doc/snippets/Tutorial_commainit_02.cpp

int rows = 5, cols = 5;
MatrixXf m(rows, cols);
m << (Matrix3f() << 1, 2, 3, 4, 5, 6, 7, 8, 9).finished(),
    MatrixXf::Zero(3, cols - 3), MatrixXf::Zero(rows - 3, 3), MatrixXf::Identity(rows - 3, cols - 3);
cout << m;