doc/examples/Tutorial_simple_example_dynamic_size.cpp

#include <Eigen/Core>
#include <iostream>

int main() {
  for (int size = 1; size <= 4; ++size) {
    Eigen::MatrixXi m(size, size + 1);    // a (size)x(size+1)-matrix of int's
    for (int j = 0; j < m.cols(); ++j)    // loop over columns
      for (int i = 0; i < m.rows(); ++i)  // loop over rows
        m(i, j) = i + j * size;           // to access matrix coefficients,
                                          // use operator()(int,int)
    std::cout << m << "\n\n";
  }

  Eigen::VectorXf v(4);  // a vector of 4 float's
  // to access vector coefficients, use either operator () or operator []
  v[0] = 1;
  v[1] = 2;
  v(2) = 3;
  v(3) = 4;
  std::cout << "\nv:\n" << v << std::endl;
}