Error using plot. Vectors must be the same length.

Hello,

When running my code I'm getting the following error :

Error using plot
Vectors must be the same length.

Error in A5Q2 (line 45)
plot(x1,u(1,:),'*',x1,us1,'-');

Please find below my code:

function u = diff1_ex
ngrid = 50;
dx = 1/ngrid;
D=1;
s=0.4;
dt = s*dx^2/D;
tmin=0;
tmax=1;
ntime = ceil((tmax-tmin)/dt);
u = zeros(ngrid+1,ntime);
source=1;
for l=1:ntime
  for k=2:ngrid
    u(k,l+1)=u(k,l)+s*(u(k+1,l)-2*u(k,l)+u(k-1,l));
    if (k>ceil((0.4/dx)-1)) && (k<ceil(0.6/dx))
      u(k,1+l)=u(k,1+l)+ dt*source;
    end
  end
end
subplot(2,1,1);
imagesc(u);
colorbar;
xlabel('x')
ylabel('y')
x1 = 0:dx:1;
us1 = -0.3*x1;
x2 = 0.4:dx:0.6;
us2 = (-x2.^2)/2+(0.1*x2)-0.08;
x3 = 0.6:dx:1;
us3 = 0.5*(1-x3);
subplot(2,1,2);
hold on
plot(x1,u(1,:),'*',x1,us1,'-');
plot(x2,u(2,:),'*',x2,us2,'-');
plot(x3,u(3,:),'*',x3,us3,'-');
hold off
xlabel('x')
ylabel('us')
title('Stationary Solution')

Could you please help me to correct my mistake.

Please find attached the questions and my solution to the first question.

Thanks in advance

[dpb]

Use the debugger and set a breakpoint after the line

u = zeros(ngrid+1,ntime);

look at

whos u

then at the plot() line try

length(u(1,:))

[Vashist Dwarka]

What do mean by " whos u " ?

[dpb]

When the debugger prompt shows up, type it in at the prompt.

See

doc whos

PS: By "at the plot line" I meant either set another breakpoint at that line and run until there or just step through until you get there (F10)

[Vashist Dwarka]

I'm really lost with the debugging process. Could you please help me going through it ? and figuring out the mistake

[dpb]

See The Debugging Process (link)

Basically you just click on the line where you want a breakpoint in the editor and then press RUN since your function doesn't require any input arguments.

As a hint to your problem, look at how you defined the array u and then how you try to use it when you are trying to plot.

Learning to use the debugger is well worth the effort...

Answer 1

[madhan ravi]

plot(x1,u(1,1:numel(x1)),'*',x1,us1,'-'); 
plot(x2,u(2,1:numel(x2)),'*',x2,us2,'-'); 
plot(x3,u(3,1:numel(x3)),'*',x3,us3,'-');

[madhan ravi]

maybe like the above? not sure if its correct

[Vashist Dwarka]

Does not give me the right graph

[madhan ravi]

ok then no idea because your x1, x2, x3 size is 1 by 51 and your u size is 51 by 6251 , how can you plot them?

[Vashist Dwarka]

I've posted more information below. Could you please check if I did something wrong in u , us1, us2, us3, x1, x2 and x3

Answer 2

[Vashist Dwarka]

Please find attached the questions and the result of of the first question.