Copy data from std::vector to Eigen's MatrixXd in C++

Eigen is a linear algebra library in C++. I have my data (double type) in a std::vector (DataVector in the code below) type array. I try to copy it row-wise using the following code which is still giving results column-wise.

Map<MatrixXd, RowMajor> MyMatrix(DataVector.data(), M, N);

Am I doing the correct syntax here?

Solution

No. The MatrixXd object has to be defined as row/column major. See the example below.

#include <Eigen/Core>
#include <iostream>
#include <vector>

using std::cout;
using std::endl;

int main(int argc, char *argv[])
{
    std::vector<int> dat(4);
    int i = 0;
    dat[i] = i + 1; i++;
    dat[i] = i + 1; i++;
    dat[i] = i + 1; i++;
    dat[i] = i + 1;
    typedef Eigen::Matrix<int, -1, -1, Eigen::ColMajor> Cm;
    Eigen::Map<Cm> m1(dat.data(), 2, 2);
    cout << m1 << endl << endl;

    typedef Eigen::Matrix<int, -1, -1, Eigen::RowMajor> Rm;
    Eigen::Map<Rm> m2(dat.data(), 2, 2);
    cout << m2 << endl << endl;

    return 0;
}

Outputs: