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 substract values between dictionaries d1-d2 and get the result:
d3 = {'a': 9, 'b': 7, 'c': 5, 'd': 7 }
Now I'm using two loops but this solution is not too fast
for x,i in enumerate(d2.keys()):
for y,j in enumerate(d1.keys()):
I think a very Pythonic way would be using dict comprehension :
d3 = {key: d1[key] - d2.get(key, 0) for key in d1}
Note that this only works in Python 2.7+ or 3.
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:
print(d2-d1)
can yield Counter({'e': 2})
.Just an update to Haidro answer.
Recommended to use subtract method instead of "-".
d1.subtract(d2)
When - is used, only positive counters are updated into dictionary. See examples below
c = Counter(a=4, b=2, c=0, d=-2)
d = Counter(a=1, b=2, c=3, d=4)
a = c-d
print(a) # --> Counter({'a': 3})
c.subtract(d)
print(c) # --> Counter({'a': 3, 'b': 0, 'c': -3, 'd': -6})
Please note the dictionary is updated when subtract method is used.
And finally use dict(c) to get Dictionary from Counter object
Haidro posted an easy solution, but even without collections
you only need one loop:
d1 = {'a': 10, 'b': 9, 'c': 8, 'd': 7}
d2 = {'a': 1, 'b': 2, 'c': 3, 'e': 2}
d3 = {}
for k, v in d1.items():
d3[k] = v - d2.get(k, 0) # returns value if k exists in d2, otherwise 0
print(d3) # {'c': 5, 'b': 7, 'a': 9, 'd': 7}
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