Ron Adam <[EMAIL PROTECTED]> writes:
> Magnus Lycka wrote:
>> Ron Adam wrote:
[...]
>>> REVERSE ORDER STEPPING
>>> ----------------------
>>> When negative steps are used, a slice operation
>>> does the following. (or the equivalent)
>>>
>>> 1. reverse the list
>>> 2. cut the reversed sequence using start and stop
>>> 3. iterate forward using the absolute value of step.
>> I think you are looking at this from the wrong perspective.
>> Whatever sign c has:
>> For s[a:b:c], a is the index for the first item to include,
>> b is the item after the last to include (just like .end() in
>> C++ iterators for instance), and c describes the step size.
>
> Yes, and that is how it "should" work. But....
>
> With current slicing and a negative step...
>
> [ 1 2 3 4 5 6 7 8 9 ]
> -9 -8 -7 -6 -5 -4 -3 -2 -1 -0
>
> r[-3:] -> [7, 8, 9] # as expected
> r[-3::-1] -> [7, 6, 5, 4, 3, 2, 1, 0] # surprise
>
> The seven is include in both cases, so it's not a true inverse
> selection either.
Did you read what Magnus said: "a is the index for the first item to
include"? How could r[-3::x] for any x not include the 7?
Cheers,
mwh
--
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-- Jim's pedigree of operating systems, asr
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