python中[iter(list)] * 2的含义是什么?

问题描述

我在网上找到了下面的代码，结果是列表中两个元素的元组，如何理解[iter(list)]*2?

I have found below code in web, result is tuple of two elements in list, how to understand [iter(list)]*2?

lst = [1,2,3,4,5,6,7,8]
b=zip(*[iter(lst)]*2)
list(b)

[(1, 2), (3, 4), (5, 6), (7, 8)]

------------
[iter(lst)]*2
[<list_iterator at 0x1aff33917f0>, <list_iterator at 0x1aff33917f0>]

我检查了 [iter(lst)]*2，与上面相同的迭代器，所以意思是 iter 重复双倍，所以，如果我检查 num 从 2 到 3，结果应该是 [(1, 2, 3), (4, 5, 6),(7,8,NaN)]但删除 7,8

I check [iter(lst)]*2, same iterator above, so meaning iter repeat double, so, if i check num from 2 to 3, result should be [(1, 2, 3), (4, 5, 6),(7,8,NaN)] but delete 7,8

lst = [1,2,3,4,5,6,7,8]
b=zip(*[iter(lst)]*3)
list(b)
--------------
[(1, 2, 3), (4, 5, 6)]

推荐答案

解释起来相当棘手.我试一试:

Quite a tricky construct to explain. I'll give it a shot:

使用 [iter(lst)] 您可以创建一个包含一个项目的列表.该项目是列表的迭代器.

with [iter(lst)] you create a list with with one item. The item is an iterator over a list.

每当 python 试图从这个迭代器中获取一个元素时，就会返回 lst 的下一个元素，直到没有更多元素可用为止.

whenever python tries to get an element from this iterator, then the next element of lst is returned until no more element is available.

请尝试以下操作:

i = iter(lst)
next(i)
next(i)

输出应如下所示:

>>> lst = [1,2,3,4,5,6,7,8]  
>>> i = iter(lst)
>>> next(i)
1
>>> next(i)
2
>>> next(i)
3
>>> next(i)
4
>>> next(i)
5
>>> next(i)
6
>>> next(i)
7
>>> next(i)
8
>>> next(i)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

现在您创建一个包含两次完全相同迭代器的列表.你这样做itlst = [iter(lst)] * 2

Now you create a list that contains twice exactly the same iterator. You do this with itlst = [iter(lst)] * 2

尝试以下:

itlst1 = [iter(lst)] * 2
itlst2 = [iter(lst), iter(lst)]
print(itlst1)
print(itlst2)

结果将类似于:

>>> itlst1 = [iter(lst)] * 2
>>> itlst2 = [iter(lst), iter(lst)]
>>> print(itlst1)
[<list_iterator object at 0x7f9251172b00>, <list_iterator object at 0x7f9251172b00>]
>>> print(itlst2)
[<list_iterator object at 0x7f9251172b70>, <list_iterator object at 0x7f9251172ba8>]

需要注意的是，itlst1 是一个包含两个相同迭代器的列表，而 itlst2 包含两个不同的迭代器.

What is important to notice is, that itlst1 is a list containing twice the same iterator, whereas itlst2 contains two different iterators.

为了说明尝试输入:

next(itlst1[0])
next(itlst1[1])
next(itlst1[0])
next(itlst1[1])

并将其与:

next(itlst2[0])
next(itlst2[1])
next(itlst2[0])
next(itlst2[1])

结果是:

>>> next(itlst1[0])
1
>>> next(itlst1[1])
2
>>> next(itlst1[0])
3
>>> next(itlst1[1])
4
>>> 
>>> next(itlst2[0])
1
>>> next(itlst2[1])
1
>>> next(itlst2[0])
2
>>> next(itlst2[1])
2

现在到 zip() 函数( https://docs.python.org/3/library/functions.html#zip):

Now to the zip() function ( https://docs.python.org/3/library/functions.html#zip ):

尝试以下操作:

i = iter(lst)
list(zip(i, i))

zip() 有两个参数.当您尝试从 zip 获取下一个元素时，它将执行以下操作:

zip() with two parameters. Whenver you try to get the next element from zip it will do following:

• 从作为第一个参数的可迭代对象中获取一个值
• 从可迭代的第二个参数中获取一个值
• 返回一个包含这两个值的元组.

list(zip(xxx)) 将重复执行此操作并将结果存储在列表中.

list(zip(xxx)) will do this repeatedly and store the result in a list.

结果将是:

>>> i = iter(lst)
>>> list(zip(i, i))
[(1, 2), (3, 4), (5, 6), (7, 8)]

使用的下一个技巧是 *，用于将第一个元素用作函数调用的第一个参数，将第二个元素用作第二个参数，依此类推)什么是**(双星/星号)和*(星号/星号)) 为参数做些什么?

The next trick being used is the * that is used to use the first element as first parameter to a function call, the second element as second parameter and so forth) What does ** (double star/asterisk) and * (star/asterisk) do for parameters?

这么写:

itlst1 = [iter(lst)] * 2
list(zip(*itlst1))

在这种情况下与

i = iter(lst)
itlst1 = [i] * 2
list(zip(itlst1[0], itlst1[1]))

与

list(zip(i, i))

我已经解释过了.

希望这可以解释上述大部分技巧.

Hope this explains most of the above tricks.

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