Eigen library - Mathematical toolbox for C++

std::endl;

const int size = 6;

MatrixXd mat1(size, size);

mat1.topLeftCorner(size/2, size/2) = MatrixXd::Zero(size/2, size/2);

mat1.topRightCorner(size/2, size/2) = MatrixXd::Identity(size/2, size/2);

mat1.bottomLeftCorner(size/2, size/2) = MatrixXd::Identity(size/2, size/2);

mat1.bottomRightCorner(size/2, size/2) = MatrixXd::Zero(size/2, size/2);

std::cout << mat1 << std::endl << std::endl;

MatrixXd mat2(size, size);

mat2.topLeftCorner(size/2, size/2).setZero();

mat2.topRightCorner(size/2, size/2).setIdentity();

mat2.bottomLeftCorner(size/2, size/2).setIdentity();

mat2.bottomRightCorner(size/2, size/2).setZero();

std::cout << mat2 << std::endl << std::endl;

MatrixXd mat3(size, size);

mat3 << MatrixXd::Zero(size/2, size/2), MatrixXd::Identity(size/2, size/2),

    MatrixXd::Identity\(size/2, size/2\), MatrixXd::Zero\(size/2, size/2\);

std::cout << mat3 << std::endl;