On Wed, Jan 25, 2012 at 2:27 PM, Chris Barker <chris.barker@noaa.gov> wrote:

HI folks,

Is there a way to get a view of a subset of a structured array? I know that an arbitrary subset will not fit into the numpy "strides"offsets" model, but some will, and it would be nice to have a view:

For example, here we have a stuctured array:

In [56]: a Out[56]: array([(1, 2.0, 3.0, 4), (7, 8.0, 9.0, 10), (5, 123.4, 7.0, 8),       (9, 10.0, 11.0, 12), (13, 14.0, 15.0, 16)],      dtype=[('i', '<i4'), ('f1', '<f8'), ('f2', '<f8'), ('i2', '<i4')])

if I pull out one "field" a get a view:

In [57]: b = a['f1']

In [58]: b[0] = 1000

In [59]: a Out[59]: array([(1, 1000.0, 3.0, 4), (7, 8.0, 9.0, 10), (5, 123.4, 7.0, 8),       (9, 10.0, 11.0, 12), (13, 14.0, 15.0, 16)],      dtype=[('i', '<i4'), ('f1', '<f8'), ('f2', '<f8'), ('i2', '<i4')])

However, if I pull out more than one field, I get a copy:

In [60]: b = a[['f1','f2']]

In [61]: b Out[61]: array([(1000.0, 3.0), (8.0, 9.0), (123.4, 7.0), (10.0, 11.0), (14.0, 15.0)],      dtype=[('f1', '<f8'), ('f2', '<f8')])

In [62]: b[1] = (2000,3000)

In [63]: b Out[63]: array([(1000.0, 3.0), (2000.0, 3000.0), (123.4, 7.0), (10.0, 11.0),       (14.0, 15.0)],      dtype=[('f1', '<f8'), ('f2', '<f8')])

In [64]: a Out[64]: array([(1, 1000.0, 3.0, 4), (7, 8.0, 9.0, 10), (5, 123.4, 7.0, 8),       (9, 10.0, 11.0, 12), (13, 14.0, 15.0, 16)],      dtype=[('i', '<i4'), ('f1', '<f8'), ('f2', '<f8'), ('i2', '<i4')])

However, in this case, the two fields are contiguous, and thus I'm pretty sure one could build a numpy array that was a view. Is there any way to do so? Ideally without manipulating the strides by hand, but I may want to do that if it's the only way.

-Chris

that's what I would try:

...

...

b = a.view(dtype=[('i', '<i4'), ('fl',[('f1', '<f8'), ('f2', '<f8')]), ('i2', '<i4')]) b['fl'] array([(2.0, 3.0), (8.0, 9.0), (123.40000000000001, 7.0), (10.0, 11.0), (14.0, 15.0)], dtype=[('f1', '<f8'), ('f2', '<f8')]) b['fl'][2]= (200, 500) a array([(1, 2.0, 3.0, 4), (7, 8.0, 9.0, 10), (5, 200.0, 500.0, 8), (9, 10.0, 11.0, 12), (13, 14.0, 15.0, 16)], dtype=[('i', '<i4'), ('f1', '<f8'), ('f2', '<f8'), ('i2', '<i4')])

Josef

-- --

Christopher Barker, Ph.D. Oceanographer

Emergency Response Division NOAA/NOS/OR&R            (206) 526-6959   voice 7600 Sand Point Way NE   (206) 526-6329   fax Seattle, WA  98115       (206) 526-6317   main reception

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