列表理解中的双重迭代 [英] Double Iteration in List Comprehension

问题描述

在Python中，列表推导中可以有多个迭代器，例如

In Python you can have multiple iterators in a list comprehension, like

[(x,y) for x in a for y in b]

对于一些合适的序列a和b.我知道Python列表推导的嵌套循环语义.

for some suitable sequences a and b. I'm aware of the nested loop semantics of Python's list comprehensions.

我的问题是:理解中的一个迭代器可以引用另一个吗?换句话说:我能有这样的东西吗?

My question is: Can one iterator in the comprehension refer to the other? In other words: Could I have something like this:

[x for x in a for a in b]

外部循环的当前值是内部循环的迭代器吗?

where the current value of the outer loop is the iterator of the inner?

例如，如果我有一个嵌套列表:

As an example, if I have a nested list:

a=[[1,2],[3,4]]

列表理解表达式将如何获得此结果:

what would the list comprehension expression be to achieve this result:

[1,2,3,4]

?? (请只列出理解答案，因为这是我要查找的内容.)

?? (Please only list comprehension answers, since this is what I want to find out).

推荐答案

Gee，我想我找到了答案:我对哪个循环是内部循环和哪个循环是外部循环不够注意.列表理解应为:

Gee, I guess I found the anwser: I was not taking care enough about which loop is inner and which is outer. The list comprehension should be like:

[x for b in a for x in b]

以获得所需的结果，是的，一个当前值可以作为下一个循环的迭代器.

to get the desired result, and yes, one current value can be the iterator for the next loop.

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