From owner-help-octave at bevo dot che dot wisc dot edu  Thu Nov 14 21:38:54 1996
Subject: Re: Efficient multiplication by a diagonal matrix
From: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>
To: SANDS at VSDEC dot NL dot NUWC dot NAVY dot MIL
Cc: mstorti at galileo dot unl dot edu dot ar, help-octave@bevo.che.wisc.edu
Date: Thu, 14 Nov 1996 21:37:57 -0600

On 14-Nov-1996, SANDS at vsdec dot nl dot nuwc dot navy dot mil <SANDS@vsdec.nl.nuwc.navy.mil> wrote:

: B=kron(v,ones(1,m)).*A;

Yes, kron will definitely be slow because it uses interpreted loops.

: B=v(:,ones(1,m)) .* A;

This will be fairly fast.  In the test I ran, with v = rand (1000,1)
and m = 10, it was nearly 600 times faster than kron.  For some cases
though,

  v * ones(1,m) .* A

will actually be faster, at least in Octave.

If the .* operator is overloaded to do the job of row and column
scaling, then I would expect that

  v .* A

will be faster than any of the other methods.  It will also use less
memory, which might turn out to be important if length(v) columns(A)
are both large.  It might even be easier to read, too.

So, why am I writing this instead of implementing that?  :-)

jwe