[scilab-Users] Atan function

[Antoine Monmayrant]

Le 10/05/2011 15:20, Adrien Vogt-Schilb a écrit :
> it looks like a bug, which could be linked to the fact that even if
> Xr(11:12)==[0 0], Xr(11:12) are complex number for scilab (try isreal(Xr(11:12))).
>
> are you trying to get the phase of thoses complex numbers?
> if yes, i suggest you to use something like imag(log(Xr(11:12)+%eps)) as a crapy
> workaround (instead of atan(imag(xr),real(xr)))
>
>
>
> Le 10/05/2011 14:38, Laurent Berger a écrit :
> >  In this program
> >
> >  clear;
> >  N=256;
> >  n=0:N-1;
> >  Xr=zeros(1,N);
> >  f0=2;
> >  Xr(f0+1)=1/(2*%i);
> >  Xr(N-(f0+1)+2)=-1/(2*%i);
> >  Xr=Xr.*exp(%i*%pi*2*6*n/N);
> >  xr=ifft(Xr);
> >  disp (atan(imag(Xr(11:12)),real(Xr(11:12))))
> >  disp (Xr(11)-Xr(12))
> >
> >  results are
> >  0. 3.1415927
> >
> >  0
> >
> >
> >  I don't understand why two numbers with a difference of 0 can have a numerical
> >  phase of 0 or pi .
> >  In matlab results are
> >  0 0
> >  0
> >
> >
> >  !Scilab Version: 5.3.1.1299160056 !
> >  ! !
> >  !Operating System: Windows XP 5.1 !
> >  ! !
> >  !Java version : 1.6.0_18 !
> >  ! !
> >  !Java runtime information : Java(TM) SE Runtime Environment (build
> >  1.6.0_18-b07) !
> >  ! !
> >  !Java vm information : Java HotSpot(TM) Client VM (build 16.0-b13, mixed mode) !
> >  ! !
> >  !Vendor specification: Sun Microsystems Inc. !
>
>
> -- 
>
> *Adrien Vogt-Schilb*
>
> Research Fellow
>
> vogt at centre-cired.fr<mailto:vogt at centre-cired.fr>
>
> Tel: (+33) 1 43 94 73 96
>
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>
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>
Definitely a bug that I encountered last year: a complex number with an 
imaginary part that is strictly 0 is not real (or at least not always).
I ended up with a couple of thisnumberisreal=real(thisnumberisreal) 
inside my code as a workaround.
It took me some time to understand what was going on.

Antoine