# EigenSymmetric

VTKExamples/Cxx/Math/EigenSymmetric

### Description¶

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.

Question

### Code¶

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[0] = 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[0] );
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[0][0] = 1; a[0][1] = 2;  a[0][2] = 3;
a[1][0] = 2; a[1][1] = 5;  a[1][2] = 9;
a[2][0] = 3; a[2][1] = 9;  a[2][2] = 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[3];

//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[0][i] << " " << w[1][i] << " " << w[2][i] << std::endl;
}

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

return EXIT_SUCCESS;
}


### CMakeLists.txt¶

cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

project(EigenSymmetric)

find_package(VTK COMPONENTS
vtkCommonCore QUIET)
if (NOT VTK_FOUND)
message("Skipping EigenSymmetric: ${VTK_NOT_FOUND_MESSAGE}") return () endif() message (STATUS "VTK_VERSION:${VTK_VERSION}")
if (VTK_VERSION VERSION_LESS "8.90.0")
# old system
include(${VTK_USE_FILE}) add_executable(EigenSymmetric MACOSX_BUNDLE EigenSymmetric.cxx ) target_link_libraries(EigenSymmetric PRIVATE${VTK_LIBRARIES})
else ()
# include all components
target_link_libraries(EigenSymmetric PRIVATE ${VTK_LIBRARIES}) # vtk_module_autoinit is needed vtk_module_autoinit( TARGETS EigenSymmetric MODULES${VTK_LIBRARIES}
)
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

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