doc/examples/Tutorial_BlockOperations_corner.cpp

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

using namespace std;

int main() {
  Eigen::Matrix4f m;
  m << 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16;
  cout << "m.leftCols(2) =" << endl << m.leftCols(2) << endl << endl;
  cout << "m.bottomRows<2>() =" << endl << m.bottomRows<2>() << endl << endl;
  m.topLeftCorner(1, 3) = m.bottomRightCorner(3, 1).transpose();
  cout << "After assignment, m = " << endl << m << endl;
}