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 <benjaminlgreen...@gmail.com> wrote: > > Maybe, just add a note to the docs? > > On Mon, Apr 17, 2017 at 4:56 PM, Vincent St-Amour > <stamo...@eecs.northwestern.edu> 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 racket-users+unsubscr...@googlegroups.com. > > 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 racket-users+unsubscr...@googlegroups.com. > 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 racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
