python - 在 Python 字典中求和值？

Question

添加相同键时如何对python dict中的值求和？

d = {'key1':10,'key2':14,'key3':47}
d['key1'] = 20

上面的值d['key1']应该是30。

这可能吗？

Answer 1

您可以使用collections.Counter：

>>> from collections import Counter
>>> d =Counter()
>>> d.update({'key1':10,'key2':14,'key3':47})
>>> d['key1'] += 20
>>> d['key4'] += 50  # Also works for keys that are not present
>>> d
Counter({'key4': 50, 'key3': 47, 'key1': 30, 'key2': 14})

计数器有一些优点：

>>> d1 = Counter({'key4': 50, 'key3': 4})
#You can add two counters
>>> d.update(d1)
>>> d
Counter({'key4': 100, 'key3': 51, 'key1': 30, 'key2': 14})

您可以使用以下方法获取已排序项目的列表（基于值）most_common()：

>>> d.most_common()
[('key4', 100), ('key3', 51), ('key1', 30), ('key2', 14)]

时间比较：

>>> keys = [ random.randint(0,1000) for _ in xrange(10**4)]
>>> def dd():                             
    d = defaultdict(int)
    for k in keys:
        d[k] += 10
...         
>>> def count():                             
    d = Counter()
    for k in keys:
        d[k] += 10
...         
>>> def simple_dict():
...     d = {}
...     for k in keys:
...         d[k] = d.get(k,0) + 10
...         
>>> %timeit dd()
100 loops, best of 3: 3.47 ms per loop
>>> %timeit count()
100 loops, best of 3: 10.1 ms per loop
>>> %timeit simple_dict()
100 loops, best of 3: 5.01 ms per loop

Answer 2

from collections import defaultdict
d = defaultdict(int)
d['key1'] += 20

Answer 3

try:
   dict[key1]+=20 #The value you wanted
except KeyError:
   dict[key1]=10 #The initial Value

Answer 4

d = {'key1':10,'key2':14,'key3':47}

当我们添加相同的键时，在一行中对 python dict 中的值求和的解决方案：

d['key1'] = dict.get('key1', 0) + 20

解释：

dict.get('key1', 0)

如果在 dict 中找到，这将返回键值，否则返回默认值为 0