Lesson 14 Usage as temporary objects

As shown above, static methods as Zero() and Constant() can be used to initialize variables at the time of declaration or at the right-hand side of an assignment operator. You can think of these methods as returning a matrix or array; in fact, they return so-called expression objects which evaluate to a matrix or array when needed, so that this syntax does not incur any overhead.

These expressions can also be used as a temporary object. The second example in the Getting started guide, which we reproduce here, already illustrates this.

Example:	Output:
#include <iostream> #include <Eigen/Dense> using namespace Eigen; using namespace std; int main() { MatrixXd m = MatrixXd::Random(3,3); m = (m + MatrixXd::Constant(3,3,1.2)) * 50; cout << "m =" << endl << m << endl; VectorXd v(3); v << 1, 2, 3; cout << "m * v =" << endl << m * v << endl; }	m = 94 89.8 43.5 49.4 101 86.8 88.3 29.8 37.8 m * v = 404 512 261

The expression m + MatrixXf::Constant(3,3,1.2) constructs the 3-by-3 matrix expression with all its coefficients equal to 1.2 plus the corresponding coefficient of m.

The comma-initializer, too, can also be used to construct temporary objects. The following example constructs a random matrix of size 2-by-3, and then multiplies this matrix on the left with .

Example:	Output:
MatrixXf mat = MatrixXf::Random(2, 3); std::cout << mat << std::endl << std::endl; mat = ( MatrixXf(2,2) << 0, 1, 1, 0).finished() * mat; std::cout << mat << std::endl;	0.68 0.566 0.823 -0.211 0.597 -0.605 -0.211 0.597 -0.605 0.68 0.566 0.823

The finished() method is necessary here to get the actual matrix object once the comma initialization of our temporary submatrix is done.