See: http://eigen.tuxfamily.org/bz/show_bug.cgi?id=1005

eigen_sparse_bug.cpp

#include <Eigen/Sparse>
#include <iostream>

using namespace Eigen;

typedef SparseMatrix<double> sparseMatrix_t;
typedef SparseVector<double> sparseVector_t;

typedef MatrixXd denseMatrix_t;
typedef VectorXd denseVector_t;

int main(int argc, char *argv[])
{
  IOFormat ioformat = IOFormat (StreamPrecision,
                                DontAlignCols,
                                ",", ",", "", "", "(", ")");
  ioformat.rowSpacer = "";

  sparseMatrix_t m1 (1, 2);
  m1.insert (0, 0) = 1.;
  m1.insert (0, 1) = 2.;

  sparseMatrix_t m2 (2, 1);
  m2.insert (0, 0) = 1.;
  m2.insert (1, 0) = 2.;

  sparseVector_t v (2);
  v.insert (0) = 1.;
  v.insert (1) = 2.;

  std::cout << denseMatrix_t (m1).format (ioformat) << std::endl;
  std::cout << denseMatrix_t (m2).format (ioformat) << std::endl;
  std::cout << denseVector_t (v).format (ioformat) << std::endl;

  v += m1.transpose ();
  std::cout << denseVector_t (v).format (ioformat) << std::endl;

  v += m2;
  std::cout << denseVector_t (v).format (ioformat) << std::endl;

  return 0;
}