No plot is being generated

Question

Kyle Broder 2016-8-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/299071-no-plot-is-being-generated

评论： Kyle Broder 2016-8-10

I'm considering a system of differential equations given by dR/dt = U and dU/dt = (L-GM)/R^2. I'm trying to use RK4 to solve this system and generate some plots. My code is given below, however I'm not generating any plots. Is therefore something wrong with my RK4 code? Note that the code does run.

function [x,t] =nma_RK4_4(  )
    delT = 0.001; %time grid
    t    = linspace(0,5,round(5/delT)); %time
    N    = length(t);
    x     = zeros(2,N);
    x(1,1) = 100000;  %initial conditions
    x(2,1) = 100000;
    G = 6.67*power(10,-11);
    M = 1.5*1.989*power(10,41);
    L = x(1,1)*x(2,2);
    theta = -x(2,1)/(power(x(1,1),2));
    xvar = x(1,1)*cos(theta);
    yvar = x(1,1)*sin(theta);
    for i = 1:(N-1)
         k1     = delT * f( t(i)          , x(:,i));
         k2     = delT * f( t(i)+1/2*delT , x(:,i)+1/2*k1);
         k3     = delT * f( t(i)+1/2*delT , x(:,i)+1/2*k2);
         k4     = delT * f( t(i)+delT     , x(:,i)+k3);
         x(:,i+1) = x(:,i)+1/6*(k1+2*k2+2*k3+k4);
    end
       function r=f(t,x)
          r=[x(2)
              (L-G*M)/(power(x(1),2))];
       end
   figure()
   plot(xvar,yvar)
  end

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Answer 1

Walter Roberson 2016-8-10

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https://ww2.mathworks.cn/matlabcentral/answers/299071-no-plot-is-being-generated#answer_231342

Your xvar and your yvar are scalars, so plot(xvar,yvar) is plotting only one point. The default when using plot() is to not use any markers, so the one point is not showing up. It would show up if you used plot(xvar, yvar, 'r*-')

Is there a reason you are calculating x but not using it to plot?

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Kyle Broder 2016-8-10

Thanks for your comment, you've made me realise an error in my code. After fixing the error, I'm still not getting a plot. Essentially, what I want to do is solve the above system of ODEs for R and then use that value obtained by using RK4 to then plot the result. I want to plot x against y however, and this is obtained from setting x = Rcos(theta) and y=Rsin(theta). As can be seen this is not working for me.

I don't want to plot just a point, I want something like this, https://arxiv.org/pdf/1503.05861.pdf, see the first graph on page 8. Note that the plot I've given as an example in that link does not look like what I have, given that it is a different system of ODEs, but the type of plot it what I want to generate.

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