function x = tridiagsolve ( L, U, U2, b )
%
%  function x = tri_solve ( L, U, U2, b )
%
%  TRI_SOLVE solves a tridiagonal linear system factored by TRI_FACTOR.
%
%    Input, real L(N-1), the subdiagonal elements of the unit lower
%    triangular factor of A.
%
%    Input, real U(N), U2(N-1), the diagonal and superdiagonal elements
%    of the upper triangular factor of A.
%
%    Input, real b(N), the right hand side.
%
%    Output, real x(N), the solution of the linear system.
%
n = 3;%length ( U );
%
%  Solve L * y = b.
%
y = zeros(n,1);
y(1) = b(1);
for i = 2 : n
  y(i) = b(i) - L(i-1) * y(i-1);
end
%
%  Solve U * x = y.
%
%x = zeros(n,1);
for i = n : -1 : 2
  x(i) = y(i) / U(i);
  y(i-1) = y(i-1) - U2(i-1) * x(i);
end
x(1) = y(1) / U(1);
end


function x =getdiag(a,n)
j = 1;
i = 1;



while i <= n
dia(i) = a(i,j);
i++;
j++;
end;
x = dia;
end

function x =getsub(a,n)
j = 1;
i = 2;


while j <= n-1
sub(j) = a(i,j);
i++;
j++;
end;
x = sub;
end

function x =getsuper(a,n)
j = 2;
i = 1;


while i <= n-1
super(i) = a(i,j);
i++;
j++;
end;
x = super;
end


a=[1, 2, 0;
   4, 5, 6;
   0, 8, 9];

b = [1,2,3];
n = length(b);


dia = getdiag(a,n); %[1,5,9]
sub = getsub(a,n); %[4,8]
super = getsuper(a,n); % [2,6]

x = tridiagsolve(sub,dia,super,b);
write("x = ",x);