On Wed, Aug 10, 2016 at 2:45 PM, Thierry Dumont <tdum...@math.univ-lyon1.fr> wrote: > Le 10/08/2016 à 13:38, Erik Bray a écrit : >> Hi all, >> >> Sorry if this has been discussed ad-infinitum before--I looked around >> a bit but didn't find a definitive answer. >> >> I have one (well at least one) test that's failing on Cygwin due to >> tiny difference in the last digit of the result of log(3). >> >> This leads to to several questions: >> >> 1) Is it worth investigating the reason for the difference? >> 2) Is it worth trying to provide a workaround for the difference? >> 3) Or should the test just be updated to ignore the last couple digits >> of the result, and if so how (ellipses?) >> >> Thanks, >> Erik >> > Hello, > > What do you mean by log(3) ? > > Is it log(3.) or log(RDF(3)) ?
Really what I meant was `float(log(3))` which I guess in Sage ends up being same as `log(RDF(3))` > With log(RDF(3)), this is classical; changing the library version, or > the compiler version often changes the way some expressions and > functions are computed. As floating point arithmetic is not associative, > this is classical. With log(3.) (RealField()), I don't know. > When we wrote the book "Calcul Mathématique avec Sage", we had this > problem: after some release (some months) some doctests of the chapter > on numerical algebra and floating point numbers failed. The only work > around we found was to truncate the last digit in the output, to try to > test what should remain constant. I have: sage: R = RealField(100) sage: log(R(3)) 1.0986122886681096913952452369 Okay. With Cygwin's libm: sage: float(log(3)) 1.0986122886681096 While on Linux I get: sage: float(log(3)) 1.0986122886681098 It looks like the latter result is properly rounded up (with an error of 1ulp) while my result on Cygwin is just truncated. -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
