From help-octave-request at bevo dot che dot wisc dot edu Thu Feb  6 03:46:16 2003
Subject: Re: late Re: A Simple Matrix Construction Question
From: (Ted Harding) <Ted dot Harding at nessie dot mcc dot ac dot uk>
To: David Bateman <David dot Bateman at motorola dot com>
Cc: help-octave at bevo dot che dot wisc dot edu, alrl1@alu.um.es
Date: Thu, 06 Feb 2003 09:18:05 -0000 (GMT)

On 05-Feb-03 David Bateman wrote:
> If this is what you want to do, why not do for example
> 
> m = 4;
> n = 5;
> y = rand(1,n);
> x = y(ones(m,1),:);
> 
> where x will now be a m by n matrix with m copies of y,
> one per row. Indexing should definitively be more
> computationally efficient than doing a matrix multiply
> which is O(n^3).

In this case, I think pre-multiplication by a column
vector of 1s is slightly quicker, though not much.

With y already set up as above:

octave:9> tic;for i=1:50000,x = y(ones(m,1),:);endfor,toc
ans = 7.4079

octave:10> tic;for i=1:50000,x = ones(m,1)*y;endfor,toc
ans = 6.0705

Ted.


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Date: 06-Feb-03                                       Time: 09:18:05
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