Python 如何从字典中减去值
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How to subtract values from dictionaries
提问by Colargol
I have two dictionaries in Python:
我在 Python 中有两个字典：
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:
我想减去字典 d1-d2 之间的值并得到结果：
d3 = {'a': 9, 'b': 7, 'c': 5, 'd': 7 }
Now I'm using two loops but this solution is not too fast
现在我使用了两个循环，但这个解决方案并不太快
for x,i in enumerate(d2.keys()):
for y,j in enumerate(d1.keys()):
采纳答案by Erfa
I think a very Pythonic way would be using dict comprehension:
我认为一种非常 Pythonic 的方式是使用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.
请注意，这仅适用于 Python 2.7+ 或 3。
回答by TerryA
Use collections.Counter, iif all resulting values are known to be strictly positive. The syntax is very easy:
collections.Counter如果已知所有结果值都严格为正，则使用, 。语法非常简单：
>>> 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 strictlypositive:
请注意，如果不是所有值都已知为严格正值：
- 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 yieldCounter({'e': 2}).
- 结果中将省略值为零的元素
- 值变为负的元素将丢失，或替换为错误的值。例如，
print(d2-d1)可以 yieldCounter({'e': 2})。
回答by joente
Haidro posted an easy solution, but even without collectionsyou only need one loop:
Haidro 发布了一个简单的解决方案，但即使没有collections您也只需要一个循环：
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}
回答by Hemanth
Just an update to Haidro answer.
只是对 Haidro 答案的更新。
Recommended to use subtract method instead of "-".
建议使用减法而不是“-”。
d1.subtract(d2)
d1.subtract(d2)
When - is used, only positive counters are updated into dictionary. See examples below
使用 - 时，仅将正计数器更新到字典中。请参阅下面的示例
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.
请注意，当使用减法方法时，字典会更新。
And finally use dict(c) to get Dictionary from Counter object
最后使用 dict(c) 从 Counter 对象中获取 Dictionary
