C++ Spectra with RowMajor sparse matrix

Question

I'm trying to use the Spectra 3.5 Library on my Linux machine, and the SparseGenMatProd wrapper for the Matrix-Vector multiplication only seems to work when the sparse matrix is in ColMajor format. Is this normal behavior and if so, how can I fix it to take RowMajor format? I've included a basic example where the output is "Segmentation fault (core dumped)". I've gone through several other posts and the documentation, but can't seem to find an answer.

#include <Eigen/Core>
#include <Eigen/SparseCore>
#include <GenEigsSolver.h>
#include <MatOp/SparseGenMatProd.h>
#include <iostream>

using namespace Spectra;

int main()
{
    // A band matrix with 1 on the main diagonal, 2 on the below-main subdiagonal,
    // and 3 on the above-main subdiagonal
    const int n = 10;
    Eigen::SparseMatrix<double, Eigen::RowMajor> M(n, n);
    M.reserve(Eigen::VectorXi::Constant(n, 3));
    for(int i = 0; i < n; i++)
    {
        M.insert(i, i) = 1.0;
        if(i > 0)
            M.insert(i - 1, i) = 3.0;
        if(i < n - 1)
            M.insert(i + 1, i) = 2.0;
    }

    // Construct matrix operation object using the wrapper class SparseGenMatProd
    SparseGenMatProd<double> op(M);

    // Construct eigen solver object, requesting the largest three eigenvalues
    GenEigsSolver< double, LARGEST_MAGN, SparseGenMatProd<double> > eigs(&op, 3, 6);

    // Initialize and compute
    eigs.init();
    int nconv = eigs.compute();

    // Retrieve results
    Eigen::VectorXcd evalues;
    if(eigs.info() == SUCCESSFUL)
        evalues = eigs.eigenvalues();

    std::cout << *emphasized text*"Eigenvalues found:\n" << evalues << std::endl;

    return 0;
}

If you change line 15 to:

Eigen::SparseMatrix<double, Eigen::ColMajor> M(n, n);

it will work as expected.

Currently I'm working around this and converting my matrices to ColMajor, but I'd like to understand what's going on. Any help is much appreciated.

Answer 1

The API of SparseGenMatProd seems to be quite misleading. Looks like you have to specify you are dealing with row-major matrices through the second template parameter:

SparseGenMatProd<double,RowMajor> op(M);

otherwise M is implicitly converted to a temporary column-major matrix which is then stored by const reference by op but this temporary is dead right after that line of code.