doc/examples/QuickStart_example2_dynamic.cpp

#include <iostream>
#include <Eigen/Dense>

using Eigen::MatrixXd;
using Eigen::VectorXd;

int main() {
  MatrixXd m = MatrixXd::Random(3, 3);
  m = (m + MatrixXd::Constant(3, 3, 1.2)) * 50;
  std::cout << "m =" << std::endl << m << std::endl;
  VectorXd v(3);
  v << 1, 2, 3;
  std::cout << "m * v =" << std::endl << m * v << std::endl;
}