Eigen library --> initialize matrix with data from file or existing std::vector content (c++)

Question

My question is how to initialize an eigen Matrix, but NOT this way:

matrix << 1,0,1,0,
          1,0,1,0,
          1,0,1,0,
         


        

Answer 1

I used element-wise initialization (assuming we know nrows and ncols):

MatrixXf X = MatrixXf::Zero(nrows,ncols);
ifstream fin ("./data.txt");

if (fin.is_open())
{
    for (int row = 0; row < nrows; row++)
        for (int col = 0; col < ncols; col++)
        {
            float item = 0.0;
            fin >> item;
            X(row, col) = item;
        }
    fin.close();
}
cout << "X = " << endl << X << endl;