Jens Axel and Alexander have provided the answer, but in case it helps to see what they mean, try these loops that display the intermediate results:
(for/fold ([v 0]) ([x (in-range 100)]) (displayln v) (+ v (/ x 100))) (for/fold ([v 0]) ([x (in-range 100)]) (displayln v) (+ v (/ x (- 100 x)))) At Sat, 24 Jan 2015 17:39:39 +0100, Daniel Kvasnička wrote: > Hi, I'm observing the following difference in performance: > > > (time (exact->inexact (for/sum ([x (in-range 10000)]) (/ x 10000)))) > cpu time: 6 real time: 6 gc time: 0 > 4999.5 > > (time (exact->inexact (for/sum ([x (in-range 10000)]) (/ x (- 10000 x))))) > cpu time: 3624 real time: 3622 gc time: 39 > 87876.06036044382 > > The difference seems huge to me and I'm wondering whether it's "normal" and > what are the right tools to overcome it. > If I understand it correctly, it's about contract checking and the fact that > there is one more math opeartion involved does not in itself lead to such > difference. The division operator needs to be sure that the outcome of the > minus op. is of the right type, is that correct? > > If so -- and the increase in time cannot be considered a "bug" -- what is the > right way to get around that. I've employed 2 ways: Typed Racket and > switching > the portion of code to racket/flonum. Both of these approaches resulted in > big > perf. improvement and basically make the computation instant again. Are these > the right, "rackety", solutions? I've also tried to use unchecked ops but > Racket 6.1.1 is constantly segfaulting on me when running this (from my > digging I think unsafe-fl/ is what segfaults it): > > (time (exact->inexact (for/sum ([x (in-range 10000)]) (unsafe-fl/ (->fl x) > (unsafe-fx- 10000 x))))) > > Daniel > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
