[Numpy-discussion] another view puzzle

Christopher Barker Chris.Barker@noaa....
Wed Jun 3 16:57:29 CDT 2009


josef.pktd@gmail.com wrote:
> I'm very happy with plain numpy arrays, but to handle different data
> types in scipy.stats, I'm still trying to figure out how views and
> structured arrays work. And I'm still confused.

OK, I'd stay away from matrix then, no need to add that confusion

>>From the use for data handling in for example matplotlib and the
> recarray functions, I thought of structured arrays (and recarrays) as
> columns of data. Instead the analogy to database records and (1d)
> arrays of structs as in matlab might be better.

they are a bit of a mixture -- I think the record style access:

arr['x']

means that there is no "rows" or "columns", just data accessed by name.

> The numpy help and documentation is not exactly rich in examples how
> to do convert structured arrays to something that can be used for
> calculation, except for dictionary access and row iteration. And using
> views to access them is not as foolproof as I thought.

views are kind of a low-level trick -- what views do is let you make 
more than one numpy array that share the same memory data block. Doing 
this required a bit of knowledge about how data is stored in memory.

For the common use case, what I do is use struct arrays to store and 
mass data around, and simple pull out the data into a regular array to 
manipulate it:

In [45]: x
Out[45]:
array([(0.0, 1.0, 2.0, 12.0, 4.0), (1.0, 2.0, 3.0, 45.0, 5.0)],
       dtype=[('a', '<f4'), ('b', '<f4'), ('c', '<f4'), ('d', '<f4'), 
('e', '<f4')])

In [46]:

In [47]: e = x['e']

In [48]: e
Out[48]: array([ 4.,  5.], dtype=float32)

note that this is still a "view" into the original array:

In [49]: e *= 5

In [50]: x
Out[50]:
array([(0.0, 1.0, 2.0, 12.0, 20.0), (1.0, 2.0, 3.0, 45.0, 25.0)],
       dtype=[('a', '<f4'), ('b', '<f4'), ('c', '<f4'), ('d', '<f4'), 
('e', '<f4')])

#( see how the e field changed )

This is interesting:
In [51]: x[0]
Out[51]: (0.0, 1.0, 2.0, 12.0, 20.0)

In [52]: type(x[0])
Out[52]: <type 'numpy.void'>

What's a numpy.void type? I thought this would be a tuple, or a numpy 
scalar of that dtype. It can be indexed either way, though:

In [70]: x[0][2]
Out[70]: 2.0

In [72]: x[0]['c']
Out[72]: 2.0

cool.


-Chris








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