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

问题描述

下面是一段代码，它给出了输出： 0 1 2 2 。因为 a [-1] 访问列表中的数字3，所以我期望输出 3 3 3 3 。网上给出的解释是：每次迭代中 a [-1] 的值发生变化，但我不太明白如何或为什么。任何解释都会很棒！

pre $ code a = [0，1，2，3]
for a [-1 ] in a：
print（a [-1]）$ ​​b $ b

解决方案 div>

这里发生了什么是一个列表在循环过程中发生了变化。

让我们考虑下面的代码片段：

  a = [0， 1，2，3] 
为a：
中的[-1]打印
  

输出是：

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ ，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
  

$ b $立即将它分配给列表中的最后一个元素

• 最后一个元素打印在标准输出中b

  所以它是这样的：
  
  

  
  
  • 内部指针指向第一个元素，它是0，最后一个元素被该值覆盖;列表是 [0，1，2，0] ;打印的值是 0
  
  
  • 内部指针指向第二个元素，它是1，最后一个元素被该值覆盖;清单是 [0，1，2，1] ;打印的值在最后一步是 1
  
  
  • （...）
  
  
  • 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;打印的元素也不会改变。
    
  
  

  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])
  

  解决方案

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

  Let's consider following code snippet:

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

  Output is:

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

  Each iteration:

  • reads value from position currently pointed by internal pointer
  • immediately assigns it to last element in list
  • after that last element is printed on standard output

  So it goes like:

  • 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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