# VTK/Examples/Cxx/Math/EigenSymmetric

< VTK‎ | Examples‎ | Cxx

At the moment, this is a pretty messy process, so here are some helper functions that will hopefully eventually be wrapped into a class/interface.

## EigenSymmetric.cxx

```#include <vtkMath.h>

/* allocate memory for an nrow x ncol matrix */
template<class TReal>
TReal **create_matrix ( long nrow, long ncol )
{
typedef TReal* TRealPointer;
TReal **m = new TRealPointer[nrow];

TReal* block = ( TReal* ) calloc ( nrow*ncol, sizeof ( TReal ) );
m = block;
for ( int row = 1; row < nrow; ++row )
{
m[ row ] = &block[ row * ncol ];
}
return m;
}

/* free a TReal matrix allocated with matrix() */
template<class TReal>
void free_matrix ( TReal **m )
{
free ( m );
delete[] m;
}

int main(int, char *[])
{
// Create this matrix
// 1 2 3
// 2 5 9
// 3 9 8

double **a = create_matrix<double> ( 3,3 );
a = 1; a = 2;  a = 3;
a = 2; a = 5;  a = 9;
a = 3; a = 9;  a = 8;

//create a matrix for the eigenvectors to be stored in
double **w = create_matrix<double> ( 3,3 );

//create a vector for the eigenvalues to be stored in
double v;

//compute the eigenvalues/vectors
vtkMath::Jacobi ( a,v,w );

//output eigen values
std::cout << "Eigen values (decreasing order): " << std::endl;
for ( int i=0; i<3; i++ )
{
std::cout << v[i] << " ";
}

//output eigenvectors
std::cout << "Eigen vectors (same order as eigen values): " << std::endl;
for ( int i=0; i<3; i++ )
{
//the columns of w are the eigenvectors
std::cout << "Eigen vector " << i << " : " << w[i] << " " << w[i] << " " << w[i] << std::endl;
}

//clean up
free_matrix ( w );
free_matrix ( a );

return EXIT_SUCCESS;
}
```

# Please try the new VTKExamples website.

## CMakeLists.txt

```cmake_minimum_required(VERSION 2.8)

PROJECT(EigenSymmetric)

find_package(VTK REQUIRED)
include(\${VTK_USE_FILE})

if(VTK_LIBRARIES)
else()
endif()
```

```cd EigenSymmetric/build
```
• If VTK is installed:
```cmake ..
```
• If VTK is not installed but compiled on your system, you will need to specify the path to your VTK build:
```cmake -DVTK_DIR:PATH=/home/me/vtk_build ..
```

Build the project:

```make
```

and run it:

```./EigenSymmetric
```

WINDOWS USERS PLEASE NOTE: Be sure to add the VTK bin directory to your path. This will resolve the VTK dll's at run time.