I agree. I'll add a note to the docs. Vincent
On Mon, 17 Apr 2017 16:16:49 -0500, Tim Jervis wrote: > > I like the document route; anyone who is happy with the performance tradeoff > would be free to implement a more conservative version. > > On 17 Apr 2017, at 21:59, Ben Greenman <[email protected]> wrote: > > Maybe, just add a note to the docs? > > On Mon, Apr 17, 2017 at 4:56 PM, Vincent St-Amour > <[email protected]> wrote: > > The latter is easy to fix; I've submitted a pull request. > > The former is trickier. The problem is not so much due to the step size > itself, but rather to a combination of the step size and of a particular > iteration state. > > (For people following along, the issue is that there exists a value `x` > such that `(= (+ x 1e-17) x)`, and that the iteration reaches that `x` > as its state at some point, then loops.) > > Guarding against that would require an additional check at each > iteration, which may introduce overhead. > > I did some very, very crude benchmarking, and here's what I got: > > Without extra check: > > (time (for ([i (in-range 10000000000)]) i)) > cpu time: 19478 real time: 19469 gc time: 0 > > (time (for ([i (in-range 10000000000)]) i)) > cpu time: 20171 real time: 20179 gc time: 0 > > (time (for ([i (in-range 10000000000)]) i)) > cpu time: 20049 real time: 20043 gc time: 0 > > With extra check: > > (time (for ([i (in-range 10000000000)]) i)) > cpu time: 22100 real time: 22210 gc time: 0 > > (time (for ([i (in-range 10000000000)]) i)) > cpu time: 22227 real time: 22265 gc time: 0 > > (time (for ([i (in-range 10000000000)]) i)) > cpu time: 21934 real time: 22016 gc time: 0 > > The difference is non-insignificant. Admittedly, this is the worst > possible case, though. > > Vincent > > On Mon, 17 Apr 2017 09:17:32 -0500, > Tim Jervis wrote: > > > > Dear Racket Users, > > > > I’ve noticed the following procedure calls don’t return (on my 64 bit Mac > hardware): > > > > 1 (range (- 1 1e-16) 1.0 1e-17) > > 2 (range 0 1 0) > > > > While (2) is obvious, (1) tripped me up (as I hadn’t noticed my step size > had fallen to effectively zero). > > > > A small tweak to for.rkt in the racket distribution could trap the > condition of an actually or effectively zero step. for.rkt already traps the > condition where step is not real. > > > > If this makes sense, could one of the authors consider adding the tweak? > Or is there a reason for leaving it alone? > > > > Kind regards, > > > > Tim > > > > Tim Jervis > > > > http://timjervis.com/ > > > > -- > > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > > For more options, visit https://groups.google.com/d/optout. > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
