Index exceeds the number of array elements (23).

function main
clc;
clear all;
eps = 0.001;
omega = input(' enter the omega value: ');
beta = input (' enter the beta value: ');
n= 100000;
nx = 26;
ny = 26;
S(1:nx, 1:ny) = 0;
SN(1:nx, 1:ny) = 0;
S(7:nx, 1)= 100;
S(nx, 1:16) = 100;
SN(7:nx, 1)= 100;
SN(nx, 1:16) = 100;
% its number of iteration
coeff = ( 2*(1+beta^2));
for iterations = 1:n
    for j = 2:ny-1
        a(1:nx-2) = -coeff;
        b(1:nx-3)=  omega;
        c(1:nx-3)= omega;
        for i = 2:nx-1 
            r(i-1)= -coeff*(1-omega)*S(i,j)- omega*beta^2*S(i,j+1)-omega*beta^2*SN(i,j-1);
        end
        r(1)= r(1)-omega*SN(1,j);
        r(nx-2)= r(nx-2)- omega*SN(nx,j);
        y = Tridiagonal (c,a,b,r);
        for k = 1:nx-2
            SN(k+1,j) = y(k);
        end
    end
    for i = 2:nx-1
        a(1:ny-2) = -coeff;
        b(1:ny-3) = beta*beta;
        c(1:ny-3) = beta*beta;
        for j = 2:ny-1
            r(j-1) = -coeff*(1-omega)*S(i,j)- omega*S(i+1,j)-omega*SN(i-1,j);
        end
        r(1) = r(1)-SN(i,1);
        r(ny-2)= r(ny-2)- SN(i,ny);
        y = Tridiagonal (c,a,b,r);
        for k = 1:ny-2
            SN (i,k+1)= y(k);
        end
    end
    error = abs(SN-S);
    totalerror = sum(error,'all');
    if totalerror <= eps
        break
    end
    S = SN;
end
iterations
contour(SN');
end

function x = Tridiagonal(e,f,g,r)
% Tridiagonal: Tridiagonal equation solver banded system
% x = Tridiagonal(e,f,g,r): Tridiagonal system solver.
% input:
% e = subdiagonal vector
% f = diagonal vector
% g = superdiagonal vector
% r = right hand side vector
% output:
% x = solution vector
n=length(f);
% forward elimination
for k = 2:n
    factor = e(k)/f(k-1);
    f(k) = f(k) - factor*g(k-1);
    r(k) = r(k) - factor*r(k-1);
end
% back substitution
x(n) = r(n)/f(n);
for k = n-1:-1:1
    x(k) = (r(k)-g(k)*x(k+1))/f(k);
end
end

COMMAND WINDOW;

enter the omega value: 1.3

enter the beta value: 1

Index exceeds the number of array elements (23).

Error in adior1>Tridiagonal (line 71)

factor = e(k)/f(k-1);

Error in adior1 (line 28)

y = Tridiagonal (c,a,b,r);

Answer 1

[Walter Roberson]

nx = 26;

        a(1:nx-2) = -coeff;

You do not otherwise initialize a before you call the function, so a is going to be length nx-2 which would be 26-2 = 24

        c(1:nx-3)= omega;

You do not otherwise initialize c before you call the function, so c is going to be length nx-3 which would be 26-3 = 23

        y = Tridiagonal (c,a,b,r);

c (shorter vector) gets passed in first position, a (longer vector) gets passed in second position

        function x = Tridiagonal(e,f,g,r)

first position (shorter vector) gets named e inside the function; second position (longer vector) gets named f inside the function.

        n=length(f);

That is length() of the second position, the longer vector. So it would be n = 24

        for k = 2:n

k can be a maximum of 24

            factor = e(k)/f(k-1);

e is the shorter vector, and would be indexed up to n = 24. But e is shorter and only has 23 elements.

[Aman Murkar]

What changes should I do to run this code? Because I am new at matlab so I am just trying to solve probems

[Aman Murkar]

because code of tridigagonal matrix is correct