Question

有一个包含许多列表的字典，例如

  list_dic= {
    q1:[1,2,3,4,5]
    q2:[2,3,5]
    q3:[2,5]
    }

我希望得到每个列表的所有常用项目，例如q1和q2的公共项目计数是3 =（2,3,5）

q1={q2:3, q3:2}
q2={q1:3,q3:2}
q3={q1:2, q2:2}

我执行此任务的代码是：

result = {}
for name, source_list in list_dic.items():
    for target_name, target_list in list_dic.items():
        count = 0
        for item in source_list:
            if item in target_list:
                count+=1
    result[name][target_name] = count

但是这个算法效率很低，我想知道一个更好的算法来完成这个任务

Answer 1

我认为应该这样做：

import itertools
import collections

q1 = 'q1'
q2 = 'q2'
q3 = 'q3'

dic_list = {
     q1:[1,2,3,4,5],
     q2:[2,3,5],
     q3:[2,5]
     }

#sets are much more efficient for this sort of thing.  Create a dict
#of the same structure as the old one, only with `set` as values 
#instead of `list`
dic_set = {k:set(v) for k,v in dic_list.items()}

new_dic = collections.defaultdict(dict)
for k1,k2 in itertools.combinations(dic_set,2):
     #to get the count, we just need to know the size of the intersection
     #of the 2 sets.
     value = len(dic_set[k1] & dic_set[k2]) 
     new_dic[k1][k2] = value
     new_dic[k2][k1] = value

print (new_dic)

如果您关注评论，则combinations略快于permutations：

import itertools
import collections

q1 = 'q1'
q2 = 'q2'
q3 = 'q3'


dic_list = {
     q1:[1,2,3,4,5],
     q2:[2,3,5],
     q3:[2,5]
     }

dic_set = {k:set(v) for k,v in dic_list.items()}

def combo_solution():
     new_dic = collections.defaultdict(dict)
     for k1,k2 in itertools.combinations(dic_set,2):
          value = len(dic_set[k1] & dic_set[k2])
          new_dic[k1][k2] = value
          new_dic[k1][k2] = value
     return new_dic

def perm_solution():
     new_dic = collections.defaultdict(dict)
     for k1, k2 in itertools.permutations(dic_set,2):
          new_dic[k1][k2] = len(dic_set[k1] & dic_set[k2])
     return new_dic

import timeit
print timeit.timeit('combo_solution()','from __main__ import combo_solution',number=100000)
print timeit.timeit('perm_solution()','from __main__ import perm_solution',number=100000)

结果：

0.58366894722    #combinations
0.832300901413   #permutations

这是因为set.intersection是一个O（min（N，M））操作 - 这很便宜，但如果你做的次数是你需要的两倍，可以加起来。

Answer 2

from collections import defaultdict
#Create a default dict. You don;t have to handle KeyError condition
result = defaultdict(dict)
list_dic= {
    'q1':[1,2,3,4,5],
    'q2':[2,3,5],
    'q3':[2,5],
    }
#Convert the value list to set list
set_dict = {k:set(v) for k,v in list_dic.items()}
# For both way mapping, you need permutation i.e. (q1, q2) and (q2, q1)
for k1, k2 in permutations(set_dict.keys(),2):
    # Now `&` is Set Intersection. The Len will return the length of the common elements
    result[k1][k2] = len(set_dict[k1] & set_dict[k2])


result
defaultdict(<type 'dict'>, {'q1': {'q3': 2, 'q2': 3}, 'q3': {'q1': 2, 'q2': 2}, 'q2': {'q1': 3, 'q3': 2}})

Answer 3

如果您不打扰列表是否可以包含重复的数字，您可以使用set（）类型

来执行此操作

>>> s1 = set([1,2,3,4,5])
>>> s2 = set([3,4,5,6,7,8])
>>> s1 & s2
{3, 4, 5}

如何获取列表的常用项目数

3 个答案: