On Fri, Jan 27, 2012 at 1:40 PM, Duncan Murdoch <murdoch.dun...@gmail.com> wrote: > On 27/01/2012 12:32 PM, Prof Brian Ripley wrote: >> >> On 27/01/2012 13:26, Duncan Murdoch wrote: >> > On 12-01-27 7:23 AM, Hans W Borchers wrote: >> >> I have a question concerning the new Windows toolchain for R>= 2.14.2. >> >> When trying out my package 'pracma' on the win-builder development >> >> version >> >> it will stop with the following error message: >> >> >> >> > f3<- function(x, y) sqrt((1 - (x^2 + y^2)) * (x^2 + y^2<= 1)) >> >> > dblquad(f3, -1, 1, -1, 1) # 2.094395124 , i.e. 2/3*pi , err = 2e-8 >> >> Warning in sqrt((1 - (x^2 + y^2)) * (x^2 + y^2<= 1)) : NaNs produced >> >> Warning in sqrt((1 - (x^2 + y^2)) * (x^2 + y^2<= 1)) : NaNs produced >> >> Error in integrate(function(y) f(x, y), ya, yb, subdivisions = >> >> subdivs, : >> >> non-finite function value >> >> Calls: dblquad ... >> >> <Anonymous> -> f -> do.call -> mapply -> <Anonymous> -> >> >> integrate >> >> Execution halted >> >> ** running examples for arch 'x64' ... ERROR >> >> Running examples in 'pracma-Ex.R' failed >> >> >> >> This probably means that the following expression got negative for >> >> some >> >> values x, y: >> >> >> >> (1 - (x^2 + y^2)) * (x^2 + y^2<= 1) >> > >> > I think you're right, it's a bug, hopefully easy to fix. Here's a >> > simpler version: >> > >> > x<- 0*(-1) >> > sqrt(x) >> > >> > x is a "negative zero", and the sqrt() function incorrectly produces a >> > NaN in the new toolchain. >> >> Well, for some definition of 'incorrectly'. It is clearly what the >> author of that piece of code intended. >> >> It would be helpful if people would cite definitive references. Someone >> is going to have to report this on the bugtracker, and at present I >> don't have enough evidence to do so: the C99/C11 standards do not seem >> to mandate a particular value (they do say what happens for values less >> than zero, but C compilers are allowed to have or not have signed >> zeroes). (Various Unix-alikes say what they do, usually -0, but that's >> not evidence that other answers are 'incorrect'.) > > > This page: > > http://pubs.opengroup.org/onlinepubs/9699919799/functions/sqrt.html > > also says that the correct answer is -0. I don't know if that has any > authority at all... > > Duncan Murdoch
Section 5.4.1 "Arithmetic operations" of "IEEE Standard for Floating- Point Arithmetic", IEEE Std 754-2008 says: "The operation squareRoot(x) computes √ x. It has a positive sign for all operands ≥ 0, except that squareRoot(−0) shall be −0." - Steve ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
