Slice objects with negative increment

[Alex Martelli]

Paul Hughett wrote:

> Alex Martelli <aleax at aleax.it> wrote:
> : Paul Hughett wrote:
> :         ...
> :> Hence my question: Is there an established meaning within Python for
> :> slice objects with a negative increment?  E.g. slice(1, 10, -2) ?
> 
> : Sure!  [Use the same convention as Numeric]
> 
> Okay, I've looked up the convention for Numeric, but the description
> isn't entirely clear in the case of negative increments.  It states that
> the default values for start and stop when the increment is negative are
> "the beginning" and "the end" of the axis.  Suppose that the axis has
> n elements.  Does ::-k default to
> 
>    n:0:-k        which refers to element n, which doesn't exist
>    (n-1):0:-k    which skips element 0
>    n:-1:-k       which refers to element n, and apparently to -1 => n-1,
>    or
>    (n-1):-1:-k   which is not generally the reversal of 0:n:k for k > 1

>>> r=Numeric.array(range(13))
>>> r
array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12])
>>> r[::-1]
array([12, 11, 10,  9,  8,  7,  6,  5,  4,  3,  2,  1,  0])
>>> r[::-2]
array([12, 10,  8,  6,  4,  2,  0])
>>> r[::-3]
array([12,  9,  6,  3,  0])
>>>


> By the way, I don't see anything in "Numerical Python" about the
> meaning of a negative start or stop.  Do they follow the usual Python
> convention of meaning the nth element from the end?

>>> r[-3::]
array([10, 11, 12])
>>> r[-3::-1]
array([10,  9,  8,  7,  6,  5,  4,  3,  2,  1,  0])
>>>


Alex