source: pacpussensors/trunk/Vislab/lib3dv-1.2.0/lib3dv/eigen/doc/snippets/MatrixBase_diagonal.cpp

Matrix3i m = Matrix3i::Random();
cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here are the coefficients on the main diagonal of m:" << endl
     << m.diagonal() << endl;