Matlab grader discretization help

Hello,

I am stuck on a Matlab grader problem about discretization. I am wondering if anyone is able to help me figure out what is wrong with my code/what I need to change or add? The asignment is attached in a file called grader asignment. My code and the asignment code is below. I wrote the function, the main program is given.

N = 10;
q = @(x) x;
k = 4;
Tl = 1;
Tr = 2;
[A,HL] = diskretisering_temperatur(N,q,k,Tl,Tr);

Error using horzcat
Dimensions of arrays being concatenated are not consistent.

Error in solution>diskretisering_temperatur (line 44)
A = spdiags([off_diagonal main_diagonal off_diagonal], -1:1, N-1, N-1);

% solve the problem
sol = A\HL;
% plot solution with boundary values
h = 1/N;
x = [h:h:1-h]';
figure
plot([0; x; 1],[Tl; sol; Tr])

function [A,HL] = diskretisering_temperatur(N,q,k,Tl,Tr)
   
   h = 1/N;
   x = [h:h:1-h]';
   
  

   
   % Create the coefficient for the finite difference
   coefficient = k / h^2;

   % Initialize the right-hand side vector HL
   HL = zeros(N-1, 1);

   % Apply the heat source function q(x) to the right-hand side vector HL
   for i = 1:N-1
       HL(i) = q(x(i));
   end

   % Apply the boundary conditions to the system
   HL(1) = HL(1) - Tl * coefficient;  % Apply the left boundary condition
   HL(N-1) = HL(N-1) - Tr * coefficient; % Apply the right boundary condition

   % Initialize the diagonals of the system matrix A
   main_diagonal = -2 * coefficient * ones(N-1, 1);
   off_diagonal = coefficient * ones(N-2, 1);

   % Construct the sparse matrix A using the diagonals
   A = spdiags([off_diagonal main_diagonal off_diagonal], -1:1, N-1, N-1);
   

end

Answer 1

[Torsten]

Use

off_diagonal = coefficient * ones(N-1, 1);

instead of

off_diagonal = coefficient * ones(N-2, 1);

See the example "Create Tridiagonal Matrix" under

https://uk.mathworks.com/help/matlab/ref/spdiags.html