嵌套for循环如何工作? [英] How do nested for loops work?

问题描述

这是我编写的代码，可以很好地工作:

This is the code I wrote, works perfectly fine:

box = [1, 2, 3, 4, 5]

for i in box:
    for x in box:
        print (i,x)

它输出以下内容:

1 1
1 2
1 3
1 4
1 5
2 1
2 2

等

有趣的部分是:我确实得到了想要的东西，但是我真的不理解逻辑.对于我编写的代码，我希望得到以下输出:

The funny part is: I got exactly what I want, however I really don't understand the logic. For the code I wrote, I would expect the following output:

1 1
2 2
3 3
4 4 
5 5 

为什么?因为根据我的想法，我说了以下内容:

Why? Because by my idea I stated the following:

对于框中的每个值 i (左列)，对于每个值 x (右列)，都应同时打印.

For each value i (left column) in box and for each value x (right column) print both.

实际上，我认为该代码应充当:

In fact, the code I believe should act as:

for i,y in zip(box,box):
    print(i,y)

我的问题是，为什么函数中的嵌套for循环的行为与我预期的不同?

My question is, why did the nested for-loop in the function behave differently then I expected?

它怎么知道创建 number * number 行?它是如何对数字进行排序的，那里到底发生了什么关系?

How did it know to create number * number rows? How did it order the numbers as it has, what relation acutally happened there?

推荐答案

这是逻辑. i in box ， i 将开始为 1 并向前移动，直到成为 5 (列表中的所有元素)).因此，如果在此循环中打印 i ，您会看到 i 首先是 1 ，然后 i 是 2 ...直到 i 为 5 (框中的最后一个元素).但是，如果您嵌套另一个for循环，则每次 i 与列表中的元素不同时，它都会执行相同的操作(从1开始并向前移动直到5).因此，当 i 为 1 时，在更改为另一个 i 之前，它将开始第二个循环(对于x in box >)，因此当 i 为 1 x 时，其更改为 x == 1 ， x == 2 ... x == 5 .当此嵌套循环完成时， i 将从列表中更改为另一个元素，因此现在 i 将为 2 ，并且嵌套循环再次开始，因此 x == 1 ， x == 2 ...， x == 5 .

It is logic. for i in box, i will start being 1 and moving forward until being 5 (all elements from the list). So if you print i inside this loop, you will see that i is first 1, then i is 2 ... until i is 5 (last element from box). But if you nest another for loop, it will do the same (start from 1 and moving forward until 5) for each time i is a different element from the list. So when i is 1, before changing to another different i it will start the second loop (for x in box), so then when iis 1 x will change being x == 1, x == 2... x == 5. When this nested loop finishes, then i changes to another element from the list, so now i would be2, and the nested loop starts again, so x == 1, x == 2..., x == 5.

我想您可以尝试一下

box = [1, 2, 3, 4, 5]


for i in box: #it will go through all elements in the list
    print i, "this is the first loop" #for each different 'i' in box
    for x in box: #it will go through all elements in the list
        print x, "this is the second loop" #you will get all elements from the box
        print (i,x) #this is what you get

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