It’s not a matter of floating point error accumulation… At least on my machine, once a Double hits +/-∞, there’s no way that I know of to get back to normal floating point numbers. That is to say, for *all* normal, finite values of x, "-Double.infinity + x" will just return “-inf". If x is to equal Double.infinity, Double.NaN, or Double.quietNaN, then it’ll return “nan” (which, incidentally, will fail the regular equality test… Double.NaN isn’t even equal to itself; I think checking the floating point class is the way to do it).
I could easily be missing something, but AFAICT the only way to always get the correct sequence (without splitting the floating point types off into their own thing) is either have a negative stride swap start and end *before* the StrideTo starts generating values (that is, *not* by just calling `.reverse()` on something with a positive stride), or to allow “0 ..< -Double.infinity” to be a valid range (with the negative stride being implied). - Dave Sweeris > On Apr 9, 2016, at 6:59 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > Yikes. Not too concerned about the infinite loop issue, as floating point > strides when fixed to avoid error accumulation will necessarily enforce a > finite number of steps. However, you're talking a regular, not-at-all-lazy > Array being returned? That would be not good at all... > > On Sun, Apr 10, 2016 at 12:29 AM Dave via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: > >> On Apr 9, 2016, at 4:33 AM, Haravikk via swift-evolution >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> >> While I’m in favour of the basic idea I think the operator selection is too >> complex, and I’m not sure about the need for negative strides. Really all I >> want are the following: >> >> (0 ... 6).striding(by: 2) // [0, 2, 4, 6] x from 0 to 6 >> (0 ..< 6).striding(by: 2) // [0, 2, 4] x from 0 while >> <6 >> (6 ... 0).striding(by: 2) // [6, 4, 2, 0] x from 6 to 0 >> (6 ..> 0).striding(by: 2) // [6, 4, 2] x from 6 while >> >0 >> >> Everything else should be coverable either by flipping the order, or using >> .reverse(). The main advantage is that there’s only one new operator to >> clarify the 6 ..> 0 case, though you could always just reuse the existing >> operator if you just interpret it as “x from 6 to, but not including, 0" > > `.reverse()` returns an array, though, not a StrideTo<>, which means it’ll > get in an infinite loop on infinite sequences. This works fine: > for i in stride(from: 0.0, to: Double.infinity, by: M_PI) { > if someTestInvolving(i) { break } > ... > } > > But this never even starts executing the loop because of the infinite loop > inside `.reverse()`: > for i in stride(from: -Double.infinity, to: 0.0, by: M_PI).reverse() { > if someTestInvolving(i) { break } > ... > } > > - Dave Sweeris > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution>
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