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

If the array is two-dimensional, one needs to pay attention to the storage order. By default, Eigen stores matrices in column-major order. However, a row-major order is needed for the direct conversion of an array into an Eigen matrix. If such conversions are performed frequently in the code, it might be helpful to use a corresponding typedef.

using namespace Eigen;
typedef Matrix RowMatrixXi;

With such a definition one can obtain an Eigen matrix from an array in a simple and compact way, while preserving the order of the original array.

From C array to Eigen::Matrix

int nrow = 2, ncol = 3;
int arr[nrow][ncol] =  { {1 ,2, 3},  {4, 5, 6} }; 
Map eig(&arr[0][0], nrow, ncol);

std::cout << "Eigen matrix:\n" << eig << std::endl;

// Eigen matrix:
// 1 2 3
// 4 5 6

In the opposite direction, the elements of an Eigen matrix can be transferred directly to a C-style array by using Map.

From Eigen::Matrix to C array

int arr2[nrow][ncol];
Map(&arr2[0][0], nrow, ncol) = eig;

std::cout << "C array:\n";
for (int i = 0; i < nrow; ++i) {
  for (int j = 0; j < ncol; ++j) {
    std::cout << arr2[i][j] << " ";
  }
  std::cout << "\n";
}

// C array:
// 1 2 3 
// 4 5 6 

Note that in this case the original matrix eig does not need to be stored in row-major layout. It is sufficient to specify the row-major order in Map.