问题描述
以下是我发现的Python程序，它使用来查找素数。它使用过滤器和生成器。我无法理解。
Below is a python program I found to find prime numbers using Sieve of Eratosthenes. It uses filter and generator. I'm not able to understand it.
def _odd_iter():
n = 1
while True:
n = n + 2
yield n
def _not_divisible(n):
return lambda x: x % n > 0
def primes():
yield 2
it = _odd_iter()
while True:
n = next(it)
yield n
it = filter(_not_divisible(n), it)
for n in primes():
if n < 1000:
print(n)
else:
break
我不明白的是 it = filter（_not_divisible（n），它）。例如，对于数字105，如何用单行代码排除它？
What I don't understand is it = filter(_not_divisible(n), it). For example for number 105, how is it excluded by this single line of code?
推荐答案
对于找到的每个素数， filter 应用于迭代器，所使用的过滤器是一个功能，它排除素数的所有倍数。
For each prime number found a filter is applied to the iterable, the filter used is a function that excludes all multiples of the prime number.
将iterable包裹在与您发现质数一样多的过滤器中，例如，排除了数字105，因为它可以被3整除，并且在找到质数3时添加了3的所有倍数的过滤器。
So your iterable is wrapped in as many filters as you found prime numbers, for example the number 105 is excluded because it's divisible by 3 and the filter for all multiples of 3 was added when you found the prime number 3.
如果添加一些 print 语句，它将更加清晰（我希望如此）：
If you add some print statements it will be a bit clearer (I hope):
def _odd_iter():
n = 1
while True:
n = n + 2
yield n
def _not_divisible(n):
print('add filter for all multiples of', n)
return lambda x: print('check if', x, 'is divisible by', n, 'result: ', not (x % n > 0)) or x % n > 0
def primes():
yield 2
it = _odd_iter()
while True:
n = next(it)
yield n
it = filter(_not_divisible(n), it)
for n in primes():
if n < 20:
print(n)
else:
break
哪些打印：
2
3
add filter for all multiples of 3
check if 5 is divisible by 3 result: False
5
add filter for all multiples of 5
check if 7 is divisible by 3 result: False
check if 7 is divisible by 5 result: False
7
add filter for all multiples of 7
check if 9 is divisible by 3 result: True
check if 11 is divisible by 3 result: False
check if 11 is divisible by 5 result: False
check if 11 is divisible by 7 result: False
11
add filter for all multiples of 11
check if 13 is divisible by 3 result: False
check if 13 is divisible by 5 result: False
check if 13 is divisible by 7 result: False
check if 13 is divisible by 11 result: False
13
add filter for all multiples of 13
check if 15 is divisible by 3 result: True
check if 17 is divisible by 3 result: False
check if 17 is divisible by 5 result: False
check if 17 is divisible by 7 result: False
check if 17 is divisible by 11 result: False
check if 17 is divisible by 13 result: False
17
add filter for all multiples of 17
check if 19 is divisible by 3 result: False
check if 19 is divisible by 5 result: False
check if 19 is divisible by 7 result: False
check if 19 is divisible by 11 result: False
check if 19 is divisible by 13 result: False
check if 19 is divisible by 17 result: False
19
add filter for all multiples of 19
check if 21 is divisible by 3 result: True
check if 23 is divisible by 3 result: False
check if 23 is divisible by 5 result: False
check if 23 is divisible by 7 result: False
check if 23 is divisible by 11 result: False
check if 23 is divisible by 13 result: False
check if 23 is divisible by 17 result: False
check if 23 is divisible by 19 result: False
这篇关于使用过滤器和生成器在python中生成无穷素数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！