From help-octave-request at bevo dot che dot wisc dot edu  Sat Dec  4 22:03:43 1999
Subject: RE: Dot product (nx1).*(nxm)
From: Heber Farnsworth <FARNSWORTH at olin dot wustl dot edu>
To: "'Andre Bonfrer'" <a dot bonfrer at mbs dot unimelb dot edu dot au>
Cc: "'help-octave at bevo dot che dot wisc dot edu'" <help-octave@bevo.che.wisc.edu>
Date: Sat, 4 Dec 1999 22:03:40 -0600 

I'm assuming that what you mean by this is that you the ith row of your nxm
matrix to be multiplied by the ith element of your n vector.  If this is the
case then you should write this as

p(:,ones(1,m)).*v

where p is nx1 and v is nxm.  What this does is to multiply v by a nxm
natrix which has p as every column.

Kron is very slow.  It's implemented as an m-file with lots of loops so you
want to avoid it when you need speed.  I think the next version of octave
will have a much faster implementation.  If you need a fast version of kron
now then look on the archive for a message I sent 

	http://www.che.wisc.edu/octave/mailing-lists/help-octave/1999/829

which contains the code for an oct-file which is very fast.

Heber Farnsworth
 
> Hi!
> I'm trying to minimise a function that requires a lot of dot 
> products of
> the
> form  (dimensions)
> (nx1).*(nxm)
> e.g. p .* v
> where
> p is size nx1
> v is size nxm
> 
> Right now I'm using kron(p,ones(1,m)).*v but it's very slow.
> I'm using Octave 2.1.9 on RH6.1, and I've searched the faq's, archives
> etc. but
> don't seem to be able to find any way of doing this.
> 
> Can anyone help?  Are there any functions out there that may 
> be a little
> 
> faster than the kron product?
> Andre
> 
> --
> Andre Bonfrer
> The Coles Myer Center for Retailing and Marketing
> Melbourne Business School
> 200 Leicester Street
> Carlton, VIC 3053
> AUSTRALIA
> Tel: 61 3 9349 8194
> Fax: 61 3 9349 8193
> Email: a dot bonfrer at mbs dot unimelb dot edu dot au
> 
> 
> 
> 
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