Can anyone help me to find out why I can't replace pixel values at (x, y-1) and (x+1, y) with the value at (x, y) ?

I am using Matlab and I need your help! Assume that the 8 neighbors of the location (x,y) are:

top-left (x-1,y-1); 
top-middle (x,y-1); 
top-right (x+1,y-1);

middle-left (x-1,y); 
middle-middle(x,y); 
middle-right (x+1,y);

bottom-left (x-1,y+1); 
bottom-middle(x,y+1); 
bottom-right (x+1,y+1);

With the following example (of A = magic(5)), I can only replace the numbers or 'let's say' pixel values at (x-1, y) and (x, y-1) with the value at (x, y). But, I can’t replace the pixel values at (x, y+1) and (x+1, y) with the value at (x, y).

Can any help me to understand why ? In addition, can you please help me to write a code that would replace the pixel values at the locations (x-1, y); (x, y-1); (x, y-1) and (x+1, y) with the value of (x,y) location - without affecting other locations not nearest or adjacent to (x,y)? Many thanks in advance!

A = magic(5);
  M1 = size(A,1);
  M2 = size(A,2);
  E = [5 20];

    for x = 2:(M1-1)
       for y = 2:(M2-1)

          for k = 1:length(E)

               if E(k) == A(x,y)

                  A(x-1,y) = A(x,y); 
                  A(x,y-1) = A(x,y);

               end
           end
        end
     end

The expected results should look like this (with reference to E).

A = 
          17    5     1     8     15
          5     5     5     20    16
          4     5     20    20    20
          10    12    19    20     3
          11    18    25     2     9

Answer 1

[Stephen23]

N = 5;
E = [5,20];
%
X = magic(N);
Y = false(N+2);
for k = 1:numel(E);
    Y(2:end-1,2:end-1) = X==E(k);
    Z = Y(1:end-2,2:end-1) | Y(3:end,2:end-1) | Y(2:end-1,3:end) | Y(2:end-1,1:end-2);
    X(Z) = E(k);
end

produces this:

X =
 17    5    1    8   15
  5    5    5   20   16
  4    5   20   20   20
 10   12   19   20    3
 11   18   25    2    9

[Gobert]

Excellent! Thank you very much!