Help... Getting errors with v_plot and x_plot

g = 9.81; m = 68.1; c = 0.25; h = 2; t = 0;

% Initial guess v_eu = 0; v_true = 0; x_eu = 350; x_true = 0; err_v = 100; err_x = 0;

i = 1; fprintf('Iter\t t\t\t x_true\t\t v_ture\t\t x_Euler\t v_Euler\t Error(x)\t Error(v)\n') while x_eu > -50 % Stopping criteria fprintf('%u\t %8.2f\t %8.2f\t%8.2f\t %8.2f\t %8.2f\t %8.2f\t%8.2f\n',i,t,x_true,v_true,x_eu,v_eu,err_x,err_v) slope = g-(c/m)*v_eu^2; % v' = slope

   t = t+h; %Interval
      x_true =(m/c)*log(cosh(t*sqrt(g*c/m))); 
   x_eu = x_eu - v_eu*h;

   % Velocity
   v_true = sqrt(m*g/c)*tanh(t*sqrt(g*c/m));
   v_eu = v_eu + slope*h;
   err_v = abs((v_true - v_eu)/v_true*100);
   err_x = abs((x_true -(x_eu))/x_true*100);
   *v_plot(1)=0;
   v_plot(i+1)=v_eu;

**x_plot(1)=350;
x_plot(i+1)= x_eu*;*
*   
 i = i +1;

end t_plot=0:h:t; plot(t_plot,v_plot,t_plot,x_plot) title('(Problem 1(a) Euler Method') xlabel('Time (s)'); ylabel('V and X');

Answer 1

[Wick]

Your code was quite difficult to understand without proper formatting and extra '*' spread around. I cleaned up the lines and ran the following code and it ran just fine.

Because you're making your vectors larger in the loop you should really consider clearing them before you start the script. That way, the value of the vector from the run before doesn't influence the results of this run. I added a 'clearvars' to the start for that purpose. Otherwise, this seemed to work just fine.

    clearvars
    g = 9.81;
    m = 68.1;
    c = 0.25;
    h = 2;
    t = 0;

    % Initial guess
    v_eu = 0;
    v_true = 0;
    x_eu = 350;
    x_true = 0;
    err_v = 100;
    err_x = 0;

    i = 1; fprintf('Iter\t t\t\t x_true\t\t v_ture\t\t x_Euler\t v_Euler\t Error(x)\t Error(v)\n')
    while x_eu > -50 % Stopping criteria
        fprintf('%u\t %8.2f\t %8.2f\t%8.2f\t %8.2f\t %8.2f\t %8.2f\t%8.2f\n',i,t,x_true,v_true,x_eu,v_eu,err_x,err_v)
        slope = g-(c/m)*v_eu^2; % v' = slope

        t = t+h; %Interval
        x_true =(m/c)*log(cosh(t*sqrt(g*c/m)));
        x_eu = x_eu - v_eu*h;
        % Velocity
        v_true = sqrt(m*g/c)*tanh(t*sqrt(g*c/m));
        v_eu = v_eu + slope*h;
        err_v = abs((v_true - v_eu)/v_true*100);
        err_x = abs((x_true -(x_eu))/x_true*100);
        v_plot(1)=0;
        v_plot(i+1)=v_eu;
        x_plot(1)=350;
        x_plot(i+1)= x_eu;
        i = i +1;
    end
    t_plot=0:h:t;
    plot(t_plot,v_plot,t_plot,x_plot)
    title('(Problem 1(a) Euler Method')
    xlabel('Time (s)');
    ylabel('V and X');