collections.Counter

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Counter is a dict sub class that allows you to easily count objects. It has utility methods for working with the frequencies of the objects that you are counting.

import collections
counts = collections.Counter([1,2,3])

the above code creates an object, counts, which has the frequencies of all the elements passed to the constructor. This example has the value Counter({1: 1, 2: 1, 3: 1})

Constructor examples

Letter Counter

>>> collections.Counter('Happy Birthday')
Counter({'a': 2, 'p': 2, 'y': 2, 'i': 1, 'r': 1, 'B': 1, ' ': 1, 'H': 1, 'd': 1, 'h': 1, 't': 1})

Word Counter

>>> collections.Counter('I am Sam Sam I am That Sam-I-am That Sam-I-am! I do not like that Sam-I-am'.split())
Counter({'I': 3, 'Sam': 2, 'Sam-I-am': 2, 'That': 2, 'am': 2, 'do': 1, 'Sam-I-am!': 1, 'that': 1, 'not': 1, 'like': 1})

Recipes

>>> c = collections.Counter({'a': 4, 'b': 2, 'c': -2, 'd': 0})

Get count of individual element

>>> c['a']
4

Set count of individual element

>>> c['c'] = -3
>>> c
Counter({'a': 4, 'b': 2, 'd': 0, 'c': -3})

Get total number of elements in counter (4 + 2 + 0 - 3)

>>> sum(c.itervalues())  # negative numbers are counted!
3

Get elements (only those with positive counter are kept)

>>> list(c.elements())
['a', 'a', 'a', 'a', 'b', 'b']

Remove keys with 0 or negative value

>>> c - collections.Counter()
Counter({'a': 4, 'b': 2})

Remove everything

>>> c.clear()
>>> c
Counter()

Add remove individual elements

>>> c.update({'a': 3, 'b':3})
>>> c.update({'a': 2, 'c':2})  # adds to existing, sets if they don't exist
>>> c
Counter({'a': 5, 'b': 3, 'c': 2})
>>> c.subtract({'a': 3, 'b': 3, 'c': 3})  # subtracts (negative values are allowed)
>>> c
Counter({'a': 2, 'b': 0, 'c': -1})

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