Counter Cookbook

counter_cookbook.py

"""
    The following is an attempt to gather various useful usecases of 
    collections.Counter in python. Some were once contributed to
    Stackoverflow Documentation Experiment but since its shut down
    I could not recover them.
"""

# imports
from collections import Counter
import random

# Construct Counter

## Empty Counter
c = Counter()  # Counter()

## From list
c = Counter(['a', 'b', 'a', 'c'])  # Counter({'a': 2, 'b': 1, 'c': 1})
c = Counter('abac')  # Counter({'a': 2, 'b': 1, 'c': 1})

## From dictionary
c = Counter({'a': 2, 'b': 1, 'c': 1})  # Counter({'a': 2, 'b': 1, 'c': 1})

# Modify Counter

## Add elements
c = Counter(['b'])
c['a'] += 1  # Counter({'a': 1, 'b': 1})
c.update(['a'])  # Counter({'a': 2, 'b': 1})
c.update(['a', 'b', 'c'])  # Counter({'a': 3, 'b': 2, 'c': 1})
c['c'] += 3  # Counter({'a': 3, 'b': 2, 'c': 4})

## Remove elements
c['c'] -= 2  # Counter({'a': 3, 'b': 2, 'c': 2})
c['b'] = 0  # Counter({'a': 3, 'b': 0, 'c': 2})
c.subtract('a'*4)  # Counter({'a': -1, 'b': 0, 'c': 2})

## Delete elements
c += Counter()  # Counter({'c': 2}) - remove zero and negative counts
#! Adding or subtracting any counter will have the same effect
del c['c']  # Counter()

# Counting
## Simple access
c = Counter(a=4, b=2, c=0, d=-2)
c['a']  # 4
c['e']  # 0 (missing element)
## Access 3 most common
c.most_common(3)  # [('a', 4), ('b', 2), ('c', 0)]
## Access 3 least common
c.most_common()[:-3-1:-1]  # [('d', -2), ('c', 0), ('b', 2)]

# Sampling (not necessarily optimal)
## Sample elements (ignores 0 and negative)
c = Counter(a=4, b=2, c=0, d=-2)
els = list(c.elements())  # ['a', 'a', 'a', 'a', 'b', 'b']
random.choice(els)
random.sample(els, 6)

## Sample keys
c = Counter(a=4, b=2, c=0, d=-2)
random.choice(list(c.keys()))
random.sample(c.keys(), 4)

## Sample positive keys
random.choice([x for x in c if c[x]>0])
random.sample([x for x in c if c[x]>0], 2)

## Sample one of the least common positive keys
(c + Counter()).most_common()[-1][0]