How to obtain the outermost elements of a 2D matrix ?

Hello everyone,

How can I obtain the outermost elements of a 2D matrix ?

(For example, the elements that are highlighted in the following picture)

Answer 1

[Akira Agata]

How about the following?

% Sample matrix
M = magic(5);

% Replace non-outermost element as NaN
M(2:end-1, 2:end-1) = nan;

% Extract the outermost elements
idx = isnan(M);
OuterElements = M(~idx)

OuterElements = 16×1

17 23 4 10 11 24 18 1 25 8

[Mehmet]

Thank you Akira,

I actually wanted know how to obtain the outermost elements in a given 2D matrix in clockwise manner or anti-clockwise manner?

[Akira Agata]

OK, then how about the follwing solution?

% Sample matrix
M = magic(5)

M = 5×5

17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9

% Extract the outermost elements in anti-clockwise manner
OuterElements = M(1:end-1, 1);
for kk = 1:3
  M = rot90(M, -1);
  OuterElements = [OuterElements; M(1:end-1, 1)];
end

% Show the result
OuterElements

OuterElements = 16×1

17 23 4 10 11 18 25 2 9 3