itertools.ifilter VS.过滤器VS.列表解析 [英] itertools.ifilter Vs. filter Vs. list comprehensions

问题描述

我试图更熟悉 itertools 模块，并找到了一个名为 ifilter 的函数。

据我所知，它基于给定的函数过滤和迭代，并在包含函数计算结果的迭代元素的列表上返回一个迭代器真。

问题1 ：我的理解到目前为止是否正确？

问题2 ：除了这个返回和迭代器的事实，它与内置的过滤器函数有什么不同？

问题3 哪个更快？

从我所知道的情况来看，事实并非如此。我错过了什么吗？ （我跑了下面的测试）

 >>> itertools.ifilter（lambda x：x％2，range（5））
< itertools.ifilter对象在0x7fb1a101b210> 
>>>对于我在itertools.ifilter（lambda x：x％2，range（5））中：print i 
 ... 
 1 
 3 
>>>过滤器（lambda x：x％2，范围（5））
 [1,3] 
>>> function = lambda x：x％2 
>>> [item for item in range（5）if function（item）] 
 [1,3] 
  

解决方案

下面的示例包含一个数字生成器，它在产生数字之前立即打印消息，显示 filter（）第一次建立列表，然后运行并过滤它。尽管 itertools.ifilter 过滤器，但从不建立列表。如果你过滤了500,000个重要的东西，你需要 ifilter ，所以你不建立一个列表。

 import itertools 
 
 def number_generator（）：
在范围内（0，3）：
 printyield，i 
产量i 
打印停止
 
函数= lambda x：x> 0 
 
数字= number_generator（）
打印itertools.ifilter：在itertools.ifilter（函数，数字）中输入n 
：
 print n 
 
打印\\\
filter：
 numbers = number_generator（）
 for filter in filter（function，numbers）：
 print n 
 
 
 itertools.ifilter：$ b输出：
 
 
 $ $ b收益率0 
收益率1 
 1 
收益率2 
 2 
止损
 
过滤器：
收益率0 
产量1 
产量2 
停产
 1 
 2 
 
 
I am trying to become more familiar with the itertools module and have found a function called ifilter.

From what I understand, it filters and iterable based on the given function and returns an iterator over a list containing the elements of the iterable on which the function evaluates to True. 

Question 1: is my understanding thus far correct?

Question 2: aside from the fact that this returns and iterator, how is it different from the built-in filter function?

Question 3 Which is faster?

From what I can tell, it is not. Am I missing something? (I ran the following test)
>>> itertools.ifilter(lambda x: x%2, range(5))
<itertools.ifilter object at 0x7fb1a101b210>
>>> for i in itertools.ifilter(lambda x: x%2, range(5)): print i
... 
1
3
>>> filter(lambda x: x%2, range(5))
[1, 3]
>>> function = lambda x: x%2
>>> [item for item in range(5) if function(item)]
[1,3]

解决方案
The example below includes a number generator that prints a message immediately before yielding the number, shows up how filter() first builds the list, then runs through that and filters it. Whereas itertools.ifilter filters as it goes, never building a list. If you're filtering 500,000 significant things, you want ifilter, so you're not building a list.
import itertools

def number_generator():
    for i in range(0, 3):
        print "yield", i
        yield i
    print "stopping"

function = lambda x: x > 0

numbers = number_generator()
print "itertools.ifilter:"
for n in itertools.ifilter(function, numbers):
    print n

print "\nfilter:"
numbers = number_generator()
for n in filter(function, numbers):
    print n
Output:
itertools.ifilter:
yield 0
yield 1
1
yield 2
2
stopping

filter:
yield 0
yield 1
yield 2
stopping
1
2


                        
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