Eigen study notes

1. Initialization

// initialize pointer outside

double _mu0[12]={...};
double _sigma_inv[9] = ...;     
kernels[0].mu = Vector3d(_mu0);
kernels[0].sigma_inv = Matrix3d(_sigma_inv0);

MatrixXf a(10,15); //初始化一个10行15列的矩阵

Vector2d a(5.0, 6.0); // 2维向量初始化维 5 6

// comma initialization 
Matrix3f m; 
m << . 1 , 2 , . 3 ,
      . 4 , . 5 , . 6 ,
      . 7 , . 8 , . 9 ;

2. traversal

  M MatrixXd ( 2 , 2 ); //
   m ( 0 , 1 ) = 3 ; // first row assigned 0 1 3