Blank figures appearing for plots

Question

Phoebe Tyson 2020-3-10

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/510123-blank-figures-appearing-for-plots

评论： Image Analyst 2020-3-10

I've create an adaptive time-stepping function and I'm trying to plot the graphs but it is just creating blank figures.

Here is my function:

function [tvec,yvec,dtvec]=adaptive_euler(f,ytrue,y0,t0,dt0,dtmin,dtmax,tmax,alpha,beta,gamma,TOL)
    y=zeros(10^5,1);
    y(1)=y0;
    dt=zeros(10^5,1);
    t=zeros(10^5,1);
    dt(1)=dt0;
    yt=y0;
    yt1=y0;
    t(1)=t0;
    n=1;
    while (t(n)<tmax && dt(n)<dtmax && dt(n)>dtmin)
        yt=yt+dt(n)*f(t(n),yt);
        yt1=yt1+(dt(n)/2)*f(t(n),yt1);
        yt2=yt1+(dt(n)/2)*f(t(n),yt1);
        Et2=(yt2-yt)/(dt(n));
        c=TOL/norm(Et2);
        if norm(Et2) < TOL
            y(n)=yt2;
            t(n+1)=t(n)+dt(n);
            n=n+1;
        end
        dt(n) = min(max(alpha*c*dt(n-1),gamma*dt(n-1)),beta*dt(n-1));
    end
    tvec=zeros(n,1);
    yvec=zeros(n,1);
    dtvec=zeros(n,1);
    for i=1:n
    tvec(i)=t(i);
    yvec(i)=y(i);
    dtvec(i)=dt(i);
    end
yreal=zeros(n,1);
error=zeros(n,1);
for i=1:n
    yreal(i)=ytrue(tvec(i));
    error(i)=abs(yvec(i)-yreal(i));
end   
T1=table(n,error(n));
figure(1)
plot(tvec,yvec,'.',tvec,yreal,'--');
legend('Approx:','True:');
title('Graph of the Adaptive Eulers Method approximation of dy/dx=-exp(2*t)*y^2 against real values');
figure(2)
plot(tvec,dtvec)
title('Graph of the change in dt over t');
end

And here is the script:

f=@(t,y)-exp(2*t)*y^2;
t0=0;
y0=2;
dt0=0001;
dtmin=10^-10;
dtmax=0.5;
tmax=10;
alpha=0.84;
beta=4;
gamma=0.1;
TOL=0.1;
ytrue=@(t)2*exp(-2*t);
[tvec,yvec,dtvec]=adaptive_euler(f,ytrue,y0,t0,dt0,dtmin,dtmax,tmax,alpha,beta,gamma,TOL)

Thank you!

4 个评论
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Adam 2020-3-10

But you set them up as vectors of length n and then you copy from 1 to n from another vector into these.

Phoebe Tyson 2020-3-10

Because in the original vectors, they are much longer and everything after n is a zero, n is the number of iterations my while loop runs for, while the length of the original vectors is much larger

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