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Counter

The Counter is a subclass of dict, as are defaultdict and OrderedDict.

It is designed to count iterables such as strings, lists, dictionaries, etc.

Table of Contents

Basic

There are several ways to initialize a Counter object. For example:

  1. You can count every character from a string.
  2. You can count every item in a list.
  3. You can initialize the Counter with an existed dict.
  4. You can initialize the Counter with keyword arguments.
c = Counter('gallahad')
# Counter({'a': 3, 'l': 2, 'g': 1, 'h': 1, 'd': 1})

c = Counter(["apple", "apple", "banana"])
# Counter({'apple': 2, 'banana': 1})

c = Counter({'red': 4, 'blue': 2})
# Counter({'red': 4, 'blue': 2})

c = Counter(cats=4, dogs=8)
# Counter({'dogs': 8, 'cats': 4})

The default value for elements that are not counted is 0.

c = Counter('gallahad')
c["e"]  # 0

update()

You can add values to the Counter by using update() method.

c = Counter(cats=4, dogs=8)

c.update(birds=10)
# Counter({'cats': 4, 'dogs': 8, 'birds': 10})

elements()

The method elements() will return an iterator that outputs all the elements from the Counter in the order first encountered.

c = Counter(cats=4, dogs=8)
list(c.elements())
# ['cats', 'cats', 'cats', 'cats', 'dogs', 'dogs', 'dogs', 'dogs', 'dogs', 'dogs', 'dogs', 'dogs']

most_common(int)

The method most_common(n: int) will return a list of top n key-value pairs that have been counted the most times.

If n > len(counter) OR n is omitted or None, it will return a list of all elements.

c = Counter(cats=4, dogs=8)
c.most_common(1)  # [('dogs', 8)]
c.most_common(2)  # [('dogs', 8), ('cats', 4)]
c.most_common(3)  # [('dogs', 8), ('cats', 4)]
c.most_common()   # [('dogs', 8), ('cats', 4)]

subtract()

The method subtract can do the subtraction for two Counters, and the results of counts can become negative.

c = Counter(cats=4, dogs=8)
d = Counter(cats=5, dogs=3)

c.subtract(d)  # Counter({'cats': -1, 'dogs': 5})

In the contrast, you can also use the magic method __sub__ (-) to do the subtraction, but this will remove the elements whose count is less than zero.

c = Counter(cats=4, dogs=8)
d = Counter(cats=5, dogs=3)

c - d  # Counter({'dogs': 5})

Magic method

Counter has built-in magic methods for add, subtract, intersect and union.

  • add: c[x] + d[x]
  • sub: c[x] - d[x] (keeping only positive counts)
  • intersect: min(c[x], d[x])
  • union: max(c[x], d[x])
c = Counter(cats=4, dogs=8)
d = Counter(cats=5, dogs=3)

c + d  # Counter({'cats': 9, 'dogs': 11})
c - d  # Counter({'dogs': 5})
c & d  # Counter({'cats': 4, 'dogs': 3})
c | d  # Counter({'cats': 5, 'dogs': 8})

Related Articles

Article Link
collections - Counter https://docs.python.org/3/library/collections.html#collections.Counter
Python Counter in Collections with Example https://www.guru99.com/python-counter-collections-example.html
7 More Tricks to Write Better Python Code https://youtu.be/SNTZpy0oDB8?list=LL&t=1609