for 循环和遍历列表 [英] for loops and iterating through lists

问题描述

这是给出输出的代码片段:0 1 2 2.我原以为输出 3 3 3 3 因为 a[-1] 访问列表中的数字 3.网上给出的解释说a[-1] 的值在每次迭代中都会改变"，但我不太明白如何或为什么.任何解释都会很棒！

Here is a snippet of code which gives the output: 0 1 2 2. I had expected the output 3 3 3 3 since a[-1] accesses the number 3 in the list. The explanation given online says "The value of a[-1] changes in each iteration" but I don't quite understand how or why. Any explanations would be great!

a = [0, 1, 2, 3]
for a[-1] in a:
    print(a[-1])

推荐答案

这里发生的是一个列表在循环过程中发生了变异.

What's happening here is a list is mutated during looping.

让我们考虑以下代码片段:

Let's consider following code snippet:

a = [0, 1, 2, 3]
for a[-1] in a:
    print a

输出为:

[0, 1, 2, 0]
[0, 1, 2, 1]
[0, 1, 2, 2]
[0, 1, 2, 2]

每次迭代:

• 从内部指针当前指向的位置读取值
• 立即将其分配给列表中的最后一个元素
• 在最后一个元素被打印到标准输出之后

所以它是这样的:

• 内部指针指向第一个元素，它是0，最后一个元素被那个值覆盖；列表是 [0, 1, 2, 0];打印值为 0
• 内部指针指向第二个元素，它是1，最后一个元素被那个值覆盖；列表是 [0, 1, 2, 1];打印值为 1
• (...)
• 最后一步，内部指针指向最后一个元素；最后一个元素被自己覆盖 - 列表在最后一次迭代时不会改变；打印元素也不会改变.

• internal pointer points to first element, it's 0, and last element is overwritten with that value; list is [0, 1, 2, 0]; printed value is 0
• internal pointer points to second element, it's 1, and last element is overwritten with that value; list is [0, 1, 2, 1]; printed value is 1
• (...)
• at last step, internal pointer points to last element; last element is overwritten by itself - list does not change on last iteration; printed element also does not change.

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