奇怪的循环语句 [英] Weird for loop statement

问题描述

我看到这个for循环，我不明白为什么最后一个打印是2.
为什么不是3？

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

out：

  0 
 1 
 2 
 2 
  循环使用 a [ -  code $] 1] 作为目标变量，从输入迭代中分配每个值： 
 
 
 用于< target> in< iterable> 
  
 循环的  a 列表到该目标， a [-1] 。所以  也是同一个列表中的最后一个元素。所以列表改变循环：
 
 
 >>> a = [0,1,2,3] 
>>>对于a中的[-1]：
 ...打印
 ... 
 [0,1,2,0]＃将a [0] == 0赋值给a [ -1]（或a [3]）
 [0,1,2,1]＃将a [1] == 1赋值给a [-1] 
 [0,1,2,2 ]＃分配a [2] == 2给[-1] 
 [0,1,2,2]＃赋值[3] == 2（自上一次迭代以来）为[-1] 
  
一次最后一次迭代赋予 a [2] 到 a [3] （或 a [-2] 到 a [-1] ），这就是为什么当最后一次迭代发生时，你再次看到 2    
 
 
 
见  for 循环语法;它需要一种通用的 target_list 的分配对象，就像一个 assigment声明。你不仅限于赋值中的简单名字，也不限于循环中的。
 
I saw this for loop and I didn't quite understood why the last print is 2.
Why it isn't 3 ? 
a = [0, 1, 2, 3]

for a[-1] in a:
    print(a[-1])
out:
0
1
2
2

解决方案
The for loop uses a[-1] as a target variable, assigning each value from the input iterable:
for <target> in <iterable>
The for loop assigns each value in the a list to that one target, a[-1]. That happens to also be the last element in that same list.

So the list changes with each step through the loop:
>>> a = [0, 1, 2, 3]
>>> for a[-1] in a:
...     print a
...
[0, 1, 2, 0]  # assigned a[0] == 0 to a[-1] (or a[3])
[0, 1, 2, 1]  # assigned a[1] == 1 to a[-1]
[0, 1, 2, 2]  # assigned a[2] == 2 to a[-1]
[0, 1, 2, 2]  # assigned a[3] == 2 (since the previous iteration) to a[-1]
The one-but-last iteration assigns puts a[2] into a[3] (or a[-2] into a[-1]), and that is why, when the last iteration takes place, you see 2 again.

See the for loop grammar; it takes a generic target_list for the assignment target, just like an assigment statement. You are not limited to just simple names in assignments, and neither are you in a for loop.

                        
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