[Numpy-discussion] Advanced indexing advice?

Cristi Constantin darkgl0w@yahoo....
Thu Jun 18 04:03:49 CDT 2009


Good day.

I have a question about advanced indexing.

I have 2 matrices :

>>>
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]])

b=array([[1, 0, 1],
         [0, 2, 0],
         [0, 0, 3]])
>>>

I want to put all NON-zero elements from array B into array A, but use offset!
This is how i did:

>>>
mask = b!=0      # Save all non-zero values from B.
offset = (1,1)   # Save offset.
bshape = b.shape # Save shape of B.

# Action !
a[ offset[0]:bshape[0]+offset[0], offset[1]:bshape[1]+offset[1] ][ mask ] = b[ mask ]
>>>

After this, a becomes :

array([[ 0,  1,  2,  3,  4,  5],
       [ 6,  1,  8,  1, 10, 11],
       [12, 13,  2, 15, 16, 17],
       [18, 19, 20,  3, 22, 23]])

That's exactly what i want.

Now, my method works, but is very slow when used with big arrays. I use doule indexing, one for offset and one for mask...
Can anyone suggest another method, maybe with advanced indexing to do both the offset and the mask ? I am quite newbie with Numpy.
Or maybe just don't use the mask at all ?
All non-zero values from B must be inserted into A, using offset...

I tried :
a[ offset[0]:bshape[0]+offset[0], offset[1]:bshape[1]+offset[1], mask ] = b[ mask ]
and
a[ mask, offset[0]:bshape[0]+offset[0], offset[1]:bshape[1]+offset[1] ] = b[ mask ]
but both methods transform a into :
array([[ 1,  1,  1,  3,  4,  5],
       [ 6,  2,  8,  9, 10, 11],
       [12, 13,  3, 15, 16, 17],
       [18, 19, 20, 21, 22, 23]])
and that's completely wrong.

Any advice is good.
Thank you in advance.




      
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