make_circulant2.cpp

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#include <Eigen/Core>
#include <iostream>
 
using namespace Eigen;
 
// [circulant_func]
template<class ArgType>
class circulant_functor {
  const ArgType &m_vec;
public:
  circulant_functor(const ArgType& arg) : m_vec(arg) {}
 
  const typename ArgType::Scalar& operator() (Index row, Index col) const {
    Index index = row - col;
    if (index < 0) index += m_vec.size();
    return m_vec(index);
  }
};
// [circulant_func]
 
// [square]
template<class ArgType>
struct circulant_helper {
  typedef Matrix<typename ArgType::Scalar,
                 ArgType::SizeAtCompileTime,
                 ArgType::SizeAtCompileTime,
                 ColMajor,
                 ArgType::MaxSizeAtCompileTime,
                 ArgType::MaxSizeAtCompileTime> MatrixType;
};
// [square]
 
// [makeCirculant]
template <class ArgType>
CwiseNullaryOp<circulant_functor<ArgType>, typename circulant_helper<ArgType>::MatrixType>
makeCirculant(const Eigen::MatrixBase<ArgType>& arg)
{
  typedef typename circulant_helper<ArgType>::MatrixType MatrixType;
  return MatrixType::NullaryExpr(arg.size(), arg.size(), circulant_functor<ArgType>(arg.derived()));
}
// [makeCirculant]
 
// [main]
int main()
{
  Eigen::VectorXd vec(4);
  vec << 1, 2, 4, 8;
  Eigen::MatrixXd mat;
  mat = makeCirculant(vec);
  std::cout << mat << std::endl;
}
// [main]