MATLAB: How to save matrices from specific points in a for loop

for loop

for n = 1:100
      %first for loop for i = 2:end-1, second for loop for j = 2: end-1
      for i = (2:Nx-1)
      for j = (2:Ny-1)
      %update equation   
      p_np1(i,j) = a^2 * (p_n(i+1,j) + p_n(i-1,j) ...
      - 4*p_n(i,j) + p_n(i, j+1) + p_n(i, j-1)) ...
      + 2*p_n(i,j) - p_nm1(i,j); 
      end
      end
      % after each iteration, previous value becomes present
      % pressure value
      p_nm1 = p_n;
      % after each iteration, present value becomes next pressure
      % value
      p_n = p_np1;
      % Plotting continuous plot
      % plot the  field
      imagesc(x_axis, y_axis, p_np1);
      % set the limits on the y-axis, 'YDir' and 'normal' commands set the
      % y-axis on the plot to go from low to high from bottom to top
      set(gca, 'YDir', 'normal');
      % adds  title with the iteration number
      title(['n = ' num2str(n)]);
      % keeps updating plot
      drawnow;
      % briefly pause before continuing the loop
      pause(0.1)
   end
I want to save the arrays of p_np1 when n is at certain points, 5, 30 and 70 for example. How would I go about writing code for that?

Best Answer

idx = [5 30 70] ;
iwant = zeros([],[],length(idx)) ;
count = 0 ;
for n = 1:100
    %first for loop for i = 2:end-1, second for loop for j = 2: end-1
    for i = (2:Nx-1)
        for j = (2:Ny-1)
            %update equation
            p_np1(i,j) = a^2 * (p_n(i+1,j) + p_n(i-1,j) ...
                - 4*p_n(i,j) + p_n(i, j+1) + p_n(i, j-1)) ...
                + 2*p_n(i,j) - p_nm1(i,j);
        end
    end
    if any(idx==n)
        count = count+1 ; 
        iwant(:,:,count) = p_np1 ;
    end
    % after each iteration, previous value becomes present
    % pressure value
    p_nm1 = p_n;
    % after each iteration, present value becomes next pressure
    % value
    p_n = p_np1;
    % Plotting continuous plot
    % plot the  field
    imagesc(x_axis, y_axis, p_np1);
    % set the limits on the y-axis, 'YDir' and 'normal' commands set the
    % y-axis on the plot to go from low to high from bottom to top
    set(gca, 'YDir', 'normal');
    % adds  title with the iteration number
    title(['n = ' num2str(n)]);
    % keeps updating plot
    drawnow;
    % briefly pause before continuing the loop
    pause(0.1)
end
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