How to determine outside perimeter of an array?

For example, a matrix, a:

a = randi(10,5)

a = 5×5

3 2 6 6 4 1 7 10 4 7 8 5 1 8 3 7 10 10 3 8 9 10 4 8 1

How would I obtain the elements of the matrix into an row vector, minus the center 3 x 3?

So in this case:

a_perim = [6 9 2 8 7 8 3 4 10 6 8 5 8 2 4 6]

a_perim = 1×16

6 9 2 8 7 8 3 4 10 6 8 5 8 2 4 6

A previous method I tried was defining the center as new matrix, and using ismember(). However, this did not work when elements in the center 3 x 3 were the same as the perimeter.

Answer 1

[Dyuman Joshi]

Note - a_perim vector doesn't correspond to the outer elements of a in the example you mentioned.

Here's how you can achieve that

y=spiral(5) %random matrix

y = 5×5

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

z=ones(5);
z(2:end-1,2:end-1)=0;
y(z==1)'

ans = 1×16

21 20 19 18 17 22 16 23 15 24 14 25 10 11 12 13

Answer 2

[Image Analyst]

I'm not sure how you're getting

a_perim = [6 9 2 8 7 8 3 4 10 6 8 5 8 2 4 6] ;

from this:

a = [...
    3     2     6     6     4 
    1     7    10     4     7 
    8     5     1     8     3 
    7    10    10     3     8 
    9    10     4     8     1];

Anyway, you can get the perimeter values as [top row, right column, bottom row, left column] like this

topRow = a(1,:);
rightCol = a(2:end-1, end);
bottomRow = a(end, end-1 : -1 : 2);
leftCol = a(end-1 : -1 : 2, 1);
% Get perimeter clockwise from upper left (1,1) element.
a_perim = [topRow, rightCol', bottomRow, leftCol']

a_perim = 1×14

3 2 6 6 4 7 3 8 8 4 10 7 8 1

If you want a different ordering, let me know what it is.