How to subtract values from dictionaries

Question

I have two dictionaries in Python:

d1 = {\'a\': 10, \'b\': 9, \'c\': 8, \'d\': 7}
d2 = {\'a\': 1, \'b\': 2, \'c\': 3, \'e\': 2}

I want to s

Answer 1

Use collections.Counter, iif all resulting values are known to be strictly positive. The syntax is very easy:

>>> from collections import Counter
>>> d1 = Counter({'a': 10, 'b': 9, 'c': 8, 'd': 7})
>>> d2 = Counter({'a': 1, 'b': 2, 'c': 3, 'e': 2})
>>> d3 = d1 - d2
>>> print d3
Counter({'a': 9, 'b': 7, 'd': 7, 'c': 5})

Mind, if not all values are known to remain strictly positive:

• elements with values that become zero will be omitted in the result
• elements with values that become negative will be missing, or replaced with wrong values. E.g., print(d2-d1) can yield Counter({'e': 2}).