FTCS code shows Error using zeros

The error displays:

Error using zeros

Size inputs must be scalar.

Error in ftcsHeatEquation (line 32)

Ynum = zeros(1,size(x)); % Solution at timestep n

function [Ynum, errMean] = ftcsHeatEquation(nx,nt)

L =10;                      % Length of domain [m]
D =1.76E-5;                      % Diffusivity constant [m^2/s]



%analytical
Ya=zeros(1,nx);
Ys=zeros(1,nx);
y0=0.78; %y when x=0
nf=100; %given


    
% Calculate the dx and dt terms    
x = linspace(0,L,nx);                      % x-domain [m]
dx = L/(nx-1);                     % x-domain spacing [m]
dt=1;  
sigma = D*dt/(dx^2);
    
% Initialise all necessary vectors    
Y=zeros(nt,nx);

Ynum = zeros(1,size(x)); % Solution at timestep n
Ynp1 = zeros(1,size(x)); % Solution at timestep n+1

Ynum(1:end-1) = 1; %initial
Ynum(end) = 0.78; 
Ynp1(1) = -0.22; %boundary 
Ynp1(end) = (-0.22/L)*x;
tEnd=dt*nt;


% Solve for the numerical solution
for n = 2:nt
    % Loop over internal points
    for i= 2:nx-1
        Ynp1(i) = Ynum(i)-sigma*yn(i+1)-2*Ynum(i)+Ynum(i-1);
    end
    
    % Update solution for next timestep
    Ynum = Ynp1;
end


  
end 

Answer 1

[Sriram Tadavarty]

Hi Mingyi,

The error you observe is due to the incorrect usage of zeros function.

This can be solved with any of the following options:

% Option 1: Directly using nx, instead of size
Ynum = zeros(1,nx);
Ynp1 = zeros(1,nx);

% Option 2: Using length instead of size
Ynum = zeros(1,length(x));
Ynp1 = zeros(1,length(x));

% Option 3: Using the zise of x, without any other dimensions placed
Ynum = zeros(size(x));
Ynp1 = zeros(size(x));

% if you are ware that Ynp1 is same size as Ynum, 
% you directly assign Ynp1 = Ynum;, rather than defining separately as Ynum

Hope this helps.

Regards,

Sriram

[Mingyi Shao]

Dear sriram,

I have decided to use option 2, and now it pops up with the message like this

Unable to perform assignment because the left and right sides have a different number of elements.

Error in ftcsHeatEquation (line 38)

Ynp1(end) = (-0.22/L)*x;

Kind regards

Mingyi

[Sriram Tadavarty]

The issue here is in the last element you are trying to store all the values of x. Replace x here with x(end) and that should solve.

[Mingyi Shao]

Ok! That solved it! Thanks

[Sriram Tadavarty]

Do accept the answer, if helped