I do not know what's wrong with my code

%1-D Heat equation

%example 1 at page 782

%dT/dt=c.d^2T/dx^2

%T(x,t)=temperature along the rod

%by finite difference method

%(T(x,t+dt)-T(x,t))/dt = (T(x+dx,t)-2T(x,t)+T(x-dx,t))/dx^2

%solve for T(x,t+dt) by iteration

%heat constant

close all;

%clear all;

c=1;

L=1; %length of the rod

N=5; %# of elements

%dicretize Xspace

x_vec=0:0.2:1;

dx=x_vec(2)-x_vec(1);

%discretize time

dt=0.5/50;

t_vec=0:dt:0.5;

%temperature matrix

T_mat=zeros(length(x_vec),length(t_vec));

T_mat1=zeros(length(x_vec),length(t_vec));

%boundary conditions

T_mat(1,:)=0;

T_mat(end,:)=0;

%initial conditions

T_mat(:,1)= 1-cos(pi*x_vec(idx))(400/npi)*sin*(npi.*x_vec(idx));

%T_mat(:,1)= sin*((pi.*x_vec);

%[tt,xx]=meshgrid(t_vec,x_vec);

subplot(2,1,1);

mesh(xx,tt,T_mat);

lamda=(c*dt/(dx^2));

for tdx=1:length(t_vec)-1

for idx=1:length(x_vec)

if idx==1

T_mat(idx,tdx+1)=T_mat(idx+1,tdx);

T_mat1(idx,tdx)= 1-cos(pi*x_vec(idx))(400/npi)sin(npix_vec(idx))*exp(-1*(pi^2)*t_vec(tdx)))); %modified code, 2nd line of 'if'

elseif idx==length(x_vec)

T_mat(idx,tdx+1)=T_mat(idx-1,tdx);

T_mat1(idx,tdx)= 1-cos(pi*x_vec(idx))(400/npi)sin*(npi*x_vec(idx))*exp(-1*(pi^2)*t_vec(tdx)); %modified code, 2nd line of 'elseif

else

T_mat(idx,tdx+1)=T_mat(idx,tdx)+lamda*((T_mat(idx+1,tdx)-2*T_mat(idx,tdx)+T_mat(idx-1,tdx)));

T_mat1(idx,tdx)=1-cos(pi*x_vec(idx))(400/npi)*sin(npix_vec(idx))*exp(-1*(pi^2)*t_vec(tdx)); %modified code, 2nd line of 'elseif)

end

end

end

%plot

subplot(2,1,1)

[tt,xx]=meshgrid(t_vec,x_vec);

mesh(xx,tt,T_mat);

title('Finite')

%plot

subplot(2,1,2)

[tt,xx]=meshgrid(t_vec, x_vec);

mesh(xx,tt,T_mat1);

title('Analytical');

figure

plot(x_vec,T_mat(:,1),x_vec,T_mat(:,11),x_vec,T_mat(:,21),x_vec,T_mat(:,31),x_vec,T_mat(:,41),x_vec,T_mat(:,51));

xlabel('Rod length (m)'); ylabel('Temperature (C)');

legend('Initially','At t=0.1sec','At t=0.2 secs','At t=0.3 secs',' At t=0.4 secs',' At t=0.5 secs');

figure

plot(x_vec,T_mat1(:,1),x_vec,T_mat1(:,11),x_vec,T_mat1(:,21),x_vec,T_mat1(:,31),x_vec,T_mat1(:,41),x_vec,T_mat1(:,51));

xlabel('Rod length (m)'); ylabel('Temperature (C)');

legend('Initially','At t=0.1sec','At t=0.2 secs','At t=0.3 secs',' At t=0.4 secs',' At t=0.5 secs');

title('Analytical')

xlabel('Rod Length |m'); ylabel('Temperature |C|');

legend('Initially','At t=0.1 sec','At t=0.0 sec','At t=0.3 sec','At t=0.4 sec','At t=0.5 sec');

[Raghunandan V]

https://in.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer

Answer 1

[Steven Lord]

If you receive an error when running your code, post the full text (everything displayed in red) of the error message you receive, and indicate (if it's not obvious) the line in your code on which the error occurs.

If you don't receive an error but receive a different result than you expected, it's likely going to be difficult for us to help you since there aren't a lot of comments in the code to help us understand it. This looks like it may be the solution to a homework assignment (the "%example 1 at page 782" suggests that to me) and if that's true you may want to ask your professor and/or teaching assistant to help you. If it's not a homework assignment, or if your professor and/or TA are unavailable, step through your code using the debugging tools and identify where the code gives a different result than you expected then figure out why.