Hi, I have some code written to solve LU decomposition, but the code tells me that there is an error in [n, n], why is this error? Any suggestions?

 function matrizlu(A)
[n,n]=size(A);
L=eye(n);
U=eye(n);
for k=1:n
    L(k,k)=1;
    U(1,k)=A(1,k);
    for i=k:n
        U(k,i)=A(k,i)-L(k,1:k-1)*U(1:k-1,i);
    end
    for i=k+1:n
        L(i,k)=A(i,k)-L(i,1:k-1)*U(1:k-1,k)/U(k,k);
    end
end
L
U

Answer 1

[Steven Lord]

When you're asking for help understanding the cause of an error message, please include the error message in your post.

I'm going to guess that it is something along these lines:

matrizlu()

Not enough input arguments.

Error in solution>matrizlu (line 3)
[n,n]=size(A);

If that's the case:

function matrizlu(A)

you've defined matrizlu to accept up to 1 input argument.

[n,n]=size(A);

This line of code uses the input argument, so you must call this function with at least 1 input argument to have it work.

So the function must be called with at least 1 input and it can be called with up to 1 input. Combined these mean that you must call matrizlu with exactly 1 input argument. But in the code above that threw the error I called it with 0 input arguments. That is the reason for the error. The solution is to pass the matrix whose LU factorization you want to receive into the function.

L=eye(n);
U=eye(n);
for k=1:n
    L(k,k)=1;
    U(1,k)=A(1,k);
    for i=k:n
        U(k,i)=A(k,i)-L(k,1:k-1)*U(1:k-1,i);
    end
    for i=k+1:n
        L(i,k)=A(i,k)-L(i,1:k-1)*U(1:k-1,k)/U(k,k);
    end
end
L
U
end

You probably want matrizlu to return the L and U matrices (so you can do something else with them in the workspace from which you called matrizlu) rather than just displaying them. If so update your function declaration:

% Commenting this out so MATLAB Answers can run the code above without this
% affecting it
%{
function [L, U] = matrizlu(A)
%}