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Message-ID: | <905080.83354.qm@web25508.mail.ukl.yahoo.com> |
Date: | Mon, 3 May 2010 12:42:23 +0000 (GMT) |
From: | Marco Atzeri <marco_atzeri AT yahoo DOT it> |
Subject: | R: Octave 3.2.3: wrong matrix calculation |
To: | texsit <texsit AT yahoo DOT it> |
Cc: | cygwin AT cygwin DOT com |
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--- Lun 3/5/10, texsit ha scritto: > Hi All, > I use octave 3.2.3 under Cygwin (windows 7) for my math > application.=20 > I discovered a bad calculation calling the function det(). > The step to reproduce is this: >=20 > > A =3D [1,2,3;4,5,6;7,8,9]; > > det(A) > ans =3D -9.5171e-16 >=20 > The matrix is singular so I expected the resul was 0 and > not a very very little number. I problem is the same also > with Octave 3.2.4. > At the begin I thougth it was only a round error, but I've > made the same proof with Octave under Debian Linux (lenny). >=20 > Under Linux=A0the resul has been 0 as I expected. >=20 > Can anyone suggest me something to solve that problem? >=20 > I'm using Cygwin 1.7.5-1 with Intel centrino 2 and windows > 7 64 bit. >=20 > Thank you in advance. > Regards. >=20 > Luca >=20 Dear Luca, it is not an error , it is a rounding feature ;-) On cygwin most of the calculation are performed with 80 bits but data are stored with 64 bits, while on Linux both are=20 on 64 bits. For this reason the same sequence of operations=20 could provide a slightly different outcome. Suggested reading: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D323 http://hal.archives-ouvertes.fr/hal-00128124 In any case if you ask also for the matrix reverse=20 condition number [D, RCOND] =3D det (A) D =3D -9.5171e-16 RCOND =3D 2.2030e-18 you can recognize that the matrix is very bad conditioned Regards Marco Atzeri octave cygwin maintainer -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
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