[Octave-bug-tracker] [bug #42742] polygcd fails valid test

[Michael Godfrey]

URL:
  <http://savannah.gnu.org/bugs/?42742>

                 Summary: polygcd fails valid test
                 Project: GNU Octave
            Submitted by: godfrey
            Submitted on: Sat 12 Jul 2014 01:04:56 PM GMT
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: Godfrey
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: Any

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Details:

This was first reported on the Mainbtainters List, but it appears
to be a bug in polygcd.

On 07/07/2014 06:57 PM, Rik wrote:
> All,
>
> I'm getting occasional failures from the test code in polygcd.  It seems to
> happen about 10% of the time according to the following code:
>
> for i = 1:100
>    bm(i) = test ("polygcd");
> endfor
> sum (bm)
>
> The test that fails is
>
> %!test
> %! for ii=1:10
> %!   p  = (unique (randn (10, 1)) * 10).';
> %!   p1 = p(3:end);
> %!   p2 = p(1:end-2);
> %!   assert (polygcd (poly (-p1), poly (-p2)), poly (- intersect (p1, p2)),
> sqrt (eps));
> %! endfor
>
> I tried a few different random seeds to see if I could fix the value to
> something that would always pass, but no luck.
>
> The simplest thing is to make this an %!xtest which can occasionally fail.
> But if someone understands polygcd and could suggest a way to modify the
> test that would be preferable.
>
> The error I get is a dimensional mismatch:
>
> !!!!! test failed
> ASSERT errors for:  assert (polygcd (poly (-p1), poly (-p2)),poly
> (-intersect (p1, p2)),sqrt (eps))
>
>    Location  |  Observed  |  Expected  |  Reason
>       .          O(1x1)       E(1x7)      Dimensions don't match
>
>
> --Rik
Here is where this goes wrong:

If the line [d, r] = deconv (b, a);  in polygcd returns a r vector whose first
element
is zero, then the line   a = r  /  r(1); in polygcd fails.  At this point a
ends up
as a = 1 and x =  1,  and the assert fails as above. I do not right now see
why  a = 1
due to divide by zero, but this is the cause of the failure.

The following script will, for me, reliably produce the failure as described
above:

 for ii=1:1000
   p  = (unique (randn (10, 1)) * 10).';
   p1 = p(3:end);
   p2 = p(1:end-2);
   assert (polygcd (poly (-p1), poly (-p2)), poly (- intersect (p1, p2)), sqrt
(eps));
 endfor

Obviously, the line  a = r / r(1);  should not be executed if r(1) = 0. But,
it is
not clear to me right now what should be done instead.

I hope this helps.





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