Multiplication of complex conjugate numbers wrong in Octave 3.0.5 mingw32

[FélixB]

Jaroslav Hajek-2 wrote:
> 
> On Sat, May 23, 2009 at 5:40 PM, FélixB
> <felix.buehlmann at acousticimplants.com> wrote:
>>
>>
>> Benjamin Lindner wrote:
>>>
>>> FélixB wrote:
>>>> Hello,
>>>>
>>>> I get an error when multiplying complex conjugate numbers (see example
>>>> below). The diagonal of the matrix should be real. I have not this
>>>> error
>>>> with version 3.0.3 (MSVC).
>>>>
>>>> octave:5> a = rand(10000,2)+j*rand(10000,2);
>>>> octave:6> imag(diag(a'*a))
>>>> ans =
>>>>
>>>>   -2.6396e-014
>>>>   -2.2185e-014
>>>>
>>>> Félix
>>>>
>>>
>>> Octave is not a symbolic math tool, it's a numerical math tool.
>>> Thus it uses floating-point arithmetic.
>>> Floating-point arithmetic shows roundoff errors like the ones you
>>> describe here, it's inherent.
>>> So it's not an error, this is floating point arithmetic.
>>>
>>> benjamin
>>> _______________________________________________
>>> Bug-octave mailing list
>>> Bug-octave at octave.org
>>> https://www-old.cae.wisc.edu/mailman/listinfo/bug-octave
>>>
>>>
>> Thank's Benjamin for your answer, but I think there is a bug. As said
>> before, I get the correct answer in version 3.0.3 (i.e no imaginary
>> part).
>> See please the example below which give the correct answer with a matrix
>> of
>> 10000x1, but a different result when duplicated in a 10000x2 matrix .
>>
>> I never seen such an error in previous version.
>>
>> Thank's for you help.
>>
>> octave:15> a=rand(10000,1)+j*rand(10000,1);
>> octave:16> imag(a'*a)
>> ans = 0
>> octave:17> b=[a,a];
>> octave:18> imag(b'*b)
>> ans =
>>
>>  -2.4536e-014  -2.4536e-014
>>  -2.4536e-014  -2.4536e-014
>> --
> 
> Matrix multiplication is handled by the BLAS library, so if the two
> versions come with different BLASes, the difference is understandable.
> Floating point multiplication is not associative, so reordering
> operations may break expectations.
> Example:
> octave:1> a = complex(rand(4,1),rand(4,1));
> octave:2> imag(a'*a)
> ans = 0
> octave:3> sum([imag(a).*real(a); -real(a).*imag(a)])
> ans = -1.1102e-16
> octave:4>
> 
> Is there any real problem you're having with this?
> 
> 
> -- 
> RNDr. Jaroslav Hajek
> computing expert & GNU Octave developer
> Aeronautical Research and Test Institute (VZLU)
> Prague, Czech Republic
> url: www.highegg.matfyz.cz
> 
> _______________________________________________
> Bug-octave mailing list
> Bug-octave at octave.org
> https://www-old.cae.wisc.edu/mailman/listinfo/bug-octave
> 
> 

Hi Jaroslav,

Thank's for you answer. I used before Matlab for 20 years and I never seen
such a behavior. I have now  some .m files which fail because the result is
complex.  

This behavior can also be a compatibility issue with some Matlab script.

I don't known if it's easy to change it, but if the answer is yes, I think
it will be worth to avoid different results for different platforms. I would
also except clean calculation for other operation for example  with the FFT( 
the inverse FFT of the FFT of a real signal should be real, ...).

BR 

Félix
-- 
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