[-4:]在python中是什么意思?
我在课堂上了解到,对于Python,字符从[0]开始。例如,在Monty Python中,'M'=0,'o'=1,'n'=2,'t'=3,'y'=4,'5,'P'=6,'y'=7,'t'=8,'h'=9,'o'=10,'n'=11 但当我看到NLTK上的以下操作时,我感到困惑:[-4:]在python中是什么意思?,python,nltk,Python,Nltk,我在课堂上了解到,对于Python,字符从[0]开始。例如,在Monty Python中,'M'=0,'o'=1,'n'=2,'t'=3,'y'=4,'5,'P'=6,'y'=7,'t'=8,'h'=9,'o'=10,'n'=11 但当我看到NLTK上的以下操作时,我感到困惑: genre_word = [(genre, word) for genre in ['news', 'romance'] for word in brown.wor
genre_word = [(genre, word) for genre in ['news', 'romance']
for word in brown.words(categories=genre)]
genre_word[:4]
genre_word[-4:]
我以为所有字符编号都以[0]开头。[-4:]是什么意思?对于python中的任何iterable[-4:]表示该iterable最后四项的索引。例如:
list1 = [1,2,3,4,5,6]
list1[-4:]
[3,4,5,6]
“世界”
它获取序列的最后四个元素:
>>> l = [1,2,3,4,5,6,7,8,9]
>>> l[-4:]
[6, 7, 8, 9]
>>>
正如您在python中所说,数组的索引从0开始 让我们说
my_word = 'hello world'
print(my_word[0]) #prints 'h'
my_word = 'hello world'
print(my_word[-1]) # prints 'd'
# - stands from last starting with index 1
print(my_word[-4:]) # prints 'orld'
# if you know the length you can directly use
print(my_word[7:]) # prints 'orld'
在python中,我们还有一个从上一个
让我们说
my_word = 'hello world'
print(my_word[0]) #prints 'h'
my_word = 'hello world'
print(my_word[-1]) # prints 'd'
# - stands from last starting with index 1
print(my_word[-4:]) # prints 'orld'
# if you know the length you can directly use
print(my_word[7:]) # prints 'orld'
如果不知道长度,此功能可帮助您打印从最后一个元素开始的索引值some_list[-n]语法可获取从最后一个元素开始的第n个元素。因此,一些_列表[-4]获取最后四个元素。