slicing to reverse string confusion

Question

This has been asked many times but i do not want to ask that again . I just want to know how is this happening ?

>>> st="arindam"

>>> st[::-1]   #This is the one that everyone probably uses 
'madnira'

Would someone please explain why this works?

>>> st[-1::-1]
'madnira'

How is this also reversing the string successfully? I thought this would start from 'a' and print 'adnira'

Index -1 is the last character of the string, so... – kindall Apr 02 '12 at 21:35 — kindall, Apr 02 '12 at 21:35

score 4 · Accepted Answer · answered Apr 02 '12 at 21:35

4

In Python list indexes, the index -1 refers to the last character in the string. When using a negative step index, omitting the start index uses -1 as the start (a positive step index starts at 0 by default).

answered Apr 02 '12 at 21:35

Greg Hewgill

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Ah.........That explains it......Thank you Sir – Arindam Roychowdhury Apr 02 '12 at 21:38
I think that would make `start:end:step` act just like `range(start,end,step)`? – ninjagecko Apr 02 '12 at 21:38

score 1 · Answer 2 · answered Apr 02 '12 at 21:37

1

because st[-1] is 'm':

>>> st="arindam"
>>> st[-1]
'm'
>>> st[::-1]
'madnira'
>>> st[-1::-1]
'madnira'
>>> st[-2::-1]
'adnira'

answered Apr 02 '12 at 21:37

jcomeau_ictx

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`>>> st[-1:0:-1]` `'madnir'` But `>>> st[-1::-1]` `'madnira'` How did this happen? – Arindam Roychowdhury Apr 02 '12 at 21:43
0 and None (which is implied between '::') behave differently. – jcomeau_ictx Apr 02 '12 at 21:51

slicing to reverse string confusion

2 Answers2