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'.)
Duncan Murdoch
It appears to be an often used trick in numerical analysis. One
advantage is
that a function using it is immediately vectorized while an expression
such
as, e.g., "max(0, 1 - (x^2 + y^2))" is not.
The example runs fine on Debian Linux and Mac OS X 32-/64-bit
architectures.
In my understanding the approach is correct and, as said above, often
used in
numerical applications.
Can someone explain to me why this fails for the Windows 64-bit
compiler and
what I should use instead. Thanks.
Hans Werner Borchers
ABB Corporate Research
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--
Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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