反转字符串 Python 切片表示法 [英] Reverse string Python slice notation

问题描述

string = "HELLO"
print string[::-1] #as expected
print string[0:6:-1] #empty string why ?

我很惊讶地看到在 python 中反转字符串是多么容易，但后来我发现了这一点并迷路了.有人可以解释为什么第二个反向不起作用吗?

I was amazed to see how easy it is to reverse a string in python but then I struck upon this and got lost. Can someone please explain why the second reverse does not works ?

推荐答案

切片符号的写法如下:

list_name[start_index: end_index: step_value]

python 中的列表索引与数轴上的数字不同.当step_value 为-1 时，列表索引不会在0th 索引之后转到-1st.

The list indexes in python are not like the numbers present on number line. List indexes does not go to -1st after 0th index when step_value is -1.

因此产生以下结果

<代码>>>>>打印字符串[0:6:-1]

>>>>

还有

<代码>>>>>打印字符串[0::-1]

<代码>>>>>H

所以当 start_index 为 0 时，它不能以循环顺序遍历索引到 -1, -2, -3, -4, -5, -6 for step_value 是 -1.

So when the start_index is 0, it cant go in a cyclic order to traverse the indexes to -1, -2, -3, -4, -5, -6 for step_value is -1.

同样

<代码>>>>>打印字符串[-1:-6:-1]

<代码>>>>>OLLEH

和

<代码>>>>>打印字符串[-1::-1]

<代码>>>>>OLLEH

还有

因此，当 start_index 为 -1 时，它会以循环顺序遍历索引到 -1、-2、-3、-4、-5、-6 以给出输出 OLLEH.

thus when the start_index is -1 it goes in a cyclic order to traverse the indexes to -1, -2, -3, -4, -5, -6 to give output OLLEH.

当 start_index 为 6 并且 step_value 为 -1 时，很容易理解

Its pretty straight forward to understand when start_index is 6 and step_value is -1

<代码>>>>>打印字符串[6::-1]

<代码>>>>>OLLEH

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