Lesson 12 Using block operations

The most general block operation in Eigen is called .block() . There are two versions, whose syntax is as follows:

Block operation	Version constructing a  dynamic-size block expression	Version constructing a  fixed-size block expression
Block of size (p,q), starting at (i,j)	matrix.block(i,j,p,q);	matrix.block<p,q>(i,j);

As always in Eigen, indices start at 0.

Both versions can be used on fixed-size and dynamic-size matrices and arrays. These two expressions are semantically equivalent. The only difference is that the fixed-size version will typically give you faster code if the block size is small, but requires this size to be known at compile time.

The following program uses the dynamic-size and fixed-size versions to print the values of several blocks inside a matrix.

Example:	Output:
#include <Eigen/Dense> #include <iostream> using namespace std; int main() { Eigen::MatrixXf m(4,4); m << 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12, 13,14,15,16; cout << "Block in the middle" << endl; cout << m.block<2,2>(1,1) << endl << endl; for ( int i = 1; i <= 3; ++i) { cout << "Block of size " << i << "x" << i << endl; cout << m.block(0,0,i,i) << endl << endl; } }	Block in the middle 6 7 10 11 Block of size 1x1 1 Block of size 2x2 1 2 5 6 Block of size 3x3 1 2 3 5 6 7 9 10 11