MATLAB: Access every nth occurrence of a value in a matrix

matrixoccurrence

Hello. I have a matrix with '0' and '1', of variable size. For example:
A =
     0     0     0     0     0
     0     1     0     0     0
     0     1     1     0     0
     0     0     1     0     0
     0     0     1     0     0
     0     0     1     1     0
     0     0     0     1     0
     0     1     1     0     0
     0     0     0     1     0
     0     0     0     1     1
My goal is to create B where every '1' is changed to '0', except for every nth '1' (let's assume 3rd, for this example). The code should sweep row by row, searching from left to right.
B =
     0     0     0     0     0
     0     1     0     0     0
     0     0     0     0     0
     0     0     1     0     0
     0     0     0     0     0
     0     0     0     1     0
     0     0     0     0     0
     0     0     1     0     0
     0     0     0     0     0
     0     0     0     0     1
Thank you.

Best Answer

One approach: 
Atv = reshape(A', [], 1);                       % Transpose & Reshape To Column Vector
lidx = find(Atv);                               % Find ‘1’ Positions
B = zeros(size(A))';                            % Create Transpose Of ‘B’
B(lidx(1:3:end)) = 1;                           % Set Appropriate Elements To ‘1’
B = B'                                          % Transpose To Get Desired Result
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