Computing determinant of matrix A using LU factorization

I must compute the determinant of an nxn matrix A using LU factorization. I have already calculated L and U in a different 'lufact' function. For some reason though, my funcion for computing the determinant of A, which is just equal to the product of the diagonal entries of matrix U is not working.

seed = 7;
rng(seed)
A = randn(n,n);
[L,U] = lufact(A);

function f = determ(A)
%Computing determinant of A

f = prod(diag(U));

    end

[the cyclist]

One weirdness here. If your function definition is really

function f = determ(A)
    %Computing determinant of A
    f = prod(diag(U));
end

then that is going to throw an error, because you don't pass in U. So, maybe you are not really calculating with the inputs you expect?

Answer 1

[the cyclist]

If I use the lu function from MATLAB (instead of your lufact function), I get the expected result. So, I'd double-check your LU factorization code.

seed = 7;
n = 5;
rng(seed)
A = randn(n,n);
[L,U] = lu(A);
f = prod(diag(U))

d = det(A)

f =

   -8.0745


d =

   -8.0745