Why is the graph in this code becoming distorted?

%% This is the code.
n=101;
nstep=100;
long=10.0;
h=long/(n-1);
dt=0.05;
D=0.05;
f=zeros(n,1);
y=zeros(n,1);
time=0.0;
for i=1:n
    f(i)=0.5*sin(2*pi*h*(i-1));
end
for m=1:nstep
    plot(f,'linewidth',2);
    axis([1 n -2 2])
    y=f;
    for ii=2:n-1
        f(ii)=y(ii)-0.5*(dt/h)*(y(ii+1)-y(ii-1)+D*(dt/h^2)*(y(ii+1)-2*y(ii)+y(ii-1)));
    end
    f(n)=y(n)-0.5*(dt/h)*(y(2)-y(n-1))+D*(dt/h^2)*(y(2)-2*y(n)+y(n-1));
    time=time+dt;
    title(sprintf('time = %3.2f',time))
    pause(0.2)
end

As a result of running this code, the graph becomes distorted from the right end and stretched as 'time' increases. Is there something wrong that is causing this?

Answer 1

[Mathieu NOE]

hello

maybe this ? I simply corrected one of your equation (looking at the one that follows , assumming the second was correct ) because there is a difference where you put your parenthesis

%% This is the code.
n=101;
nstep=100;
long=10.0;
h=long/(n-1);
dt=0.05;
D=0.05;
f=zeros(n,1);
y=zeros(n,1);
time=0.0;

i=1:n;
f=0.5*sin(2*pi*h*(i-1));


for m=1:nstep
    
     plot(f,'linewidth',2);
     axis([1 n -2 2])
     y=f;
     
     for ii=2:n-1
%         f(ii)=y(ii)-0.5*(dt/h)*(y(ii+1)-y(ii-1)+D*(dt/h^2)*(y(ii+1)-2*y(ii)+y(ii-1)));
        f(ii)=y(ii)-0.5*(dt/h)*(y(ii+1)-y(ii-1))+D*(dt/h^2)*(y(ii+1)-2*y(ii)+y(ii-1)); % correction based on the equation 2 lines below
     end
     f(n)=y(n)-0.5*(dt/h)*(y(2)-y(n-1))+D*(dt/h^2)*(y(2)-2*y(n)+y(n-1));

     time=time+dt;
     title(sprintf('time = %3.2f',time))
     pause(0.2)
     
end