negative stride list slices

Christopher Koppler klapotec at chello.at
Thu Sep 2 04:03:33 CEST 2004


On Wed, 01 Sep 2004 14:57:57 -0700, Shalabh Chaturvedi wrote:

> Shalabh Chaturvedi wrote:
> 
>> I'd like to add:
>> 
>>   +---+---+---+---+---+
>>   | H | e | l | p | A |
>>   +---+---+---+---+---+
>>   0   1   2   3   4   5
>>  -5  -4  -3  -2  -1
>>   |                   |
>>  start               end   (defaults for +ve step)
>>  end                 start (defaults for -ve step)
>> 
>> (Is this correct?)
> 
> Apparently not.

That should be

 +---+---+---+---+---+
 | H | e | l | p | A |
 +---+---+---+---+---+
 0   1   2   3   4   5
-6  -5  -4  -3  -2  -1
 |                   |
start               end   (defaults for +ve step)
end                 start (defaults for -ve step)

> 
>  >>> "HelpA"[-1:-2:-1]
> 'A'
> 
> I would expect 'p'. Also,
> 
>  >>> "HelpA"[:-1:-1]
> ''
> 
> And I definitely expected 'A'. Now I find it confusing too.
> 
> Does anyone else find it intuitive to expect that S[b:a:-1] be exactly 
> reverse of S[a:b:1]?

As others have explained, that's not easily possible because of Python's
zero-based indexing, which in general is great, but can lead to confusion
because using negative strides indices begin with -1.

> 
> This is only true for S[::1] and S[::-1]. In other cases, for -ve step, 
> the -ve indices seem to indicate a shifted position. I'm using Python 2.3.3.

The defaults here map to S[0:len(S):1] and S[-1:-len(S)-1:-1].

-- 
Christopher




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