eigen/doc/snippets/MatrixBase_block_int_int.cpp

RowVector4i v = RowVector4i::Random();
cout << "Here is the vector v:" << endl << v << endl;
cout << "Here is v.block(1, 2):" << endl << v.block(1, 2) << endl;
v.block(1, 2).setZero();
cout << "Now the vector v is:" << endl << v << endl;