From owner-help-octave at bevo dot che dot wisc dot edu  Thu Nov 14 16:42:07 1996
Subject: Re: Efficient multiplication by a diagonal matrix
From: SANDS at VSDEC dot NL dot NUWC dot NAVY dot MIL
To: mstorti at galileo dot unl dot edu dot ar
Cc: help-octave at bevo dot che dot wisc dot edu
Date: Thu, 14 Nov 1996 17:41:46 -0400 (EDT)


Mario Storti writes:
>I found  myself repeatedly with the following  problem. Given a matrix
>A(n,m)  and a vector  v(n), I  have to  multiply   each row  A(j,:) by
>v(j). This is equivalent to compute:
>
>B = diag(v) * A                     (1)
>
>Now, for large    n, (1) is  very  inefficient ...

Right, try using "Tony's Trick", it's been around for years. 

Assuming v is a column vector (if not do v=v(:);) try

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

This is a kind-of MATLAB specific thing but I think 
it works in octave o.k.

Scott Sands