How to check entries in a Matrix diagonal by diagonal

n=21;   %three states: blipped(2- or some other marker), not blipped (1), empty (0)
blipped=2;
notblipped=1;
empty=0;
A=randi(2,n)-1;     %WE CAN SET UP THE MATRIX IN WATEREVER WAY WE WANT AND DESIGN AN ALGORITHM OF WIPING OUT "1"S IN WHATEVER WAYS POSSIBLE


%Approach 2
%Check the cells of matrix diagonally 
for i=2:n-1 
     if A(i-1,i-1)+A(i-1,i)+A(i-1,i+1)+A(i,i-1)+A(i,i)+A(i,i+1)... 
                +A(i+1,i-1)+A(i+1,i)+A(i+1,i+1)>=3 
       A(i,i)=0;               %First loop through diagnal elements
    end
    for j=1:n-3 
        if A(i-1,i-j-1)+A(i-1,i-j)+A(i-1,i-j+1)+A(i,i-j-1)+A(i,i-j)+A(i,i-j+1)...
            +A(i+1,i-j-1)+A(i+1,i-j)+A(i+1,i-j+1)>=3
            A(i,i-j)=0;            %Loop through elements below the diagonal
        end
        if A(i-1,i+j-1)+A(i-1,i+j)+A(i-1,i+j+1)+A(i,i+j-1)+A(i,i+j)+A(i,i+j+1)...
            +A(i+1,i+j-1)+A(i+1,i+j)+A(i+1,i+j+1)>=3
            A(i,i+j)=0;            %Loop through elements above diagonal
        end
    end
end
disp(A);

I am trying to first loop through elements on the diagonal and then the ones on the subdiagonals. However, my code does not run in matlab. In addition, I am wondering how to deal with boundary conditions? In each "if" statement, I have analyzed eight neighboring entries, however, this does not apply to boundary entries. Just wondering if someone could help with this? Thanks a lot.

[Jonas]

i'am not sure what exactly you want to achieve, but maybe you should have a look at the diag() function which can easily give you the diagonal and subdiagonal elements of a matrix

[Zhukun Wang]

First, I am looping through all entries in a matrix. I am trying to loop through diagonal and subdiagonal entries and I want to show that if the sum of a certain entry and its eight neighbors are >=3, then this entry is gonna turn to 0.

Answer 1

[Walter Roberson]

[nrow, ncol] = size(A);

count_around = @(A,R,C) sum(A(max(1,R-1:R+1):min(nrow,R-1:R+1), max(1,C-1:C+1):min(ncol,C-1:C+1))) - A(R,C));

Now you can apply

count_around(A,i,j)

sliding i and j in whatever pattern you want.