[Numpy-discussion] indexing question
Robin
robince@gmail....
Thu Mar 5 15:34:36 CST 2009
On Thu, Mar 5, 2009 at 9:15 PM, Stéfan van der Walt <stefan@sun.ac.za> wrote:
> Hi Robin
>
> 2009/3/5 Robin <robince@gmail.com>:
>> On Thu, Mar 5, 2009 at 10:57 AM, Robin <robince@gmail.com> wrote:
>>> On Thu, Mar 5, 2009 at 10:40 AM, Robin <robince@gmail.com> wrote:
>>>> Hi,
>>>>
>>>> I have an indexing problem, and I know it's a bit lazy to ask the
>>>> list, sometime when people do interesting tricks come up so I hope no
>>>> one minds!
>>>>
>>>> I have a 2D array X.shape = (a,b)
>>>>
>>>> and I want to change it into new array which is shape (2,(a*b)) which
>>>> has the following form:
>>>> [ X[0,0], X[0,1]
>>>> X[1,0], X[1,1]
>>>> X[2,0], X[2,1]
>>>> ....
>>>> X[a,0], X[a,1]
>>>> X[0,1], X[0,2]
>>>> X[1,1], X[1,2]
>>>> ...
>>>> ]
>>>>
>
> >From the array you wrote down above, I assume you meant ((a*b-1), 2):
>
> In [23]: x = np.arange(16).reshape((4,4))
>
> In [24]: x
> Out[24]:
> array([[ 0, 1, 2, 3],
> [ 4, 5, 6, 7],
> [ 8, 9, 10, 11],
> [12, 13, 14, 15]])
>
> In [25]: x.strides
> Out[25]: (16, 4)
>
> In [26]: np.lib.stride_tricks.as_strided(x, shape=(3, 4, 2), strides=(4, 16, 4))
> Out[26]:
> array([[[ 0, 1],
> [ 4, 5],
> [ 8, 9],
> [12, 13]],
>
> [[ 1, 2],
> [ 5, 6],
> [ 9, 10],
> [13, 14]],
>
> [[ 2, 3],
> [ 6, 7],
> [10, 11],
> [14, 15]]])
>
> In [27]: np.lib.stride_tricks.as_strided(x, shape=(3, 4, 2),
> strides=(4, 16, 4)).reshape((12, 2))
> Out[27]:
> array([[ 0, 1],
> [ 4, 5],
> [ 8, 9],
> [12, 13],
> [ 1, 2],
> [ 5, 6],
> [ 9, 10],
> [13, 14],
> [ 2, 3],
> [ 6, 7],
> [10, 11],
> [14, 15]])
>
> Does that help?
Ah thats great thanks... I had realised it could be done with
as_strided and a reshape from your excellent slides - but I had
trouble figure out the new strides so I settled on making a list with
_ix and the hstack'ing the list.
This is much neater though.
Thanks,
Robin
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