Eigen
3.3.4

This page explains how to solve linear systems, compute various decompositions such as LU, QR, SVD, eigendecompositions... After reading this page, don't miss our catalogue of dense matrix decompositions.
The problem: You have a system of equations, that you have written as a single matrix equation
Where A and b are matrices (b could be a vector, as a special case). You want to find a solution x.
The solution: You can choose between various decompositions, depending on what your matrix A looks like, and depending on whether you favor speed or accuracy. However, let's start with an example that works in all cases, and is a good compromise:
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix3f A;
Vector3f b;
A << 1,2,3, 4,5,6, 7,8,10;
b << 3, 3, 4;
cout << "Here is the matrix A:\n" << A << endl;
cout << "Here is the vector b:\n" << b << endl;
Vector3f x = A.colPivHouseholderQr().solve(b);
cout << "The solution is:\n" << x << endl;
}
 Here is the matrix A: 1 2 3 4 5 6 7 8 10 Here is the vector b: 3 3 4 The solution is: 2 1 1 
In this example, the colPivHouseholderQr() method returns an object of class ColPivHouseholderQR. Since here the matrix is of type Matrix3f, this line could have been replaced by:
Here, ColPivHouseholderQR is a QR decomposition with column pivoting. It's a good compromise for this tutorial, as it works for all matrices while being quite fast. Here is a table of some other decompositions that you can choose from, depending on your matrix and the tradeoff you want to make:
Decomposition  Method  Requirements on the matrix  Speed (smalltomedium)  Speed (large)  Accuracy 

PartialPivLU  partialPivLu()  Invertible  ++  ++  + 
FullPivLU  fullPivLu()  None       +++ 
HouseholderQR  householderQr()  None  ++  ++  + 
ColPivHouseholderQR  colPivHouseholderQr()  None  ++    +++ 
FullPivHouseholderQR  fullPivHouseholderQr()  None       +++ 
LLT  llt()  Positive definite  +++  +++  + 
LDLT  ldlt()  Positive or negative semidefinite  +++  +  ++ 
JacobiSVD  jacobiSvd()  None         +++ 
All of these decompositions offer a solve() method that works as in the above example.
For example, if your matrix is positive definite, the above table says that a very good choice is then the LLT or LDLT decomposition. Here's an example, also demonstrating that using a general matrix (not a vector) as right hand side is possible.
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix2f A, b;
A << 2, 1, 1, 3;
b << 1, 2, 3, 1;
cout << "Here is the matrix A:\n" << A << endl;
cout << "Here is the right hand side b:\n" << b << endl;
cout << "The solution is:\n" << x << endl;
}
 Here is the matrix A: 2 1 1 3 Here is the right hand side b: 1 2 3 1 The solution is: 1.2 1.4 1.4 0.8 
For a much more complete table comparing all decompositions supported by Eigen (notice that Eigen supports many other decompositions), see our special page on this topic.
Only you know what error margin you want to allow for a solution to be considered valid. So Eigen lets you do this computation for yourself, if you want to, as in this example:
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
MatrixXd A = MatrixXd::Random(100,100);
MatrixXd b = MatrixXd::Random(100,50);
cout << "The relative error is:\n" << relative_error << endl;
}
 The relative error is: 2.31495e14 
You need an eigendecomposition here, see available such decompositions on this page. Make sure to check if your matrix is selfadjoint, as is often the case in these problems. Here's an example using SelfAdjointEigenSolver, it could easily be adapted to general matrices using EigenSolver or ComplexEigenSolver.
The computation of eigenvalues and eigenvectors does not necessarily converge, but such failure to converge is very rare. The call to info() is to check for this possibility.
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix2f A;
A << 1, 2, 2, 3;
cout << "Here is the matrix A:\n" << A << endl;
SelfAdjointEigenSolver<Matrix2f> eigensolver(A);
if (eigensolver.info() != Success) abort();
cout << "The eigenvalues of A are:\n" << eigensolver.eigenvalues() << endl;
cout << "Here's a matrix whose columns are eigenvectors of A \n"
<< "corresponding to these eigenvalues:\n"
<< eigensolver.eigenvectors() << endl;
}
 Here is the matrix A: 1 2 2 3 The eigenvalues of A are: 0.236 4.24 Here's a matrix whose columns are eigenvectors of A corresponding to these eigenvalues: 0.851 0.526 0.526 0.851 
First of all, make sure that you really want this. While inverse and determinant are fundamental mathematical concepts, in numerical linear algebra they are not as popular as in pure mathematics. Inverse computations are often advantageously replaced by solve() operations, and the determinant is often not a good way of checking if a matrix is invertible.
However, for very small matrices, the above is not true, and inverse and determinant can be very useful.
While certain decompositions, such as PartialPivLU and FullPivLU, offer inverse() and determinant() methods, you can also call inverse() and determinant() directly on a matrix. If your matrix is of a very small fixed size (at most 4x4) this allows Eigen to avoid performing a LU decomposition, and instead use formulas that are more efficient on such small matrices.
Here is an example:
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix3f A;
A << 1, 2, 1,
2, 1, 0,
1, 1, 2;
cout << "Here is the matrix A:\n" << A << endl;
cout << "The determinant of A is " << A.determinant() << endl;
cout << "The inverse of A is:\n" << A.inverse() << endl;
}
 Here is the matrix A: 1 2 1 2 1 0 1 1 2 The determinant of A is 3 The inverse of A is: 0.667 1 0.333 1.33 1 0.667 1 1 1 
The most accurate method to do least squares solving is with a SVD decomposition. Eigen provides one as the JacobiSVD class, and its solve() is doing leastsquares solving.
Here is an example:
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
MatrixXf A = MatrixXf::Random(3, 2);
cout << "Here is the matrix A:\n" << A << endl;
VectorXf b = VectorXf::Random(3);
cout << "Here is the right hand side b:\n" << b << endl;
cout << "The leastsquares solution is:\n"
}
 Here is the matrix A: 0.68 0.597 0.211 0.823 0.566 0.605 Here is the right hand side b: 0.33 0.536 0.444 The leastsquares solution is: 0.67 0.314 
Another methods, potentially faster but less reliable, are to use a Cholesky decomposition of the normal matrix or a QR decomposition. Our page on least squares solving has more details.
In the above examples, the decomposition was computed at the same time that the decomposition object was constructed. There are however situations where you might want to separate these two things, for example if you don't know, at the time of the construction, the matrix that you will want to decompose; or if you want to reuse an existing decomposition object.
What makes this possible is that:
For example:
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix2f A, b;
LLT<Matrix2f> llt;
A << 2, 1, 1, 3;
b << 1, 2, 3, 1;
cout << "Here is the matrix A:\n" << A << endl;
cout << "Here is the right hand side b:\n" << b << endl;
cout << "Computing LLT decomposition..." << endl;
llt.compute(A);
cout << "The solution is:\n" << llt.solve(b) << endl;
A(1,1)++;
cout << "The matrix A is now:\n" << A << endl;
cout << "Computing LLT decomposition..." << endl;
llt.compute(A);
cout << "The solution is now:\n" << llt.solve(b) << endl;
}
 Here is the matrix A: 2 1 1 3 Here is the right hand side b: 1 2 3 1 Computing LLT decomposition... The solution is: 1.2 1.4 1.4 0.8 The matrix A is now: 2 1 1 4 Computing LLT decomposition... The solution is now: 1 1.29 1 0.571 
Finally, you can tell the decomposition constructor to preallocate storage for decomposing matrices of a given size, so that when you subsequently decompose such matrices, no dynamic memory allocation is performed (of course, if you are using fixedsize matrices, no dynamic memory allocation happens at all). This is done by just passing the size to the decomposition constructor, as in this example:
Certain decompositions are rankrevealing, i.e. are able to compute the rank of a matrix. These are typically also the decompositions that behave best in the face of a nonfullrank matrix (which in the square case means a singular matrix). On this table you can see for all our decompositions whether they are rankrevealing or not.
Rankrevealing decompositions offer at least a rank() method. They can also offer convenience methods such as isInvertible(), and some are also providing methods to compute the kernel (nullspace) and image (columnspace) of the matrix, as is the case with FullPivLU:
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix3f A;
A << 1, 2, 5,
2, 1, 4,
3, 0, 3;
cout << "Here is the matrix A:\n" << A << endl;
FullPivLU<Matrix3f> lu_decomp(A);
cout << "The rank of A is " << lu_decomp.rank() << endl;
cout << "Here is a matrix whose columns form a basis of the nullspace of A:\n"
<< lu_decomp.kernel() << endl;
cout << "Here is a matrix whose columns form a basis of the columnspace of A:\n"
<< lu_decomp.image(A) << endl; // yes, have to pass the original A
}
 Here is the matrix A: 1 2 5 2 1 4 3 0 3 The rank of A is 2 Here is a matrix whose columns form a basis of the nullspace of A: 0.5 1 0.5 Here is a matrix whose columns form a basis of the columnspace of A: 5 1 4 2 3 3 
Of course, any rank computation depends on the choice of an arbitrary threshold, since practically no floatingpoint matrix is exactly rankdeficient. Eigen picks a sensible default threshold, which depends on the decomposition but is typically the diagonal size times machine epsilon. While this is the best default we could pick, only you know what is the right threshold for your application. You can set this by calling setThreshold() on your decomposition object before calling rank() or any other method that needs to use such a threshold. The decomposition itself, i.e. the compute() method, is independent of the threshold. You don't need to recompute the decomposition after you've changed the threshold.
Example:  Output: 

#include <iostream>
#include <Eigen/Dense>
using namespace std;
using namespace Eigen;
int main()
{
Matrix2d A;
A << 2, 1,
2, 0.9999999999;
FullPivLU<Matrix2d> lu(A);
cout << "By default, the rank of A is found to be " << lu.rank() << endl;
lu.setThreshold(1e5);
cout << "With threshold 1e5, the rank of A is found to be " << lu.rank() << endl;
}
 By default, the rank of A is found to be 2 With threshold 1e5, the rank of A is found to be 1 