Question

I have an unsorted vector of eigenvalues and a related matrix of eigenvectors. I’d like to sort the columns of the matrix with respect to the sorted set of eigenvalues. (e.g., if eigenvalue[3] moves to eigenvalue[2], I want column 3 of the eigenvector matrix to move over to column 2.)

I know I can sort the eigenvalues in O(N log N) via std::sort. Without rolling my own sorting algorithm, how do I make sure the matrix’s columns (the associated eigenvectors) follow along with their eigenvalues as the latter are sorted?

Answer 1

Typically just create a structure something like this:

struct eigen { 
    int value;
    double *vector;

    bool operator<(eigen const &other) const { 
        return value < other.value;
    }
};

Alternatively, just put the eigenvalue/eigenvector into an std::pair — though I’d prefer eigen.value and eigen.vector over something.first and something.second.