On Mon, Apr 6, 2009 at 22:31, <josef.p...@gmail.com> wrote:
> I ran again into a problem where numpy created a view (which I didn't
> realize) and an operation works differently on the view than if it
> were a copy.
>
> I try to construct an example array, which, however, is only a view
>
>>>> x,y = np.mgrid[0:3,0:3]
>>>> xx = np.vstack((x.flatten(), y.flatten(), np.ones(9))).T
>>>> xx
> array([[ 0., 0., 1.],
> [ 0., 1., 1.],
> [ 0., 2., 1.],
> [ 1., 0., 1.],
> [ 1., 1., 1.],
> [ 1., 2., 1.],
> [ 2., 0., 1.],
> [ 2., 1., 1.],
> [ 2., 2., 1.]])
>>>> xx.flags
> C_CONTIGUOUS : False
> F_CONTIGUOUS : True
> OWNDATA : False
> WRITEABLE : True
> ALIGNED : True
> UPDATEIFCOPY : False
>>>> xx.base
> array([[ 0., 0., 0., 1., 1., 1., 2., 2., 2.],
> [ 0., 1., 2., 0., 1., 2., 0., 1., 2.],
> [ 1., 1., 1., 1., 1., 1., 1., 1., 1.]])
>>>> xx == xx.base
> False
>
> When I convert it to a view as a structured array, it produces a
> strange result. I didn't get what I thought I should get
>
>>>> xx.view([('',xx.dtype)]*xx.shape[1])
> array([[(0.0, 0.0, 0.0), (0.0, 1.0, 2.0), (1.0, 1.0, 1.0)],
> [(1.0, 1.0, 1.0), (0.0, 1.0, 2.0), (1.0, 1.0, 1.0)],
> [(2.0, 2.0, 2.0), (0.0, 1.0, 2.0), (1.0, 1.0, 1.0)]],
> dtype=[('f0', '<f8'), ('f1', '<f8'), ('f2', '<f8')])
>
> if I make a copy and then construct a view as structured array, I get
> what I want
>
>>>> xx2 = xx.copy()
>>>> xx2.view([('',xx2.dtype)]*xx2.shape[1])
> array([[(0.0, 0.0, 1.0)],
> [(0.0, 1.0, 1.0)],
> [(0.0, 2.0, 1.0)],
> [(1.0, 0.0, 1.0)],
> [(1.0, 1.0, 1.0)],
> [(1.0, 2.0, 1.0)],
> [(2.0, 0.0, 1.0)],
> [(2.0, 1.0, 1.0)],
> [(2.0, 2.0, 1.0)]],
> dtype=[('f0', '<f8'), ('f1', '<f8'), ('f2', '<f8')])
>
> Are there rules for this behavior or a description in the docs,
> because my mistakes in this are quite difficult to debug? Or do I have
> to make a copy by default as in matlab?
.view() makes a view onto the memory, not onto the
memory-as-if-it-were-normalized-to-C-contiguous-order. Use
ascontiguous() if you need to ensure the latter.
--
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
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