[SciPy-User] Some thing wrong in scipy reference

Warren Weckesser warren.weckesser@enthought....
Sat Jul 21 06:26:45 CDT 2012


On Sat, Jul 21, 2012 at 3:02 AM, Ralf Gommers
<ralf.gommers@googlemail.com>wrote:

>
>
> 2012/7/21 王瑞期 <shahuwang@qq.com>
>
>>
>> http://docs.scipy.org/doc/scipy/reference/generated/scipy.cluster.vq.whiten.html#scipy.cluster.vq.whiten
>>
>>  In the bottom of this page,the result of the whiten function is wrong.
>>
>
> You're right, the current output as documented is off by a constant factor:
> >>> res = whiten(features)
> >>> res * 3.41250074 / res[0, 0]  # reproduce docstring output
> array([[ 3.41250074,  2.20300046,  5.88897275],
>        [ 2.69407953,  2.39456571,  7.62102356],
>        [ 1.43684242,  0.57469577,  5.88897275]])
>
> Does anyone know why, and is this result correct?
>
> Ralf
>
>

whiten(obs) returns (in effect) obs / obs.std(axis=0).  It appears that
the docstring shows obs / obs.std(axis=0, ddof=1):

In [14]: obs
Out[14]:
array([[ 1.9,  2.3,  1.7],
       [ 1.5,  2.5,  2.2],
       [ 0.8,  0.6,  1.7]])

In [15]: vq.whiten(obs)
Out[15]:
array([[ 4.17944278,  2.69811351,  7.21248917],
       [ 3.29956009,  2.93273208,  9.33380951],
       [ 1.75976538,  0.7038557 ,  7.21248917]])

In [16]: obs / obs.std(axis=0)
Out[16]:
array([[ 4.17944278,  2.69811351,  7.21248917],
       [ 3.29956009,  2.93273208,  9.33380951],
       [ 1.75976538,  0.7038557 ,  7.21248917]])

In [17]: obs / obs.std(axis=0, ddof=1)
Out[17]:
array([[ 3.41250074,  2.20300046,  5.88897275],
       [ 2.69407953,  2.39456571,  7.62102355],
       [ 1.43684242,  0.57469577,  5.88897275]])


The whiten function has not changed in a long time.  Has the
default value of ddof in the numpy std() method changed in
the last five (or more) years?

Warren



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