[Numpy-discussion] the neighbourhood of each element of an array
Fri Feb 23 13:39:00 CST 2007
On Fri, 2007-02-23 at 17:38 +0100, firstname.lastname@example.org wrote:
> Given a (possibly masked) 2d array x, is there a fast(er) way in Numpy to obtain
> the same result as the following few lines?
> d = 1 # neighbourhood 'radius'
> Nrow = x.shape
> Ncol = x.shape
> y = array([[x[i-d:i+d+1,j-d:j+d+1].ravel() for j in range(d,Ncol-d)] \
> for i in range(d,Nrow-d)])
how about something like
er = Nrow - d
ec = Ncol - d
y = array([x[i:er+i, j:ec+j] for j in arange(-d,d)
for i in arange(-d,d)])
now you're looping over a small array and combining slices of the big
array (as opposed to looping over the big array and combining slices
from a small one). This should be faster for large Nrow, Ncol.
> What you get is an array containing all the elements in a neighbourhood for each
> element, disregarding the edges to avoid out-of-range problems. The code above
> becomes quite slow for e.g. a 2000x2000 array. Does anyone know a better
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