1、要访问字符串中的字符，例如最后一个字符，需要知道它在字符串中的位置。
从0到小于字符串长度的1开始编号或索引。
str = 'Antarctica is really cold.'
print('str = ', str)
#first character
print('str[0] = ', str[0])
#last character
print('str[-1] = ', str[-1])
#slicing 2nd to 5th character
print('str[1:5] = ', str[1:5])
#slicing 6th to 2nd last character
print('str[5:-2] = ', str[5:-2])
2、如果索引从左到右遵循递增顺序模式，则从右到左遵循递减顺序模式。
即从-1、-2、-3等。因此，如果您想访问最后一个字符，您可以使用两种方法。
str = 'Antarctica is really cold.'
a = len(str)
print('length of str ', a)
#last character with the help of length of the string
print('str[a] ', str[a-1])
#last character with the help of indexing
print('str[-1] ',str[-1])
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