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Index exceeds matrix dimensions

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Harsha
Harsha on 7 Nov 2020
Edited: Harsha on 11 Nov 2020
Hi everyone,
I am trying to model a problem for FTCS and I am using this code:
% --- Define constants and initial condition
L =1; % length of domain in x direction
tmax= 0.8; % end time
nx =10; % number of nodes in x direction
nt =10; % number of time steps
dx = L/(nx-1);
dt = tmax/(nt-1);
r = 0.5; r2 = 1 - 2*r;
% --- Loop over time steps
t = 0;
u =100; % initial condition
for m=1:nt
uold = u; % prepare for next step
t = t + dt;
for i=2:nx-1
u(i) = r*uold(i-1) + r2*uold(i) + r*uold(i+1);
end
end
But I couldn't figure out whats the mistake here, I am new to this so anyhelp is much appreciated.

Answers (2)

Reshma Nerella
Reshma Nerella on 10 Nov 2020
Hi,
In the code, you are assigning a value to variable 'u'
u =100 % 1x1 Double
Implies you can only index till 1(since we have only one element).
for i=2:nx-1
u(i) = r*uold(i-1) + r2*uold(i) + r*uold(i+1); %You are indexing 'u' from 2, beyond dimensions.
end
If you want to find 'u' matrix, consider preallocating it for speed and assign required values to it.
u = zeros(1,nx-1); %creates a matrix of size 1 x (nx-1) with zeros
For the initial condition, if you want first element of 'u' to be 100, You can assign it this way.
u(1) = 100;
You may get the issue with 'uold'
uold = u; % assigning a single value
for i=2:nx-1
u(i) = r*uold(i-1) + r2*uold(i) + r*uold(i+1); %Indexing it beyond dimensions
end
Hope this helps!

Harsha
Harsha on 10 Nov 2020
Edited: Harsha on 11 Nov 2020
Thank you very much for your reply,
I actually modified the code to the follwoing:
c>> n = 10;
Lx = 1;
dx = Lx/(n-1);
x = 0:dx:Lx;
% 2. Parameters for the t vector
m = 80;
tf = 0.8;
dt = tf/(m-1);
t = 0:dt:tf;
% 3. Other parameters needed for the solution
% The value of alpha
Fo = .5; % a mutiplicative constant that should be < = 1/2
% to insure stability
% Initial and boundary conditions
f = @(x)100;
g1 = @(t)0;
g2 = @(t)0;
u = zeros(n,m);
u(2:n,1) = f(x(2:n)); % Put in the initial condition starting from
% 2 to n-1 since f(0) = 0 and f(N) = 1
u(1,:) = g1(t); % The boundary conditions, g1 and g2 at
u(n,:) = g2(t); % x = 0 and x = 1
% Implementation of the explicit method
for k = 2:m-1 % Time Loop
for i= 2:n-1 % Space Loop
u(i,k+1) = Fo*(u(i-1,k)+u(i+1,k))+(1-2*Fo)*u(i,k);
end
end
figure
hold all
for i=1:4:numel(t)
plot(x,u(:,i),'linewidth',1.5,'DisplayName',sprintf('t = %1.2f',t(i))); fprintf(x,u(:,i),'linewidth',1.5,'DisplayName',sprintf('t = %1.2f',t(i)));
end
a = ylabel('Heat');
set(a,'Fontsize',14);
a = xlabel('x');
set(a,'Fontsize',14);
a=title(['Using The Explicit Method - Fo =' num2str(Fo)]);
set(a,'Fontsize',16);
legend('-DynamicLegend','location','bestoutside');
grid;
figure
[X, T] = meshgrid(x,t);
s2 = mesh(X',T',u);
title(['3-D plot of the 1D Heat Equation using the Explicit Method - Fo =' num2str(Fo)])
set(s2,'FaceColor',[0 0 1],'edgecolor',[0 0 0],'LineStyle','--');
a = title('Exact solution of the 1D Diffusivity Equation');
set(a,'fontsize',14);
a = xlabel('x');
set(a,'fontsize',20);
a = ylabel('y');
which I got it from here: https://www.mathworks.com/matlabcentral/answers/404194-question-on-heat-equation-1d-forward-in-time-centered-in-space
Now my only problem is to get the values as a table like the attached pic, any idea for doing that?
Edit I also keep getting zeros for all t .
Very grateful for your help!

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