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'.)
Section 6.3 of IEEE 754-2008 says
Except that squareRoot(−0) shall be −0, every numeric squareRoot result
shall have a positive sign.
Duncan Murdoch
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