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

Question

Let's assume that we have this code:

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] ?

Answer 1

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

1. It takes the string line and 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 .

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

>>> 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'
>>>

Answer 2

str.split returns a list:

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

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

>>> '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).

Answer 3

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

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

Answer 4

Split将创建列表，然后使用[-1]获取最后一个元素

Answer 5

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

For example:

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

 After splitting the string, it becomes a list. As shown below • ['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.

 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.eg 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'