Head function in eigen3 produces wrong result

Question

I want to get a subset of the vector s, the following code didn't give the right result. But if I write

    VectorXd e=s.head(svp)     

e is right. Why the following code is wrong? Thanks!

    Eigen::JacobiSVD<Eigen::MatrixXd> svd(input, Eigen::ComputeThinU |
            Eigen::ComputeThinV);
    VectorXd s=svd.singularValues();
    int svp=5;
    s=s.head(svp);  //Wrong!

Answer 1

You should be using conservativeResize for tasks like this. The error is a type of aliasing; you're overwriting the data which you're reading from (although, in this case, I believe eigen will throw an explicit error for it)