Index in position 2 exceeds array bounds. Index must not exceed 7. Error in ftcsimplicit (line 39) plot(x,u(:​,7200),'-'​,x,u(:,144​00),'-',x,​u(:,21600)​,'-',x,u(:​,28800),x,​u(:,3

% Chlorine decay using FTCS Implicit method

clear;

% Parameters to define the chlorine dacay in the pipe and the range in space and time

L= 5.; % Length of the pipe

T= 43200.; % Final time

% Parameters needed to fully solve the equation within the implicit method

maxk = 6; % Number of time steps

dt = T/maxk;

n = 50.; % Number of space steps

dx = L/n;

cond = 1. % Conductivity

b = cond*dt/(dx^2);

% Initial temperature of the wire: a sinus

for i = 1:n+1

x(i)= (i-1)*dx;

u(i,1) = sin(pi*x(i));

end

% Temperature at the boundary (T=0)

for k=1:maxk+1

u(1,k) = 0.;

u(n+1,k)=0.;

time(k) = (k-1)*dt;

end

aa(1:n-2)=-b;

bb(1:n-1)=1.+2.*b;

cc(1:n-2)= -b;

MM = inv(diag(bb,0)+diag(aa,-1)+diag(cc,1));

% Implementation of the implicit method

for k=2:maxk %Time loop

uu=u(2:n,k-1);

u(2:n,k)= MM*uu;

end

% Graphical representation of the temperature at different selected times

figure (1)

plot(x,u(:,7200),'-',x,u(:,14400),'-',x,u(:,21600),'-',x,u(:,28800),x,u(:,36000),x,u(:,43200),'-')

title('Temperature within the fully implicit method')

xlabel('X')

ylabel('T')

figure(2)

mesh(x,time,u')

title('Temperature within the fully implicit method')

xlabel('X')

ylabel('Temperature')

[Mujtaba Farrukh]

Sir, varible "u" size is 51x7 means it has 7 columns (means you can only access 7 columns not more than that ) and in the line 39, you are extracting the columns 7200, 14400 and etc, which exceeds the index value of u's column. Hence results in error.

[Mathieu NOE]

the plot time steps should be normalized by your dt = 7200 to make the plot work

plot(x,u(:,7200/dt),'-',x,u(:,14400/dt),'-',x,u(:,21600/dt),'-',x,u(:,28800/dt),x,u(:,36000/dt),x,u(:,43200/dt),'-')