Indexes and values of surrounding cells in 3d matrix - matlab

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{:});

Answer 2

Here is a tidied up version. Cheers.
mat = rand(5,5,5);
N = size(mat)
if length(N) < 3 || length(N) > 3; error('Input must be 3 dimensional'); end;
pos = [1 3 5]
surrounding_val = mat(max(pos(1)-1, 1):min(pos(1)+1, N(1)), max(pos(2)-1, 1):min(pos(2)+1, N(2)), max(pos(3)-1, 1):min(pos(3)+1, N(3)))
EDIT: Added an error trap.

Answer 3

I found this post because I needed to grab the surrounding indices of a chosen point in a Matrix. It seems like the answers here are returning a matrix of the surrounding values, but the question is also interested in the surrounding indices. I was able to do this with "try/catch" statements, which I previously did not know existed in MATLAB. For a 2D Matrix, Z:
%Select a node in the matrix
Current = Start_Node;
%Grab its x, y, values (these feel reversed for me...)
C_x = rem(Start_Node, length(Z));
if C_x ==0
C_x =length(Z);
end
C_y = ceil(Start_Node/length(Z));
C_C = [C_x, C_y];
%Grab the node's surrounding nodes.
try
TRY = Z(C_x - 1, C_y);
Top = Current -1;
catch
Top = Inf;
end
try
TRY = Z(C_x + 1, C_y);
Bottom = Current +1;
catch
Bottom = Inf;
end
try
TRY = Z(C_x, C_y + 1);
Right = Current + length(Z);
catch
Right = Inf;
end
try
TRY = Z(C_x, C_y - 1);
Left = Current - length(Z);
catch
Left = Inf;
end
surround = [Top, Bottom, Left, Right];
m = numel(surround == inf);
k = 1;
%Eliminate infinites.
surround(surround==inf) =[];
I hope someone finds this information relevant.