Eigen

Install

• Releases

wget https://gitlab.com/libeigen/eigen/-/archive/3.4.0/eigen-3.4.0.tar.bz2

Build

find_package (Eigen3 3.3 REQUIRED NO_MODULE)
target_link_libraries (example Eigen3::Eigen)

Use

Matrix storage order is column-major.

• Import

#include <Eigen/Dense>

• Static size matrix

Matrix3f matrix;
// equivalent to:
Matrix <float 3, 3> matrix;

Vector3f vector;
// equivalent to:
Matrix <float 3, 1> vector;

RowVector2i row_vector;
Matrix <int 1, 2> vector;

• Dynamic size matrix

Matrix <float, Dynamic, Dynamic> matrix;
// or short:
MatrixXf matrix;

// <rows, cols>
MatrixXd matrix(10, 10)

• Initialize matrix

MatrixXd matrix;

matrix = MatrixXf::Zero(2, 2);
matrix = MatrixXf::Ones(2, 2);
matrix = MatrixXf::Constant(2, 2, 1.343); // <row, col, value>
matrix = MatrixXf::Identity(2, 2);
matrix = MatrixXf::Random(2, 2);


matrix.setZero(2, 2);
matrix.setOnes(2, 2);
matrix.setConstant(2, 2, 1,343);
matrix.setIdentity(2, 2);

Vector3f vector {{1, 2, 3}};
MatrixXd diagonalMatrix;
diagonalMatrix = vector.asDiagonal();

• Value Assignment

// <row, col>
matrix(0, 0) = 1;
matrix(0, 1) = 2;
matrix(1, 0) = 3;
matrix(1, 1) = 4;
// or
matrix << 1, 2,
          3, 4;
// or
MatrixXi matrix {      // construct a 2x2 matrix
      {1, 2},          // first row
      {3, 4}           // second row
};

• Value Access

// <start row index, start col index, height, width
matrix.block(1, 1, 2, 2);

matrix.row(0);

matrix.col(0);

matrix.topRows(1);
matrix.bottomRows(1);
matrix.leftCols(1);
matrix.rightCols(1);

• Basic Operations

Matrix3d matrix, matrixA, matrixB;

// Elementwise add
matrixA + matrixB;

// Matrix multiplication
matrixA * matrixB;

// Coefficient wise multiplication
matrixA.cwiseProduct(matrixB); 

matrix.transpose();
matrix.transposeInPlace();

matrix.inverse();

Eigen::Matrix2d mat;
mat << 1, 2,
       3, 4;
mat.sum();
mat.prod();
mat.mean();
mat.minCoeff();
mat.maxCoeff();
mat.trace();

• Sizing

MatrixXf matrix(10, 10);

matrix.rows();
matrix.cols();
matrix.size();

matrix.resize(20, 20);