[Numpy-discussion] OWNDATA flag and reshape() -- views vs. copies
Eric Firing
efiring@hawaii....
Sat Apr 19 21:33:47 CDT 2008
Guillaume Desjardins wrote:
> I'm pretty new to Python and numpy (longtime c / matlab programmer),
> but after a read through some of the past threads and Travis' "Guide
> to Numpy", I think I have a fairly good understanding of how the
> reshape() function / methods work, with regards to views and copies.
> For what its worth (and to make things clearer), my understanding is
> that:
>
> * Function reshape: create a view, or if it can't (i.e. resulting
> array cannot be expressed with constant stride parameters), a copy
> * Method obj.reshape(): creates a view, or throws an exception if it can't
>
> Now assuming that is the case, I do not understand the behaviour of
> the following code.
>
> a = array([[1,2,3,4],[5,6,7,8],[9,10,11,12]]);
> a.flags
> C_CONTIGUOUS : True
> F_CONTIGUOUS : False
> OWNDATA : True
> WRITEABLE : True
> ALIGNED : True
> UPDATEIFCOPY : False
>
> b = a[1:,1:3] # creates view of a
>>> array([[ 6, 7],
> [10, 11]])
> b.flags
> C_CONTIGUOUS : False
> F_CONTIGUOUS : False
> OWNDATA : False
> WRITEABLE : True
> ALIGNED : True
> UPDATEIFCOPY : False
>
> c = reshape(b,(4,1)) # RESHAPE FUNCTION: should copy, as view of this
> is impossible (? isn't it ?)
> c
>>> array([[ 6],
> [ 7],
> [10],
> [11]])
> c.flags
> C_CONTIGUOUS : True
> F_CONTIGUOUS : False
> OWNDATA : False
> WRITEABLE : True
> ALIGNED : True
> UPDATEIFCOPY : False
>
> First question: if c is a copy of b, then why isn't the OWNDATA set to true ?
It looks like creation of c is done in two steps: first a copy is made
of b, and then c is returned as a view of that copy. I don't know if
this is intentional; it sees a bit odd. I agree with you that I would
have expected c to own its data rather than refer to a base hidden in
the shadows.
In [62]:b.base
Out[62]:
array([[ 1, 2, 3, 4],
[ 5, 6, 7, 8],
[ 9, 10, 11, 12]])
In [63]:b
Out[63]:
array([[ 6, 7],
[10, 11]])
In [64]:c.base
Out[64]:
array([[ 6, 7],
[10, 11]])
In [65]:c
Out[65]:
array([[ 6],
[ 7],
[10],
[11]])
Eric
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