[SciPy-User] Some thing wrong in scipy reference

[Warren Weckesser]

On Sat, Jul 21, 2012 at 9:39 AM, Ralf Gommers
<ralf.gommers at googlemail.com>wrote:

>
>
> On Sat, Jul 21, 2012 at 1:26 PM, Warren Weckesser <
> warren.weckesser at enthought.com> wrote:
>
>>
>>
>> On Sat, Jul 21, 2012 at 3:02 AM, Ralf Gommers <
>> ralf.gommers at googlemail.com> wrote:
>>
>>>
>>>
>>> 2012/7/21 王瑞期 <shahuwang at 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?
>>
>
> Four years ago, in 62e99493, a ddof kw was added. But that looks like a
> backwards-compatible commit to me.  I'll change the output in the whiten
> docstring.
>
> Ralf
>
>
While you're at it, could you fix the spelling mistake ("devation") in the
description of the return value?   Thanks!

Warren



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