Please, help us to better know about our user community by answering the following short survey: https://forms.gle/wpyrxWi18ox9Z5ae9 Eigen  3.3.9 Eigen::VectorBlock< VectorType, Size > Class Template Reference

## Detailed Description

### template<typename VectorType, int Size> class Eigen::VectorBlock< VectorType, Size >

Expression of a fixed-size or dynamic-size sub-vector.

Template Parameters
 VectorType the type of the object in which we are taking a sub-vector Size size of the sub-vector we are taking at compile time (optional)

This class represents an expression of either a fixed-size or dynamic-size sub-vector. It is the return type of DenseBase::segment(Index,Index) and DenseBase::segment<int>(Index) and most of the time this is the only way it is used.

However, if you want to directly maniputate sub-vector expressions, for instance if you want to write a function returning such an expression, you will need to use this class.

Here is an example illustrating the dynamic case:

#include <Eigen/Core>
#include <iostream>
using namespace Eigen;
using namespace std;
template<typename Derived>
segmentFromRange(MatrixBase<Derived>& v, int start, int end)
{
return Eigen::VectorBlock<Derived>(v.derived(), start, end-start);
}
template<typename Derived>
segmentFromRange(const MatrixBase<Derived>& v, int start, int end)
{
return Eigen::VectorBlock<const Derived>(v.derived(), start, end-start);
}
int main(int, char**)
{
Matrix<int,1,6> v; v << 1,2,3,4,5,6;
cout << segmentFromRange(2*v, 2, 4) << endl; // calls the const version
segmentFromRange(v, 1, 3) *= 5; // calls the non-const version
cout << "Now the vector v is:" << endl << v << endl;
return 0;
}

Output:

6 8
Now the vector v is:
1 10 15  4  5  6

Note
Even though this expression has dynamic size, in the case where VectorType has fixed size, this expression inherits a fixed maximal size which means that evaluating it does not cause a dynamic memory allocation.

Here is an example illustrating the fixed-size case:

#include <Eigen/Core>
#include <iostream>
using namespace Eigen;
using namespace std;
template<typename Derived>
firstTwo(MatrixBase<Derived>& v)
{
return Eigen::VectorBlock<Derived, 2>(v.derived(), 0);
}
template<typename Derived>
firstTwo(const MatrixBase<Derived>& v)
{
return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0);
}
int main(int, char**)
{
Matrix<int,1,6> v; v << 1,2,3,4,5,6;
cout << firstTwo(4*v) << endl; // calls the const version
firstTwo(v) *= 2; // calls the non-const version
cout << "Now the vector v is:" << endl << v << endl;
return 0;
}

Output:

4 8
Now the vector v is:
2 4 3 4 5 6

See also
class Block, DenseBase::segment(Index,Index,Index,Index), DenseBase::segment(Index,Index) Inheritance diagram for Eigen::VectorBlock< VectorType, Size >:

## Public Member Functions

VectorBlock (VectorType &vector, Index start)

VectorBlock (VectorType &vector, Index start, Index size)

## ◆ VectorBlock() [1/2]

template<typename VectorType , int Size>
 Eigen::VectorBlock< VectorType, Size >::VectorBlock ( VectorType & vector, Index start, Index size )
inline

Dynamic-size constructor

## ◆ VectorBlock() [2/2]

template<typename VectorType , int Size>
 Eigen::VectorBlock< VectorType, Size >::VectorBlock ( VectorType & vector, Index start )
inline

Fixed-size constructor

The documentation for this class was generated from the following file:
Eigen
Namespace containing all symbols from the Eigen library.
Definition: Core:309
Eigen::VectorBlock
Expression of a fixed-size or dynamic-size sub-vector.
Definition: VectorBlock.h:60
Eigen::Matrix
The matrix class, also used for vectors and row-vectors.
Definition: Matrix.h:180
Eigen::MatrixBase
Base class for all dense matrices, vectors, and expressions.
Definition: MatrixBase.h:50