Returning and plotting values from while loop results

This is my code so far:

M = 39;
% Number of gridpoints
deltax = 1/(M+1);
% Defines deltax
u = zeros(M,1);
count = 0;
x = 1;

for j = 1:M
m(j) = deltax*j;
b(j) = 6*(deltax^3)*j;
exact(j) = (deltax*j)^3;
end

for r = 1:M
    if r == 1
        res(r) = b(r) - (u(r+1) - 2*u(r) + 0);
    else if r == M
        res(r) = b(r) - (1 - 2*u(r) + u(r-1));
    else 
        res(r) = b(r) - (u(r+1) - 2*u(r) + u(r-1));
    end
    end
end

res0 = norm(res);
residual(1) = res0/res0;
% Defines the initial residual equal to 1

while residual(x) > 1.0e-2
       for i = 1:M
        if i == 1
            d = u(i+1)+0;
        else if i == M
            d = 1+u(i-1);
        else
            d = u(i+1) + u(i-1);
            end
            u(i) = -(1/2)*((b(i)) - d);
        end

           for r = 1:M
      if r == 1
          res(r) = b(r) - (u(r+1) - 2*u(r) + 0);
      else if r == M
          res(r) = b(r) - (1 - 2*u(r) + u(r-1));
      else 
          res(r) = b(r) - (u(r+1) - 2*u(r) + u(r-1));
          end
      end
                  resx = norm(res);
          residual(x) = resx/res0
      end
           end
     count = count + 1

end

When I run it, it runs the while loop till my residual term drops below the specified value (which is what I want it to do).

My questions is, how can I get it to save all my residual values so I can plot their history (i.e. residual 1, residual 2, ..., to last residual in while loop). I want to plot decreasing residual versus number of iterations but right now my code just returns the last updated residual value. I want it to still do that but also give me all residuals to plot.

Thanks in advance

Colin

Answer 1

[G A]

I would modify your while loop as:

   while residual > 1.0e-2 
   ...
        resx = norm(res);
        residual = resx/res0;
        residualToPlot(count+1)=residual;
         end
       end
    count = count + 1;
   end

x as index is redundant

Answer 2

[Colin]

Thanks this helped and is exactly what I needed