 Eigen  3.3.7 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, Index size)

VectorBlock (VectorType &vector, Index start)

## ◆ 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 files: