Convert Eigen Matrix to C array

Question

The Eigen library can map existing memory into Eigen matrices.

float array[3];
Map(array, 3).fill(10);
int data[4] = 1, 2, 3, 4;
Matrix2i mat         


        

Answer 1

To convert normal data type to eigen matrix type

  double *X; // non-NULL pointer to some data

You can create an nRows x nCols size double matrix using the Map functionality like this:

  MatrixXd eigenX = Map( X, nRows, nCols );

To convert eigen matrix type into normal data type

  MatrixXd resultEigen;   // Eigen matrix with some result (non NULL!)
  double *resultC;        // NULL pointer <-- WRONG INFO from the site. resultC must be preallocated!
  Map( resultC, resultEigen.rows(), resultEigen.cols() ) =   resultEigen;

In this way you can get in and out from eigen matrix. Full credits goes to http://dovgalecs.com/blog/eigen-how-to-get-in-and-out-data-from-eigen-matrix/