Hi, I am new to Eigen but I can't seem to figure out the problem that I'm having. I am using Eigen's SVD to calculate some singular values from a matrix "R" (I think it's a 3x3 matrix):

Eigen::JacobiSVD<Eigen::MatrixXd> svd(R, Eigen::ComputeThinU | Eigen::ComputeThinV); //SVD Matrix
Eigen::MatrixXd U;
Eigen::MatrixXd S;
Eigen::MatrixXd V;
double UdVd;

U=svd.matrixU();
S=svd.singularValues();
V=svd.matrixV();

However, I noticed that I get different behavior when I then do:

UdVd=U.determinant()*V.determinant();
std::cout << U << std::endl;

---OUTPUT---

4.20253e-07 0.707107 -0.707107
9.86222e-10 0.707107 0.707107
1 -2.97861e-07 2.96466e-07


versus, simply:

std::cout << U << std::endl;

---OUTPUT---
0 0 1
1 3.14303e-05 0
3.14303e-05 -1 0


That is, given the same input, R, the matrix U that gets printed out is different if I omit the determinant function call. I expect U to be the same since the input is the same. Is this assumption incorrect or am I missing something here?

Thanks!

In fact, I get different results again if I do:

U.determinant();
std::cout << U << std::endl;

or

V.determinant();
std::cout << U << std::endl;

In the latter case, I don't really understand why calculating the determinant of V could affect U. I wonder if this has anything to do with aliasing?

You are right, computing the determinant should not change the matrix U. Unfortunately, I cannot reproduce the problem you are seeing, and I have no idea what's causing it. Could you please give us a self-contained program exhibiting the issue, and tell us the version of Eigen and the compiler that you are using?

Indeed that does not make sense. If you are running linux or osx, I'd strongly recommend to run your program under valgrind ($ valgrind path/to/executable) to check possible memory issues.

My mistake. It had to do with:

Eigen::JacobiSVD<Eigen::MatrixXd> svd(R, Eigen::ComputeThinU | Eigen::ComputeThinV);

I declared the "svd" matrix but "R" had not been filled yet (R had been declared but not filled). If I declare "svd" AFTER "R" has been filled then all is fine! I apologize for this oversight.