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From: Gordon on 20 Oct 2009 02:36
I would like to quickly determine the nearest neighbour to an element in a matrix that contains a value>=1.

For example if I have the matrix
data=[1 1 1 1 0 0; 0 1 1 0 0 0; 1 1 1 1 0 0];
and the reference matrix element (3,6)
then the desired output would be element number (3,4)

similarly for reference matrix element (2,4)
then the desired output would be elements (1,4);(2,3);(3,4)

I can implement this by calculating the distance matrix between the input reference matrix element (3,6) and all of the values in the data matrix which have a value of 1, and then seeing which is the closest to the point.

But given the nature of the data, being in a matrix, it seems to me that there should be a more efficient way of doing this search. For example, start searching the elements closest to the reference matrix element, moving progressively outwards until the desired output is found.

Note that I will be doing this search on very large matrices, so ideally I am looking for fairly efficient code.

Thanks for any help in advance
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