EigenSymmetric

#include <vtkMath.h>

namespace {
/* 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 = static_cast<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;
}

} // namespace

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;
}

cmake_minimum_required(VERSION 3.12 FATAL_ERROR)

project(EigenSymmetric)

find_package(VTK COMPONENTS 
  CommonCore
)

if (NOT VTK_FOUND)
  message(FATAL_ERROR "EigenSymmetric: Unable to find the VTK build folder.")
endif()

# Prevent a "command line is too long" failure in Windows.
set(CMAKE_NINJA_FORCE_RESPONSE_FILE "ON" CACHE BOOL "Force Ninja to use response files.")
add_executable(EigenSymmetric MACOSX_BUNDLE EigenSymmetric.cxx )
  target_link_libraries(EigenSymmetric PRIVATE ${VTK_LIBRARIES}
)
# vtk_module_autoinit is needed
vtk_module_autoinit(
  TARGETS EigenSymmetric
  MODULES ${VTK_LIBRARIES}
)

cd EigenSymmetric/build

cmake ..

cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

make

./EigenSymmetric