继续假设你有这个列表：

Here is what's going on. Suppose you have this list:

['a', 'b', 'c', 'd']

您正在循环列表中的每个元素。假设你目前在索引位置1：

and you are looping over every element in the list. Suppose you are currently at index position 1:

['a', 'b', 'c', 'd']
       ^
       |
   index = 1

...并删除索引位置1的元素，给出你这样：

...and you remove the element at index position 1, giving you this:

['a',      'c', 'd']
       ^
       |
    index 1

删除该项后，其他项目向左滑动，给您这是：

After removing the item, the other items slide to the left, giving you this:

['a', 'c', 'd']
       ^
       |
    index 1

然后当循环再次运行时，循环将索引增加到2，给你这个：

Then when the loop runs again, the loop increments the index to 2, giving you this:

['a', 'c', 'd']
            ^ 
            |
         index = 2

看看你如何跳过'c'？课程是：永远不要从您正在循环的列表中删除一个元素。

See how you skipped over 'c'? The lesson is: never delete an element from a list that you are looping over.