[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
## Re: late Re: A Simple Matrix Construction Question

**From**: |
Ted Harding |

**Subject**: |
Re: late Re: A Simple Matrix Construction Question |

**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.
--------------------------------------------------------------------
E-Mail: (Ted Harding) <address@hidden>
Fax-to-email: +44 (0)870 167 1972
Date: 06-Feb-03 Time: 09:18:05
------------------------------ XFMail ------------------------------
-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.
Octave's home on the web: http://www.octave.org
How to fund new projects: http://www.octave.org/funding.html
Subscription information: http://www.octave.org/archive.html
-------------------------------------------------------------