Matrix and Vector Operators and Functions

Matrix and Vector Operators

The ROOT::Math::SVector and ROOT::Math::SMatrix classes defines the following operators described below. The m1,m2,m3 are vectors or matrices of the same type (and size) and a is a scalar value:

m1 == m2           // returns whether m1 is equal to m2 (element by element comparison)
m1 != m2           // returns whether m1 is NOT equal to m2 (element by element comparison)
m1 < m2            // returns whether m1 is less than m2 (element wise comparison)
m1 > m2            // returns whether m1 is greater than m2 (element wise comparison)
// in the following m1 and m3 can be general and m2 symmetric, but not vice-versa
m1 += m2           // add m2 to m1
m1 -= m2           // subtract m2 to m1
m3 = m1 + m2       // addition
m1 - m2            // subtraction
 
// Multiplication and division via a scalar value a
m3 = a*m1; m3 = m1*a; m3 = m1/a;

Vector-Vector multiplication

The operator * defines an element by element multiplication between vectors. For the standard vector-vector multiplication, \( a = v^T v \), (dot product) one must use the ROOT::Math::Dot function. In addition, the Cross (only for vector sizes of 3), ROOT::Math::Cross, and the Tensor product, ROOT::Math::TensorProd, are defined.

Matrix - Vector multiplication

The operator * defines the matrix-vector multiplication, \( y_i = \sum_{j} M_{ij} x_j\):

// M is a  N1xN2 matrix, x is a N2 size vector, y is a N1 size vector
y = M * x

It compiles only if the matrix and the vectors have the right sizes.

Matrix - Matrix multiplication

The operator * defines the matrix-matrix multiplication, \( C_{ij} = \sum_{k} A_{ik} B_{kj}\):

// A is a N1xN2 matrix, B is a N2xN3 matrix and C is a N1xN3 matrix
C = A * B

The operation compiles only if the matrices have the right size. In the case that A and B are symmetric matrices, C is a general one, since their product is not guaranteed to be symmetric.

Special note on using the C++ auto keyword

Special care must be taken when using the C++ auto keyword with expression templates. Some expression can lead to temporary objects that the compiler might remove. One example is when dealing with an expression like:

auto D = (A * B) * C; 

while instead declaring directly the matrix as

SMatrix<double, N, N> D = (A * B) * C; 

will be fine, because it will force the evaluation of the expression template when constructing the matrix D. This is a limitation of the package, see ROOT-6731 and present in other similar libraries such as Eigen.

Matrix and Vector Functions

The most used matrix functions are:

• ROOT::Math::Transpose(M) : return the transpose matrix, \( M^T \)
• ROOT::Math::Similarity( v, M) : returns the scalar value resulting from the matrix- vector product \( v^T M v \)
• ROOT::Math::Similarity( U, M) : returns the matrix resulting from the product \( U M U^T \). If M is symmetric, the returned resulting matrix is also symmetric
• ROOT::Math::SimilarityT( U, M) : returns the matrix resulting from the product \( U^T M U \). If M is symmetric, the returned resulting matrix is also symmetric

See Matrix Template Functions for the documentation of all existing matrix functions in the package. The major Vector functions are:

• ROOT::Math::Dot( v1, v2) : returns the scalar value resulting from the vector dot product
• ROOT::Math::Cross( v1, v2) : returns the vector cross product for two vectors of size 3. Note that the Cross product is not defined for other vector sizes
• ROOT::Math::Unit( v) : returns unit vector. One can use also the v.Unit() method.
• ROOT::Math::TensorProd(v1,v2) : returns a general matrix M of size N1xN2 resulting from the Tensor Product between the vector v1 of size N1) and v2 of size N2

See Vector Template Functions for the list and documentation of all of them.

Matrix and Vector I/O

One can print (or write in an output stream) Vectors (and also Matrices) using the Print method or the << operator, like:

v.Print(std::cout);
std::cout << v << std::endl;

In the ROOT distribution, the CINT dictionary is generated for SMatrix and SVector for double types and sizes up to 5. This allows the storage of them in a ROOT file.