i need help with this question

I am supposed to find the number of timesteps i.e the number of steps taken but my answer is incorrect any inputs on my code will be a major help thanks.Also has the advection equation with the center spaced method used correctly ?

N=200;
x=linspace(-10,10,N);
dx=x(2)-x(1);
c=-0.9;
t=0;
dt=0.2;
timestep=0;
f=@(x)exp(-5*x.^2);
y=f(x)';
figure(1),plot(x,y)
title(['Current time=',num2str(t)])
drawnow

%using centered space 
A=eye(N)-(0.5*c*(dt/dx))*((diag(ones(N-1,1),1)-diag(ones(N,1),0)));


while abs(y(end))<0.001 && abs(y(1))<0.001
    y=A*y;
    t=t+dt;
    timestep=timestep+1;
    figure(1),cla
    plot(x,y)
    hold on
    plot(x,f(x-c*t),'r')
    title(['Current time=',num2str(t)])
    drawnow
end

[Jan]

All we know is the code. We cannot know, why you assume, that the answer is wrong. Without any further details it is impossible to miodify a code, such that it produces the output you expect. So please edit the question, exoplain, which problem you want to solve and why you assume, that there is a bug.

[dulanga]

with c= -0.9 on -10<x<10 with the Gaussian initial condition f,The code runs from t = 0 until a time in which |y|=0.001 on one of the boundaries,solve this problem using the centred-in-space approximation

So my assignment has a method of checking if my answer is correct where input my final answer and it says if it is correct or not and in this case it is incorrect.

the below is the centered approach method i should implement in matlab

thanks

[Torsten]

A is wrong. The factor 0.5 is missing, and the lower diagonal is replaced by a main diagonal.

Maybe this A was written for Euler forward instead of centered.

[dulanga]

oh sorry i forgot the 0.5 where the c is at what do u mean by lower diagonal is replaced by main ?

%using centered space

A=eye(N)-(0.5*c*(dx/dt))*diag(ones(N-1,1),1)-diag(ones(N-1,1),-1);

[Torsten]

You forgot the parenthesis around the difference of the diag-expressions.

And it must be dt/dx instead of dx/dt.

[dulanga]

thanks alot how would u implemnt this

N=1000;
x=linspace(-10,10,N);
dx=x(2)-x(1);
c=2.6;
t=0;
dt=0.2;
timestep=0;
f=@(x)exp(-5*x.^2);
y=f(x)';
figure(1),plot(x,y)
title(['Current time=',num2str(t)])
drawnow

%using lax 
A=eye(N)-(0.5*c*(dt/dx))*diag(ones(N-1,1),1)-diag(ones(N-1,1),-1);

while abs(y(end))<0.001 && abs(y(1))<0.001
    y=A*y;
    t=t+dt;
    timestep=timestep+1;
    figure(1),cla
    plot(x,y)
    hold on
    plot(x,f(x-c*t),'r')
    title(['Current time=',num2str(t)])
    drawnow
end