Get all non-zero values of a dense Eigen::Matrix object

Question

Asuming you have a dynamic size Eigen::Matrix object and want to do some computation on only non-zero values, how can you get a vector or list representation of all non-zero values?

Matrix3f m;
m << 1, 0, 0,
     0, 5, 6,
     0, 0, 9;
VectorXf v = get_non_zero_values(m);
cout << v;

should give you

1 5 6 9

How can this be done with Eigen (most efficiently)?

Answer 1

After a lot of research in the web and inspired by this stackoverflow post I came up with my own solution

template <typename T>
Eigen::Matrix<T, Eigen::Dynamic, 1> get_non_zeros(Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic>& _input)
{
    Eigen::Matrix<T, Eigen::Dynamic, 1> reduced(Eigen::Map<Eigen::Matrix<T, Eigen::Dynamic, 1>>(_input.data(), _input.size()));
    Eigen::Matrix<bool, Eigen::Dynamic, 1> empty = (reduced.array() == 0).rowwise().all();

    size_t last = reduced.rows() - 1;
    for ( size_t i = 0; i < last + 1;) {
        if ( empty(i) ) {
            reduced.row(i).swap(reduced.row(last));
            empty.segment<1>(i).swap(empty.segment<1>(last));
            --last;
        }
        else {
            ++i;
        }
    }

    reduced.conservativeResize(last + 1, reduced.cols());

    return reduced;
}