Question

我正在运行一个Python脚本，它迭代两个巨大的列表并找到匹配的对。

然而，它似乎需要永远。如何加快这个脚本？

import sys
import random
import itertools

def main(args):
    target_num = int(999999999)
    num_list = range(1, target_num)
    rand_list = []
    hit_list = []

    for _ in itertools.repeat(None, target_num):
        rand_list.append(random.randint(1, target_num))

    for num in num_list:
        for rand_num in rand_list:
            if num == rand_num:
                print "hit"

if __name__ == "__main__":
    main(sys.argv[1:])

Answer 1

使用套装

import sys
import random
import itertools

def main(args):
    target_num = int(999999999)
    num_list = set(range(1, target_num))
    rand_list = []
    hit_list = []

    for _ in itertools.repeat(None, target_num):
        rand_list.append(random.randint(1, target_num))


    for num in rand_list:
        if num in num_list: # O(1)
            print "hit"

if __name__ == "__main__":
    main(sys.argv[1:])

使用第一个列表的集合意味着检查该项目是否在该列表中现在减少为O（1）

在我写这篇文章时，我意识到你甚至可以做得更好。 python 3中的range函数返回一个序列，因此下一部分需要python 3

import sys
import random
import itertools

def main(args):
    target_num = int(999999999)
    num_list = range(1, target_num) # this is a generator
    rand_list = []
    hit_list = []

    for _ in itertools.repeat(None, target_num):
        rand_list.append(random.randint(1, target_num))


    for num in rand_list:
        if num in num_list: # Stil O(1)
            print ("hit")

if __name__ == "__main__":
    main(sys.argv[1:])

更好的是，使用范围并在第一个循环中进行检查？

for _ in itertools.repeat(None, target_num):
    rand_num = random.randint(1, target_num)
    rand_list.append(rand_num)
    if rand_num in num_list:
        print ("hit")

Answer 2

如果使用Python 2，请使用xrange（），它返回类似于生成器的对象。

# requires Python 2
import random

target_num = 99 # 999999999 are too much items for testing

# target_num random numbers in range 1 .. target_num-1
random_numbers = set(random.randint(1, target_num) for _ in xrange(target_num)) 

hits = set()
for num in xrange(1, target_num):  # check for all numbers in range 1 .. target_num-1
    if num in random_numbers:   # num in set() is O(1)
        hits.add(num)

if len(random_numbers - hits) == 0:
     print "all random numbers are hits!"

# so:
for num in random_numbers:
    print num
# is the same result

如何加速Python脚本迭代嵌套循环？

2 个答案: