Lesson 1 A simple first program

The Eigen/Dense header file defines all member functions for the MatrixXd type and related types 

#include <iostream>#include <Eigen/Dense>using Eigen::MatrixXd;int main(){MatrixXd m(2, 2);m(0, 0) = 3;m(1, 0) = 2.5;m(0, 1) = -1;m(1, 1) = m(1, 0) + m(0, 1);std::cout << m << std::endl;}