bugGNU Octave - Bugs: bug #65170, [octave forge] (statistics)...


bug #65170: [octave forge] (statistics) Failing BIST in fitcknn

Submitter:  Rafael Laboissière <rlaboiss>
Submitted:  Tue 16 Jan 2024 05:07:27 PM UTC
Category:  Octave Package Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Unexpected Error or Warning
Status:  Fixed Assigned to:  None
Originator Name:  Open/Closed:  * Closed
Release:  * other Operating System:  * GNU/Linux
Fixed Release:  None Planned Release:  None
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Thu 07 Mar 2024 06:34:58 PM UTC, comment #5: 

This can be closed

Andreas Bertsatos <pr0m1th3as>
Sun 21 Jan 2024 03:23:18 PM UTC, comment #4: 
Andreas Bertsatos <pr0m1th3as>
Wed 17 Jan 2024 11:09:52 AM UTC, comment #3: 

Although the notion of bug-to-bug compatibility appeals to my sense of humor, we 'd rather avoid it in Octave's functionality. I will push the changes (hopefully today or tomorrow) to statistics and make a new release, so that the release bundled in Octave 9.1 does not maintain bug compatibility with Matlab.

Andreas Bertsatos <pr0m1th3as>
Tue 16 Jan 2024 11:02:11 PM UTC, comment #2: 

I think it is one of those "WTF Matlab?" bugs and Andreas wants to be bug-for-bug compatible with matlab.

octave:32> c = cov(x)
c =

   6.250000000000000e+00   6.750000000000000e+00   7.250000000000000e+00
   6.750000000000000e+00   8.250000000000000e+00   9.750000000000000e+00
   7.250000000000000e+00   9.750000000000000e+00   1.225000000000000e+01

octave:33> 4*c
ans =

   25   27   29
   27   33   39
   29   39   49

and one can easily calculate with pen and paper (or perhaps with some symbolic math software) that det(4*c) is exactly 0.
So "c" is NOT positive definite.


Dmitri A. Sergatskov <dasergatskov>
Tue 16 Jan 2024 05:24:51 PM UTC, comment #1: 

possibly related to
https://octave.discourse.group/t/octave-possible-bug-inconsistencies-with-chol-and-isdefinite/5162/69 ?  is there already a bug report on chol issues?

Nicholas Jankowski <nrjank>
Group Member
Tue 16 Jan 2024 05:07:27 PM UTC, original submission:  

One of the BISTs of function fitcknn of the statistics package, version 1.6.1, is failing:

octave:1> pkg load statistics
octave:2> test fitcknn
***** test
 x = [1, 2, 3; 4, 5, 6; 7, 8, 9; 3, 2, 1];
 y = ["a"; "a"; "b"; "b"];
 C = cov (x);
 a = fitcknn (x, y, "Cov" , C, "distance", "mahalanobis");
 assert (class (a), "ClassificationKNN");
 assert ({a.DistParameter}, {C})
 assert ({a.NSMethod, a.Distance}, {"exhaustive", "mahalanobis"})
 assert ({a.BucketSize}, {50})
!!!!! test failed
ClassificationKNN: Cov must be a symmetric positive definite matrix.

According to the documentation, the argument "Cov" must be a positive definite matrix. Now, in the BIST above, the C matrix is not positive definite. Indeed:

octave:3> x = [1, 2, 3; 4, 5, 6; 7, 8, 9; 3, 2, 1];
octave:4> C = cov (x);
octave:5> [~, p] = chol (C)
p = 3

Rafael Laboissière <rlaboiss>


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    Date Changed by Updated Field Previous Value => Replaced by
    2024-03-07 nrjank StatusNone Fixed
        Open/ClosedOpen Closed

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