[Numpy-discussion] field names on numpy arrays

Pierre GM pgmdevlist@gmail....
Wed Jun 3 18:20:43 CDT 2009


On Jun 3, 2009, at 5:03 PM, Robert Kern wrote:

> On Wed, Jun 3, 2009 at 15:26,  <josef.pktd@gmail.com> wrote:
>> 2009/6/3 Stéfan van der Walt <stefan@sun.ac.za>:
>>> Hi Jon
>>>
>>> 2009/6/3 D2Hitman <j.m.girven@warwick.ac.uk>:
>>>> I understand record arrays such as:
>>>> a_array =
>>>> np.array([(0.,1.,2.,3.,4.),(1.,2.,3.,4.,5.)],dtype=[('a','f'), 
>>>> ('b','f'),('c','f'),('d','f'),('e','f')])
>>>> do this with field names.
>>>> a_array['a'] = array([ 0.,  1.], dtype=float32)
>>>> however i seem to lose simple operations such as multiplication  
>>>> (a_array*2)
>>>> or powers (a_array**2).
>> Why does it not preserve "shape", to do e.g. np.mean by axis?
>
> It does preserve the shape. The input and output are both 1D. If you
> need a different shape (e.g. re-interpreting the record as another
> axis), you need to reshape it yourself. numpy can't guess what you
> want.

Or, as all fields have the same dtype:

 >>> a_array.view(dtype=('f',len(a_array.dtype)))
array([[ 0.,  1.,  2.,  3.,  4.],
        [ 1.,  2.,  3.,  4.,  5.]], dtype=float32)

Ain't it fun ?




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