'r': 0, 'e': 0, 'b': 0, 'l': 0, 'u': 0, 'o': -1, 'a': -1, 'n': -1, 'g': -1})\n<\/pre><\/div>\n\n\n
7. update(iterable\/mapping)<\/h3>\n\n\n\n
This is similar to subtract()<\/code>, but only adds counts instead of subtracting them.<\/p>\n\n\n
\n>>> a = Counter('12321')\n>>> b = Counter('23432')\n>>> a\nCounter({'1': 2, '2': 2, '3': 1})\n>>> b\nCounter({'2': 2, '3': 2, '4': 1})\n\n>>> # Add counts into a\n>>> a.update(b)\n>>> a\nCounter({'2': 4, '3': 3, '1': 2, '4': 1})\n<\/pre><\/div>\n\n\n
Other Counter() methods<\/h2>\n\n\n\n