ok. it confused me a bit that d prints as "1.0", but d==1.0 evaluates to false. but sprintf shows that it's just float#to_s that is truncating the output:
z = 0.034228469424 r = Math.sqrt z*z d = z/r puts d # => 1.0 puts d==1.0 # => false puts sprintf "%.20f",d # => "1.00000000000000088818" what is the precision of macruby floats? On 07/06/2010, at 04.35, Laurent Sansonetti wrote: > Hi Emil, > > This is sort-of expected. MacRuby floats have less precision than MRI because > we pack them as immediate values, for performance reasons. As a more generic > issue, floating point arithmetic is almost never accurate. > > Laurent > > On Jun 5, 2010, at 9:59 AM, Emil Tin wrote: > >> >> hey, >> thanks for all the great work with macruby! >> >> i'm experiencing a difference in floating point behaviour between mri and >> the latest macruby 0.7 nightly: >> >> ~$ ruby -v >> ruby 1.8.7 (2009-06-08 patchlevel 173) [universal-darwin10.0] >> >> ~$ ruby -e "z = 0.034228469425; puts Math.sqrt(z*z) < z" >> false >> >> ~$ ruby -e "z = 0.034228469424; puts Math.sqrt(z*z) < z" >> false >> >> >> ~$ macruby -v >> MacRuby 0.7 (ruby 1.9.2) [universal-darwin10.0, x86_64] >> >> ~$ macruby -e "z = 0.034228469425; puts Math.sqrt(z*z) < z" >> false >> >> ~$ macruby -e "z = 0.034228469424; puts Math.sqrt(z*z) < z" >> true >> >> >> notice how that last script outputs true instead of false. also i'm seeing >> this difference with floats: >> >> ~$ ruby -e "puts 8.1" >> 8.1 >> >> ~$ macruby -e "puts 8.1" >> 8.09999999999999 >> >> >> >> >> this difference is causing trouble because asin() might fail even when the >> input seems to be 1.0: >> >> z = 0.034228469424 >> r = Math.sqrt z*z >> puts d = z/r # => 1.0 >> puts d>1.0 # false with mri, but true with macruby >> puts Math.asin d # => 1.5707963267949 with mri, but error with macruby >> (since d>1.0) >> >> >> >> -emil >> >> _______________________________________________ >> MacRuby-devel mailing list >> MacRuby-devel@lists.macosforge.org >> http://lists.macosforge.org/mailman/listinfo.cgi/macruby-devel > > _______________________________________________ > MacRuby-devel mailing list > MacRuby-devel@lists.macosforge.org > http://lists.macosforge.org/mailman/listinfo.cgi/macruby-devel
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