[Numpy-discussion] overlapping rows
Timothy Hochberg
tim.hochberg@ieee....
Wed Mar 7 16:19:52 CST 2007
On 3/7/07, Matthew Koichi Grimes <mkg@cs.nyu.edu> wrote:
>
> I would like to twiddle with the strides of a matrix such that the rows
> overlap each other. I've gotten this far:
>
> In [1]: import numpy as N
>
> In [2]: mat = N.arange(12).reshape(3,4)
>
> In [3]: mat
> Out[3]:
> array([[ 0, 1, 2, 3],
> [ 4, 5, 6, 7],
> [ 8, 9, 10, 11]])
>
> In [4]: mat.strides
> Out[4]: (16, 4)
>
> In [5]: omat = mat[...]
>
> In [6]: omat.strides = (8,4)
>
> In [7]: omat
> Out[7]:
> array([[0, 1, 2, 3],
> [2, 3, 4, 5],
> [4, 5, 6, 7]])
>
> So far so good, but I'd like to have "omat"'s first dimension grow to
> size 6 so that all of the data is used. In other words, I'd like for
> omat to look like:
>
> array([[0, 1, 2, 3],
> [2, 3, 4, 5],
> [4, 5, 6, 7],
> [6, 7, 8, 9],
> [8, 9, 10, 11]])
>
> However, when I try to resize the first dimension to 6 I get the
> following error:
>
> In [8]: omat.shape = (6,4)
>
> ---------------------------------------------------------------------------
> exceptions.ValueError Traceback (most
> recent call last)
>
> /home/mkg/<ipython console>
>
> ValueError: total size of new array must be unchanged
>
>
> Is there any workaround to this? It seems like something I should be
> able to do, since I'm only growing omat into memory that's owned
> by omat.base.
>
> Any help would be much appreciated,
I think you're on thin ice here, but:
>>> a = np.arange(12)
>>> b = np.ndarray([6,4], int, a, strides=[8,4])
>>> b
array([[ 0, 1, 2, 3],
[ 2, 3, 4, 5],
[ 4, 5, 6, 7],
[ 6, 7, 8, 9],
[ 8, 9, 10, 11],
[ 10, 11, 458762, 19464224]])
(See the docs for ndarray).
Hmm. Perhaps you meant shape [5,4[?
>>> c = np.ndarray([5,4], int, a, strides=[8,4])
>>> c
array([[ 0, 1, 2, 3],
[ 2, 3, 4, 5],
[ 4, 5, 6, 7],
[ 6, 7, 8, 9],
[ 8, 9, 10, 11]])
That seems more plausible.
--
//=][=\\
tim.hochberg@ieee.org
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