doc/examples/Tutorial_BlockOperations_block_assignment.cpp

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

int main()
{
  Eigen::Array22f m;
  m << 1,2,
       3,4;
  Eigen::Array44f a = Eigen::Array44f::Constant(0.6);
  std::cout << "Here is the array a:\n" << a << "\n\n";
  a.block<2,2>(1,1) = m;
  std::cout << "Here is now a with m copied into its central 2x2 block:\n" << a << "\n\n";
  a.block(0,0,2,3) = a.block(2,1,2,3);
  std::cout << "Here is now a with bottom-right 2x3 block copied into top-left 2x3 block:\n" << a << "\n\n";
}