[SciPy-User] Can I get the "rings" of a 2D array?

[Jeremy Conlin]

On Mon, Apr 12, 2010 at 9:19 PM,  <josef.pktd at gmail.com> wrote:
> On Mon, Apr 12, 2010 at 10:23 PM, Jeremy Conlin <jlconlin at gmail.com> wrote:
>> On Mon, Apr 12, 2010 at 5:16 PM, Charles R Harris
>> <charlesr.harris at gmail.com> wrote:
>>>
>>>
>>> On Mon, Apr 12, 2010 at 4:55 PM, Jeremy Conlin <jlconlin at gmail.com> wrote:
>>>>
>>>> I need to get the "rings" of a 2D array; i.e. the first ring would be
>>>> the first and last column in addition to the first and last row, the
>>>> second ring would be the second and second-from-last row and the
>>>> second and second-from-last column---except for what is not in the
>>>> first ring.  Is there a way to slice an array to get this data?
>>>>
>>>
>>> So will the rings be different sizes or all the same size with the other
>>> elements zeroed?
>>
>> Good question.  The better option would be to keep the size of the
>> array the same and zero all the other elements.
>
> The following seems to work for building the index arrays for the rings,
> tested on only 2 examples
>
>
> import numpy as np
> a = np.arange(30).reshape(6,5)
> n = np.array(a.shape)
> from collections import defaultdict
> rings = defaultdict(list)
> for i in np.ndindex(*n):
>    #print i,
>    imin = min(i)
>    imax = min(n-i-1)
>    #print imin,imax
>    rings[min(imin,imax)].append(i)
>
> for r in rings:
>    print r
>    print a[np.array(rings[r])[:,0],np.array(rings[r])[:,1]]
>
>
> 0
> [ 0  1  2  3  4  5  9 10 14 15 19 20 24 25 26 27 28 29]
> 1
> [ 6  7  8 11 13 16 18 21 22 23]
> 2
> [12 17]
>>>> a
> array([[ 0,  1,  2,  3,  4],
>       [ 5,  6,  7,  8,  9],
>       [10, 11, 12, 13, 14],
>       [15, 16, 17, 18, 19],
>       [20, 21, 22, 23, 24],
>       [25, 26, 27, 28, 29]])
>
> Josef
>

Thanks.  I just might be able to put that to work.

Jeremy