Re: [OctDev] control-2.3.51 released in package forum - please upload

[c.]

On 4 Jun 2012, at 23:56, Thomas Weber wrote:

> Eh no, not quite. We usually look at the errors and decide. But here we
> are talking about a factor of 10000 between expected and actual result.
> That's a bug.

As far as I understand from Lukas' explanation, the sign in the result is not 
relevant.
The failures I get are of the form:

  ***** assert (Mo, Me, 1e-4);
!!!!! test failed
assert (Mo,Me,1e-4) expected

  -0.23910   0.30720   1.16300   1.19670  -1.04970
  -2.97090  -0.23910   2.62700   3.10270  -3.70520
   0.00000   0.00000  -0.51370  -1.28420   0.82230
   0.00000   0.00000   0.15190  -0.51370   0.74350
  -0.44660   0.01430  -0.47800  -0.20130   0.02190

but got

  -0.23915   0.30723   1.16297  -1.19671   1.04965
  -2.97091  -0.23915   2.62702  -3.10273   3.70515
   0.00000   0.00000  -0.51368   1.28421  -0.82227
   0.00000   0.00000  -0.15189  -0.51368   0.74348
   0.44660  -0.01427   0.47803  -0.20129   0.02190

maximum absolute error 7.41035 exceeds tolerance 0.0001

If I take the abs () of the results I have 

>> abs (Mo) - abs (Me) 
ans =

  -5.0000e-05  -3.0000e-05   3.0000e-05  -1.0000e-05   5.0000e-05
  -1.0000e-05  -5.0000e-05  -2.0000e-05  -3.0000e-05   5.0000e-05
   0.0000e+00   0.0000e+00   2.0000e-05  -1.0000e-05   3.0000e-05
   0.0000e+00   0.0000e+00   1.0000e-05   2.0000e-05   2.0000e-05
   0.0000e+00   3.0000e-05  -3.0000e-05   1.0000e-05   0.0000e+00

so results are well within the expected tolerance.
Of course if the abs is left out of the test the error looks much larger as 
x - (- x) = 2 * x so I'd really prefer the assert above to be changed to 
"assert (abs (Mo), abs (Me), 1e-4);" to avoid confusing error messages.

> I also don't buy the argument that ATLAS and BLAS will produce wildly
> differing results that are both correct. Whenever the differences
> between ATLAS and BLAS were huge in the past, there was a bug in the
> library or in the calling code.

As to why the sign of the result is not predictable, I do not know enough about 
the algorithm being implemented here to comment. Lukas, could you point to 
the specific BLAS routine that is giving different results here?

c.