[Numpy-discussion] another view puzzle

Christopher Barker Chris.Barker@noaa....
Wed Jun 3 15:46:52 CDT 2009


josef.pktd@gmail.com wrote:
>>>> import numpy as np
>>>> x = np.array([(0.0, 1.0, 2.0, 3.0, 4.0), (1.0, 2.0, 3.0, 4.0, 5.0)],
>      dtype=[('a', '<f4'), ('b', '<f4'), ('c', '<f4'), ('d', '<f4'),
> ('e', '<f4')])
> 
>>>> xvm = x.view(np.matrix)
>>>> xvm
> matrix([[(0.0, 1.0, 2.0, 3.0, 4.0), (1.0, 2.0, 3.0, 4.0, 5.0)]],
>        dtype=[('a', '<f4'), ('b', '<f4'), ('c', '<f4'), ('d', '<f4'),
> ('e', '<f4')])

I think you need to tell numpy more about how you want to turn the 
struct array into a matrix -- a custom data type could be anything, 
numpy sees x as a (2,) sized array of a custom structure -- there is no 
one way to turn it into a matrix. I think this may be what you want:

In [32]: x
Out[32]:
array([(0.0, 1.0, 2.0, 3.0, 4.0), (1.0, 2.0, 3.0, 4.0, 5.0)],
       dtype=[('a', '<f4'), ('b', '<f4'), ('c', '<f4'), ('d', '<f4'), 
('e', '<f4')])

In [33]: x.shape
Out[33]: (2,)

In [34]: x2 = x.view(dtype=np.float32).reshape((2,5))

In [35]: x2
Out[35]:
array([[  0.00000000e+00,   4.60060299e-41,   8.96831017e-44,
           2.30485571e-41,   4.60074312e-41],
        [  4.60060299e-41,   8.96831017e-44,   2.30485571e-41,
           4.60074312e-41,   5.74868682e-41]], dtype=float32)

In [36]: xvm = np.asmatrix(x2)

In [37]: xvm
Out[37]:
matrix([[  0.00000000e+00,   4.60060299e-41,   8.96831017e-44,
            2.30485571e-41,   4.60074312e-41],
         [  4.60060299e-41,   8.96831017e-44,   2.30485571e-41,
            4.60074312e-41,   5.74868682e-41]], dtype=float32)

In [38]: xvm * 2
Out[38]:
matrix([[  0.00000000e+00,   9.20120598e-41,   1.79366203e-43,
            4.60971143e-41,   9.20148624e-41],
         [  9.20120598e-41,   1.79366203e-43,   4.60971143e-41,
            9.20148624e-41,   1.14973736e-40]])


-Chris



-- 
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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