matrix inverse bug

[macrom]

Hi,

We are working with labview8 and the matrix inverse (at least the 4 by 4) differs from the result of matlab, is there something that I could be doing wrong?

[smercurio_fc]

Do you have a matrix for which you get the wrong answer? I tried it with a few matrices and I was getting the same answer.

[altenbach]

You are using the "Inverse Matrix" primitive from the linear algebra palette, not the 1/x primitive of the numeric palette, right?

[macrom]

here... thanks for the quick response!

[GerdW]

Hi macrom,

atleast the MathScript node gives the same result as the attachment shows...

Edited: changed the mathscript command from "a^-1" to "inv(a)" - both give the same result (within DBL precision).


Message Edited by GerdW on 07-03-2008 01:25 PM

[macrom]

Hi GerdW,

Thanks for the reply.

Im sorry I cant open the vi file, I only have the labview 8 and the file is saved as 8.5

I've tried the inverseMatrix.vi and the mathscript with both a^-1 and inv(a), all three are consistent with each other but still inconsistent with Matlab and Mathematica. A mathematician friend of mine said that it could be a numerical inestability of the algorithm. Has anyone heard of Labview having this problem? It could still be that its Matlab or Mathematica.

Anyway, I decided to use a matrix that works well in all the cases.

Thank you very much!!!

[altenbach]

Well, Mathscript is basically LabVIEW under the hood, so I don't expect them to be different.

How different are the results between LabVIEW and matlab?

I did some test and the LabVIEW version seems OK.

1. If you take the inverse twice, you get the input matrix back within 1e-17.
2. I also tried the scilab script node, the results differ from LabVIEW by about 1e-16, or about the precision of DBL. So clearly they use an entirely different algorithm, but give the same result!

Message Edited by altenbach on 07-03-2008 08:13 AM

[GrantM]

Hello macrom,

If it is just a small difference, it is most likely due to algorithmic differences.  Can you post the result matrix you get from your computation in the MATLAB® software?

MATLAB® is a registered trademark of The MathWorks, Inc.

Grant M.
Staff Software Engineer | LabVIEW Math & Signal Processing | National Instruments