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Re: Octave QR factorization
From: |
A S Hodel |
Subject: |
Re: Octave QR factorization |
Date: |
Wed, 17 Dec 2003 14:19:02 -0600 |
I am unable to access your script. You should check
norm(Q*R - A)/norm(A), which will (typically) be around m*n*1e-15,
where [m,n] = size(A).
You cannot expect perfect equality in numerical calculations.
Well, actually, you *can* expect it, but you'd be often disappointed by
your results.
On Wednesday, December 17, 2003, at 02:04 PM, Bart Vandewoestyne wrote:
Hello Octave-list,
While I was checking the correctness of an exercise given to my
students during one of my algebra exercise sessions, i found the
following curiosity:
The purpose of the exercise was for students to calculate the QR
factorization of a given matrix A. Now all I wanted to do is enter
the factorization we found by hand into Octave and see if the product
of Q and R is again the original matrix A.
What I found is that when I multiply the matrices Q and R we've found
by hand and then check for Q*R==A, Octave tells me that Q*R is not
equal to A, most probably due to rounding errors I guess...
If I run the same script in Matlab, Matlab *does* give me that
Q*R=A... so it doesn't experience the rounding errors as much as
Octave does???
So now my question is: why does Matlab gives different results than
Octave? Is there a big difference in the implementation of QR between
Matlab and Octave? Is there a way to decrease the rounding errors
made by Octave so we get the same accuracy as Matlab?
The script I used can be found at
http://www.cs.kuleuven.ac.be/~bartv/p370_oef1_6_17.m
When replying, please also reply to me personally, as I am not
subscribed to the list...
Thanks,
Bart
--
Bart Vandewoestyne Bart.Vandewoestyne_at_pandora.be
Naamsesteenweg 328 bus 201 GSM: +32 (0)478 397 697
B-3001 Heverlee http://osswin.sourceforge.net
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Octave is freely available under the terms of the GNU GPL.
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Subscription information: http://www.octave.org/archive.html
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Re: Octave QR factorization,
A S Hodel <=