python 排列组合函数

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在使用python进行运算的时候，经常会遇到对list或者dict中任意两个元素，或者任意多个元素之间进行运算，这就涉及到了排列组合的知识。比较幸运的是在python的itertools包中提供了和排列组合相关的函数

上代码：

from itertools import combinations,permutations

a=[1,2,3,4]
b=[2,4,6,8]
c = list(combinations(a,2))
d = list(permutations(b,3))
print(c)
print(d)


运算结果：
[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]
[(2, 4, 6), (2, 4, 8), (2, 6, 4), (2, 6, 8), (2, 8, 4), (2, 8, 6), (4, 2, 6), (4, 2, 8), (4, 6, 2), (4, 6, 8), (4, 8, 2), (4, 8, 6), (6, 2, 4), (6, 2, 8), (6, 4, 2), (6, 4, 8), (6, 8, 2), (6, 8, 4), (8, 2, 4), (8, 2, 6), (8, 4, 2), (8, 4, 6), (8, 6, 2), (8, 6, 4)]

很好用吧，下面附带一个例子供大家学习

import math
import itertools
from itertools import combinations

print(unique)
## [ 1  2  3  4  5 56 78 23]

i=0
for combination in combinations(unique, 2):
    print(combination)
    i += 1
print(i) 
## 输出为28

print(math.factorial( len(unique) )/(2 * math.factorial( len(unique)-2 )))
## 输出为28

## 笛卡尔积
d = 0
uniques = [unique, unique]
for combination in itertools.product(*uniques):
    print(combination)
    d += 1
print(d)
## 输出为64