# Eigen Linear Algebra in C++

To install download the .zip file from the website. Extract to a folder (C:/Eigen).

To include Eigen in a Cmake project add:

find_package (Eigen3 3.3 NO_MODULE)

if (TARGET Eigen3::Eigen)
  # Use the imported target
endif (TARGET Eigen3::Eigen)

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If don't work just include

#include "C:/eigen/Eigen/Dense"
#include "C:/eigen/Eigen/Sparse"

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First matrix:

int main()
{
    Eigen::MatrixXd m;
}

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Capital X means the size of the matrix in unknown and d is the type, double; or we could have f for float.

This is what we call a dynamic matrix, because the size is unknown, and the size is not set, you can reset during runtime of the program.

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using namespace Eigen;

On the other hand we have the Fixed size matrix

Eigen has a defined 2x2 and 4x4 matrix, so just put a 2 or a 4. After that comes the letter for the type.

Matrix4d

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Dynamic matrices are stored on the heap and fixed size on the stack. To access the stack memory is faster

To find the size of the matrix use f.size for a matrix called f.

Resizing a Matrix:

d.resize(4, 6);

Initializing a matrix

// Initializing the matrix

f << 1, 2, 3,
     4, 5, 6,
     7, 8, 9;

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Initializing a matrix randomly

f = Matrix3d::Random();

To give a clean format to the matrix

    IOFormat CleanFmt(4, 0, ", ", "\n", "[", "]");
    std::cout <<  f.format(CleanFmt) << std::endl;

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