[SciPy-Dev] DST I precision (was ANN: SciPy 0.11.0 release candidate 1)

[Ralf Gommers]

On Sat, Jul 21, 2012 at 9:57 PM, Ralf Gommers
<ralf.gommers at googlemail.com>wrote:

>
>
> On Sat, Jul 21, 2012 at 6:06 PM, John Hassler <hasslerjc at comcast.net>wrote:
>
>>
>> On 7/21/2012 10:43 AM, Ralf Gommers wrote:
>>
>>
>> On Fri, Jul 20, 2012 at 9:57 PM, John Hassler <hasslerjc at comcast.net>wrote:
>>
>>  ======================================================================
>>> FAIL: test_definition (test_real_transforms.TestDSTIDouble)
>>>
>>> ----------------------------------------------------------------------
>>> Traceback (most recent call last):
>>>    File
>>> "C:\Python32\lib\site-packages\scipy\fftpack\tests\test_real_transforms.py",
>>> line 213, in test_definition
>>>     err_msg="Size %d failed" % i)
>>>   File "C:\Python32\lib\site-packages\numpy\testing\utils.py", line 800,
>>> in assert_array_almost_equal
>>>     header=('Arrays are not almost equal to %d decimals' % decimal))
>>>   File "C:\Python32\lib\site-packages\numpy\testing\utils.py", line 636,
>>> in assert_array_compare
>>>     raise AssertionError(msg)
>>> AssertionError:
>>> Arrays are not almost equal to 15 decimals
>>> Size 256 failed
>>> (mismatch 3.515625%)
>>>  x: array([  1.00000000e+00,  -5.03902743e-01,   3.33300126e-01,
>>>         -2.51913719e-01,   1.99940224e-01,  -1.67900640e-01,
>>>          1.42771743e-01,  -1.25881543e-01,   1.11000399e-01,...
>>>  y: array([  1.00000000e+00,  -5.03902743e-01,   3.33300126e-01,
>>>         -2.51913719e-01,   1.99940224e-01,  -1.67900640e-01,
>>>          1.42771743e-01,  -1.25881543e-01,   1.11000399e-01,...
>>>
>>
>> This looks like the test precision being a little too high. Could you
>> adjust it to decimal=14 or lower, and tell us when the test passes on your
>> machine?
>>
>> Thanks,
>> Ralf
>>
>>
>> decimal = 14 passes.
>> john
>>
>
> This test and the failure look a bit odd, so I just want to make sure: is
> it OK to lower the test precision? DST II passes with decimal=15 and DST
> III with decimal=16, so not sure why DST I is worse. Looks like this test
> could use some improvement:
>
>             # XXX: we divide by np.max(y) because the tests fail
> otherwise. We
>             # should really use something like assert_array_approx_equal.
> The
>             # difference is due to fftw using a better algorithm w.r.t
> error
>             # propagation compared to the ones from fftpack.
>             assert_array_almost_equal(y / np.max(y), yr / np.max(y),
> decimal=self.dec,
>                     err_msg="Size %d failed" % i)
>
>
Actually I get a failure myself for DSTIIIDouble, only with Python 2.4. It
seems that the DST test tolerances are set much higher than the DCT ones.
Lowering them all to decimal=14 would make sense to avoid further failures.
Any objections?

Ralf
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