Re: [Help-gsl] angle between two vectors

[Jerome BENOIT]

are we dealing with direction or vectors ?

Liam Healy wrote:
> I'm a big fan of atan2, but in this case there's no benefit to using it
> because an angle between two vectors will always be less than or equal
> to 180 degrees, so the quadrant is unambiguous from the cosine.
> Dot product and cosine is the right way to go.
>
> Liam
>
> On Sun, Mar 28, 2010 at 11:11 AM, Jerome BENOIT <address@hidden> wrote:
> > Hello List,
> >
> > a better idea is to build some code around the atan2 function.
> >
> > Jerome
> >
> > John D Lamb wrote:
> > > On Sun, 2010-03-28 at 11:07 +0200, Jordi wrote:
> > > > I am quite new to maths and gsl I would like to get the angle between two
> > > > vectors.
> > > In C it should be something like the following. (You may have to check
> > > where acos is defined, check norm_u * norm_v != 0; and you may want a
> > > direction with the angle.) For floats use snrm and sdot instead of dnrm
> > > and ddot.
> > >
> > > #include<gsl/gsl_blas.h>
> > > …
> > >  gsl_vector* u, v;
> > >  double norm_u, norm_v, x, angle;
> > > …
> > >  norm_u = gdl_blas_dnrm( u );
> > >  norm_v = gdl_blas_dnrm( v );
> > >  gsl_blas_ddot( u, v, &x );
> > >  angle = acos( x / (norm_u * norm_v ) );
> > --
> > Jerome BENOIT
> > jgmbenoit-at+rezozer*dot_net

--
Jerome BENOIT
jgmbenoit-at+rezozer*dot_net