[Numpy-discussion] Documentation for dtypes with named fields

[Skipper Seabold]

On Tue, Mar 16, 2010 at 11:46 AM,  <josef.pktd at gmail.com> wrote:
> On Tue, Mar 16, 2010 at 11:34 AM, Skipper Seabold <jsseabold at gmail.com> wrote:
>> On Tue, Mar 16, 2010 at 11:20 AM, Sam Tygier
>> <Sam.Tygier at hep.manchester.ac.uk> wrote:
>>> Thanks for those responses.
>>>
>>> could the dtype pages in the numpy reference link to the basics.rec page in the user guide?
>>>
>>> there seem to be some gotchas in list within a list notation.
>>>
>>> if i have
>>> a = array([0,0.1,0.2,0.3,0.4])
>>> b =  array((0,0.1,0.2,0.3,0.4), dtype=[('a','f'), ('b','f'), ('c','f'), ('d','f'),('f','f')])
>>>
>>> then
>>>>>> a[[0,1,4]]
>>> array([ 0. ,  0.1,  0.4])
>>>>>> a[[4,1,0]]
>>> array([ 0.4,  0.1,  0. ])
>>>
>>> but
>>>>>> b[['a','b','f']]
>>> (0.0, 0.10000000149011612, 0.40000000596046448)
>>>>>> b[['f','b','a']]
>>> (0.0, 0.10000000149011612, 0.40000000596046448)
>>>
>>> so i always get the vales back in the original order. is the by design, or a bug?
>>>
>>
>> I've been bitten by this before too and asked the same question with
>> no response.  I think it's just a limitation of the design of
>> structured arrays.
>
> It might be by historical design, structured arrays are not really
> designed for slicing but I think more like sets of variables.
>
> But it means it cannot be used directly for the old pattern
>
> [arr(name) for name in listofnames]
>
> Skipper, Is this subset selection documented anywhere? I only know
> about it because you showed the example.
>

Just added it and a link to the cookbook for recarrays.  I don't think
it will show up until the doc wiki changes are applied(?).

http://docs.scipy.org/numpy/docs/numpy.doc.structured_arrays/

Skipper