[swift-evolution] Feature proposal: Range operator with step

[Thorsten Seitz]

> Am 06.04.2016 um 20:26 schrieb Erica Sadun via swift-evolution <swift-evolution at swift.org>:
> 
>>> On Apr 6, 2016, at 12:23 PM, Stephen Canon via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>>> On Apr 6, 2016, at 11:20 AM, Stephen Canon via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> On Apr 6, 2016, at 11:16 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> One question that I *do* think we should answer, is whether the elements
>>>> of
>>>> 
>>>>    (0..<199).striding(by: -2)
>>>> 
>>>> are even or odd.
>>> 
>>> Odd.  I don’t believe that many real use cases care, but odd is more efficient from a performance perspective.  Needs to be documented clearly, however.
>> 
>> Sorry, I was thinking of (0…199).striding(by: -2).
>> 
>> For the (0..<199) case, Erica’s assessment seems about right, though it isn’t at all obvious how it generalizes to floating point strides.
> 
> (l..<h).striding(by: -dX) is undefined for floating point strides as there cannot be a starting value.

Why?
The starting value could quite naturally be s = l + k*dX for the largest value of k in the natural numbers for which s < h, wouldn't it?

-Thorsten 
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