[Numpy-discussion] Slices of structured arrays

[Robert Kern]

On Mon, Jun 1, 2009 at 17:32, Robert Ferrell <ferrell at diablotech.com> wrote:
> Is there a way to get slices of a structured array and keep the field
> names?  For instance, I've got dtype=[('x','f4'),('y','f4'),
> ('z','f4')] and I want to get just the x & y slices into a new array
> with dtype=[('x','f4'),('y','f4')].
>
> I can just make a new dtype, and extract what I need, but I'm
> wondering if there's some simple way to do this that I haven't found.
>
> Here's what I know works:
>
> # Make a len 10 array with 3 fields, 'x', 'y', 'z'
> In [647]: xyz = np.array(zip(*np.random.random_integers(low=10,
> size=(3,10))), dtype=[('x', 'f4'), ('y', 'f4'), ('z', 'f4')])
>
> # Get just the 'x' and 'y' fields
> In [648]: xy = np.array( zip(xyz['x'], xyz['y'] ), dtype=[('x','f4'),
> ('y', 'f4')])
>
> In [649]: xyz['x']
> Out[649]: array([ 4.,  1.,  1.,  5.,  1.,  2.,  9.,  8.,  1.,  9.],
> dtype=float32)
>
> In [650]: xy['x']
> Out[650]: array([ 4.,  1.,  1.,  5.,  1.,  2.,  9.,  8.,  1.,  9.],
> dtype=float32)
>
> That works, but just feels like there's probably an elegant solution I
> don't know.  I couldn't find anything in the docs, but I may not have
> been using the right search words.

In numpy 1.4, there will be a function that does this,
numpy.lib.recfunctions.drop_fields(). In the meantime, you can
copy-and-paste it into your own code:

  http://svn.scipy.org/svn/numpy/trunk/numpy/lib/recfunctions.py

Or use it from it's original source,
matplotlib.mlab.rec_drop_fields(), if you have matplotlib.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco