Indexes and values of surrounding cells in 3d matrix

Question

I'd like to return the indexes and values of the 8 cells surrounding a cell in a 3d matrix.

mat = rand(5,5,5);

% Cell of interest
pos = [3 3 3]
pos_val = mat(pos(1), pos(2), pos(3))

% Surrounding cells
surrounding_pos = [pos(1)-1:pos(1)+1; pos(2)-1:pos(2)+1; pos(2)-1:pos(2)+1]
surrounding_val = mat(surrounding_pos(1,:), surrounding_pos(2,:), surrounding_pos(3,:))

This works fine for values in the centre of a matrix, but it breaks if pos is on the edge. (E.g. if pos was [3,4,5], surrounding_pos would include [3,4,6], which is out of bounds)

I could obviously remove surrounding_pos values <0 or >size(mat), but this doesn't seem like a terribly MATLABian method. Any ideas?

Answer 1

Same solution as discussed here, but extended to multiple (any) dimensions:

mat = randi(10,5,5,5);
siz = size(mat );
N = numel(siz); % number of dimensions
M = 1; % surrounding region size

pos = [3 3 3];
pos_val = mat(pos(1), pos(2), pos(3));

surrounding_pos = cell(N,1);
for ii=1:N
    surrounding_pos{ii} = max(1,pos(ii)-M):min(siz(ii),pos(ii)+M);
end
surrounding_val2 = mat(surrounding_pos{:});

The important part is the last four lines, it avoids having to c/p the max, min thing for every dimension..

Or if you like short code, the loop changed to an arrayfun:

surrounding_pos = arrayfun(@(ii) max(1,pos(ii)-M):min(siz(ii),pos(ii)+M), 1:N,'uni',false);
surrounding_val2 = mat(surrounding_pos{:});