problem with back-substitution code

I want to make an algorithm for back substitution and am testing it on some U and some b. when I call the function though it says U is not defined. Can anyone see where I have made an error? I am very new to MATLAB!

%--INPUTS------------------------------------------------------------------

%

% U - upper triangular matrix

% b - column vector

%

%--OUTPUT------------------------------------------------------------------

%

% b - solution x of Ux=b, stored in b

%

function b=myBackwardSubstitution(U,b)

d=size(U,1);

U=[1 2 3; 4 5 6; 7 8 9];

b=[1;2;3];

for i=1:d

for j=i+1:d

b(i)=b(i)-L(i,j)*b(j);

end

b(i)=b(i)/L(i,i);

end

Answer 1

[Alan Stevens]

Define U and b outside function to which you pass them.

Define L.

%--INPUTS------------------------------------------------------------------
%
% U          -     upper triangular matrix
% b          -     column vector
%
%--OUTPUT------------------------------------------------------------------
%
% b          -     solution x of Ux=b, stored in b
%

% Define U and b outside of function.
U=[1 2 3; 4 5 6; 7 8 9];
b=[1;2;3];

% Call function
b = myBackwardSubstitution(U,b);

function b=myBackwardSubstitution(U,b)
    d=size(U,1);
for i=1:d
    for j=i+1:d
        % You haven't defined L
        b(i)=b(i)-L(i,j)*b(j);
    end
    b(i)=b(i)/L(i,i);
end
end

[Shaima Al-Shalwi]

What does L represent in the code?