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

I tried this : passing the address of the element at (0,0) and iterating forward.

Eigen::Matrix<double, 3, 8> coordinates3d;
coordinates3d <<    0.0,  0.0,  1.0,  1.0,  0.0,  0.0,  1.0,  1.0,
                    0.0,  1.0,  1.0,  0.0,  0.0,  1.0,  1.0,  0.0,
                    1.0,  1.0,  1.0,  1.0,  0.0,  0.0,  0.0,  0.0;
double *p = &coordinates3d(0,0);
std::vector<double> x2y2;
x2y2.assign(p, p + coordinates3d.size());

for(int i=0;i < coordinates3d.size(); i++) {
    std::cout <<x2y2[i];
}

This is the output : 001011111101000010110100 The data is stored row-major it seems