Re: Two possible problems with gsl_complex behavior

[Brian Gough <bjg at network-theory dot co dot uk>]

Charlie Zender writes:
 > First, and least troublesome, is that the result of
 > gsl_complex_cos(gsl_complex z) where z=(0.0,0.0) does not appear to
 > be exactly (1.0,0.0), but rather (1.0,-0.0). 

Thanks for the bug reports.  

For cos(z) I've adjusted the code so that the sign of zero is positive
for Im(z) = 0.  

The negative sign was a result of the identity used to calculate the
imaginary part,

  cos(x+iy) = cosh(x) cos(y) - i sin(x) sinh(y)

 > Second, and more troublesome, is the behavior of gsl_complex functions
 > when used as arguments to other functions. Again, using g++-2.96, the
 > following used to work in gsl-0.7 "as expected":
 > 
 > float_complex cpx=(0.0,0.0);
 > gsl_complex ngl_cos=gsl_complex_cos(gsl_complex_rect(cpx.real(),cpx.imag())); 

I had a quick look and this function does not appear to have changed
between 0.7 and 0.9 apart from,

-- the name being changed from gsl_complex_xy() to gsl_complex_rect()

-- the addition of an inline version in the header file, wrapped with
#ifdef HAVE_INLINE (so this should not cause any change unless
compiling the test program with -DHAVE_INLINE)

I tried compiling with g++ 2.95 and it worked ok.

Before I investigate more did you try compiling with -Wall and other
warning options?  Also, you know that the initializer should be
float_complex cpx(0.0,0.0); rather than =(0.0,0.0); right?

regards
Brian Gough