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eigen.md

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Eigen

Compiling and running your codes in c++ eigen*

BASIC USAGE : For using eigen library in your code.

include <eigen3/Eigen/Dense>

using namespace Eigen;

Compile the code using g++ code.cpp

The matrix class

  • All matrices and vectors are objects of the Matrix template class.

  • The three mandatory template parameters of Matrix are: Matrix<typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime>

eg - typedef Matrix<float, 4, 4> Matrix4f;

  • typedef Matrix<int, Dynamic, 1> VectorXi; // Use dynamic if the number of rows and/or columns is not known at compile time.

  • Constructors - A default constructor is always available, never performs any dynamic memory allocation, and never initializes the matrix coefficients.

eg - Matrix3f a; MatrixXf a(10,15); VectorXf b(30);

  • Coefficient accessors - a(3,3)

  • Comma - initialization:

eg - Matrix3f m; m << 1, 2, 3, 4, 5, 6, 7, 8, 9; std::cout << m;

  • Resizing - The current size of a matrix can be retrieved by rows(), cols() and size(). These methods return the number of rows, the number of columns and the number of coefficients, respectively. Resizing a dynamic-size matrix is done by the resize() method.

The resize() method is a no-operation if the actual matrix size doesn't change; otherwise it is destructive: the values of the coefficients may change. If you want a conservative variant of resize() which does not change the coefficients, use conservativeResize()

  • Assignment and resizing - Assignment is the action of copying a matrix into another, using operator=. Eigen resizes the matrix on the left-hand side automatically so that it matches the size of the matrix on the right-hand size.