If you define a range as range[i] = first + i * stride where i is an Int then this generates an error when there are more than Int_Max steps, see code previously posted. The error is generated when the range is formed, which is ideal since an error part way along an iteration or a never ending iteration would be difficult to track down.
On Friday, 1 April 2016, Stephen Canon via swift-evolution < swift-evolution@swift.org> wrote: > > > On Mar 31, 2016, at 11:16 AM, Rainer Brockerhoff via swift-evolution < > swift-evolution@swift.org <javascript:;>> wrote: > > > > On 3/31/16 15:06, Dave Abrahams via swift-evolution wrote: > >> > >> on Thu Mar 31 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote: > >> > >>> Thoughts on an edge case: so long as it's possible to use > >>> `stride(from:to:by:)` with Double, we'll need to figure out what > >>> happens when you have `stride(from: 0.0, to: DBL_MAX, by: DBL_MIN)`. > >>> Bounds may be unknown at compile time, obviously. > >>> > >>> Currently (this is by reasoning through the code, not actually > >>> observing it run), `for i in stride(from: 0.0, to: DBL_MAX, by: > >>> DBL_MIN) { }` degenerates into an infinite loop once you reach > >>> sufficient large values such that `current + stride == current`, which > >>> for a stride of DBL_MIN should happen pretty quickly. > >>> > >>> In Erica's proposed floating point Stride, an Int is used to count > >>> iterations (and iterations need to be counted in order to avoid > >>> accumulating error). Thus, one must break from `for i in stride(from: > >>> 0.0, to: DBL_MAX, by: DBL_MIN) { }` before the iteration counter > >>> overflows or it will trap. IMO, trapping at some point is fine, but I > >>> think a limit of Int.max iterations might be rather arbitrary for a > >>> StrideTo<Double> (or whatever it will be named) and I'm not sure how > >>> one can justify why the behavior of StrideTo<Double> would change from > >>> machine to machine based on the size of Int. > >>> > >>> I've been waffling between using an Int counter as Erica does or a > >>> counter of type Strideable.Stride in `StrideTo<Strideable where > >>> Strideable.Stride : FloatingPoint>`. In the latter alternative, no > >>> trapping occurs, but error begins to accumulate when the iteration > >>> counter is too large to represent integers exactly (e.g., 2^53 for > >>> Double). In that case, `for i in stride(from: 0.0, to: DBL_MAX, by: > >>> DBL_MIN) { }` degenerates into an infinite loop eventually (once > >>> `iterationCount + 1.0 == iterationCount`) and never traps, which I'm > >>> not sure I like, but a limit of 2^53 iterations bears at least a > >>> rational connection to Double and is known at compile time independent > >>> of the supplied bounds. We could alternatively return nil on reaching > >>> 2^53 iterations, trap, etc. > >>> > >>> Comments? > >> > >> I think I want to hear Steve Canon's input on this one. I defer to him > >> on most things numeric. > > > > In particular, should Steve confirm that the IEEE754 Decimal128 format > > is being worked on, and if simple decimal constants like those in > > `for i in stride(from: 0.0, to: DBL_MAX, by: DBL_MIN) { }` > > will be type-inferred as Decimal128, all that would "just work". > > Decimal is something that I would like to see happen. However, I would > not expect any such proposal to result in that loop being type inferred to > Decimal, since the to: and by: values are explicitly (binary > floating-point) Doubles. > > I also don’t think that such a loop is particularly useful. For > floating-point types, something like stride(from: T, to: T, steps: Int) > seems safer and more workable to me (this is just my immediate reaction, I > haven’t thought this through in detail, and am likely to change my mind if > someone makes a good case). > > – Steve > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <javascript:;> > https://lists.swift.org/mailman/listinfo/swift-evolution > -- -- Howard.
_______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution