Re: adding vectors and matrices

[O. Scott Sands]

Ted Harding wrote:
> 
> ( Re Message From: Andreas Heitmann )
> >
> > I'm using octave for some time now, but I'm still wondering if there
> > is a simple way to add a fixed column vector to a number of other
> > column vectors stored in a matrix. Currently i'm using a loop like the
> > following:
> >
> >       v=[1;2;3];
> >       for i=1:columns(x)
> >               y(:,i)=x(:,i)+v;
> >       endfor
> >
> > Is it possible to compute y without a loop statement?
> 
> Hi,
> 
>       y = x + v*ones(1,columns(x));
> 
> will do it. I understand John Eaton plans to incorporate a syntax like
> 
>       y = x .+ v
> 
> for a future release which will do the same thing (and the corresponding
> thing if v is a row vector with as many elements as x has columns, i.e.
> add v to each row of x).
> 
> Best wishes,
> Ted.                                    (address@hidden)

nononononononono!

Octave supports zero-one indexing (ala MATLAB) so you
can use "Tony's Trick" (see Mathworks archives)

y=x+v(:,ones(1,columns(x)));

The indexing functions done in with Tony's Trick
are much faster than the multiply operations 
suggested by y=x+v*ones(1,columns(x)). Try it!

Since this issue appears periodically in the 
mailing list here, maybe it should be put in 
the manual?

Scott Sands