How to determine outside perimeter of an array?

Question

Sonicflash 2022-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/1773475-how-to-determine-outside-perimeter-of-an-array

回答： Image Analyst 2022-8-3

For example, a matrix, a:

a = randi(10,5)
a = 5×5
   2     6     6     4
   7    10     4     7
   5     1     8     3
  10    10     3     8
  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.

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Answer 1

Dyuman Joshi 2022-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/1773475-how-to-determine-outside-perimeter-of-an-array#answer_1020660

在 MATLAB Online 中打开

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

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Answer 2

Image Analyst 2022-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/1773475-how-to-determine-outside-perimeter-of-an-array#answer_1020750

在 MATLAB Online 中打开

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 = [...
   2     6     6     4 
   7    10     4     7 
   5     1     8     3 
  10    10     3     8 
  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