How do I save matrices from specific points in a 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?

Answer 1

[KSSV]

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

[mathman]

Thanks for the response. Could you explain the code you've added? I'd like to understand it rather than just copy it in

[Jan]

@mathman: The code is commented and this should explain the purpose already. Which part is not clear yet?

[mathman]

@Jan, the comments in the code are my own. The additions he's added are not commented.

idx = [5 30 70] ;
iwant = zeros([],[],length(idx)) ;
count = 0 ;

if any(idx==n)
        count = count+1 ; 
        iwant(:,:,count) = p_np1 ;
    end

[KSSV]

idx are the n values for which you want to save output.

iwant is initialised using zeros. It will be a 3d matrix.

if condition is met, the matrix is saved into iwant.