We have summarized the nature of collections.Counter that gets the number of elements of iterable. Besides counting, it has other useful properties that dict does not have, so I would like to actively use it.
Addition / subtraction and set calculation are possible. Keys whose value is less than or equal to zero are automatically deleted.
from collections import Counter
A = Counter('ab') # Counter({'a': 1, 'b': 1})
B = Counter('bc') # Counter({'c': 1, 'b': 1})
A + B # Counter({'b': 2, 'a': 1, 'c': 1})
A - B # Counter({'a': 1})
A & B # Counter({'b': 1})
A | B # Counter({'a': 1, 'b': 1, 'c': 1})
Unlike operators, keys with values less than or equal to zero are also retained. ʻElements` returns only keys with positive values.
from collections import Counter
A = Counter('ab')
B = Counter('bc')
C = Counter('ac')
C.update(A) # Counter({'a': 2, 'c': 1, 'b': 1})
A.subtract(B) # Counter({'a': 1, 'b': 0, 'c': -1})
A.most_common() # [('a', 1), ('b', 0), ('c', -1)]
sorted(C.elements()) # ['a', 'a', 'b', 'c']
sorted(A.elements()) # ['a']
If you change the element directly, the value below zero is allowed as in the method. References to non-existent elements return 0 without exception.
from collections import Counter
A = Counter('ab')
A['a'] += 2 # Counter({'a': 3, 'b': 1})
a['b'] -= 2 # Counter({'a': 3, 'b': -1})
A['c'] # 0
The unary operator is a shortcut for operations on the empty Counter ().
Only positive or negative values can be extracted after the operation for each element.
A # Counter({'a': 3, 'b': -1})
+A # Counter({'a': 3})
-A # Counter({'b': 1})
Recommended Posts