在 line.split('+')[-1] 中，方括号中的 -1 在 Python 中表示什么

In line.split('+')[-1] what does the -1 in the square brackets indicate in Python

提问by user3245453

Let's assume that we have this code:

让我们假设我们有这个代码：

name = line.split('+')[-1]

What does the -1 do? I have seen it in various codes but not sure on what it does? And what would the difference be if there was a [0]or a [1]?

-1 有什么作用？我在各种代码中看到过它，但不确定它的作用是什么？如果有 a[0]或 a会有什么区别[1]？

采纳答案by user3245453

The line of code you gave is basically doing three things:

您提供的这行代码基本上做了三件事：

1. It takes the string lineand splits it on +'s using str.split. This will return a list of substrings:

   >>> line = 'a+b+c+d'
   >>> line.split('+')
   ['a', 'b', 'c', 'd']
   >>>
   

2. The [-1]then indexes that list at position -1. Doing so will return the last item:

   >>> ['a', 'b', 'c', 'd'][-1]
   'd'
   >>>
   

3. It takes this item and assigns it as a value for the variable name.

1. 它获取字符串line并将其拆分为+'s using str.split。这将返回子字符串列表：

   >>> line = 'a+b+c+d'
   >>> line.split('+')
   ['a', 'b', 'c', 'd']
   >>>
   

2. 在[-1]随后指数在位置列表-1。这样做将返回最后一项：

   >>> ['a', 'b', 'c', 'd'][-1]
   'd'
   >>>
   

3. 它接受此项并将其分配为变量的值name。

Below is a more complete demonstration of the concepts mentioned above:

下面是对上述概念的更完整的演示：

>>> line = 'a+b+c+d'
>>> line.split('+')
['a', 'b', 'c', 'd']
>>> lst = line.split('+')
>>> lst[-1]
'd'
>>> lst[0]
'a'
>>> lst[1]
'b'
>>> lst[2]
'c'
>>> lst[3]
'd'
>>>

回答by falsetru

str.splitreturns a list:

str.split返回一个列表：

>>> '1+2+3'.split('+')
['1', '2', '3']

list[-1]yields the last item (negative index starts from -1)

list[-1]产生最后一项（负索引从 -1 开始）

>>> '1+2+3'.split('+')[-1]
'3'
>>> '1+2+3'.split('+')[0] # the first item (Python index starts from 0)
'1'
>>> '1+2+3'.split('+')[1]
'2'

See Lists- Python tutorial(contains indexing, slicing).

请参阅Lists- Python 教程（包含索引、切片）。

回答by Jayaram

Splitwill create list and from that you are getting the last element using [-1]

Split将创建列表，并从中获取最后一个元素 [-1]

回答by óscar López

Negative indexes in Python are syntactic sugar for accessing the elements in reverse order, from right-to-left, starting in -1. So -1is the last item, -2is the second-to-last item, and so on - the first item would be lst[-len(lst)]. For example:

Python 中的负索引是一种语法糖，用于以相反的顺序访问元素，从右到左，从-1. 所以-1是最后一项，倒数-2第二项，依此类推 - 第一项是lst[-len(lst)]. 例如：

lst = [1, 2, 3]
lst[-1]
=> 3
lst[-2]
=> 2
lst[-3]
=> 1

回答by Carolyne Mwende

Okay, in order to understand what is happening here, we need to understand lists, split() and slicing functions of a list.

好的，为了理解这里发生的事情，我们需要了解列表、列表的 split() 和切片函数。

For example:

例如：

? Given the string below, let us split it: ? line ='a+b+c+d' ? name=line.split(‘+')

? 给定下面的字符串，让我们拆分它：行 ='a+b+c+d' ? name=line.split('+')

? After splitting the string, it becomes a list. As shown below ? ['a','b','c','d']

? 字符串拆分后，就变成了一个列表。如下所示 ？['A B C D']

? Please note: '+' is called the separator : meaning, the string will be separated based on the number of '+' available, hence the list above.

? 请注意：'+' 被称为分隔符：意思是，字符串将根据可用的 '+' 的数量进行分隔，因此是上面的列表。

? Square brackets after the separator(always in normal parenthesis) is accessing the elements in the list Lists are accessed using index, positive index begins with 0 (which is the first element of a list.e.g name[0] is accessing first element of the list which is ‘a') and it operates from left to right while negative indexes starts from -1, from right to left(in the example above 'd' is index -1) Using your sample question, name = line.split('+')[-1] , this will return last item in the list, that is ‘d' name = line.split('+')[0], this will return first item in the list, that is ‘a' name = line.split('+')[1], this will return second item in the list, that is ‘b'

? 分隔符后的方括号（总是在正常括号中）访问列表中的元素列表使用索引访问，正索引从 0 开始（这是列表的第一个元素。例如 name[0] 访问列表的第一个元素list 是 'a') 并且它从左到右操作，而负索引从 -1 开始，从右到左（在上面的示例中，'d' 是索引 -1）使用您的示例问题，name = line.split( '+')[-1] ，这将返回列表中的最后一项，即 'd' name = line.split('+')[0] ，这将返回列表中的第一项，即 'a ' name = line.split('+')[1], 这将返回列表中的第二项，即 'b'