将特征向量复制到C数组？

Question

How do I copy an Eigen vector mesh to a C array r ?

double *r;
typedef Eigen::VectorXd RealVector;
Eigen::Map<RealVector>(r, 1, mesh.cols()) = mesh;

gives an assert from Eigen

DenseBase::resize() does not actually allow to resize.

The same message comes from either

Eigen::Map<RealVector>(r, mesh.cols()) = mesh;

or

Eigen::Map<RealVector>(r, mesh.cols(), 1) = mesh;

I need the values to be copied, not just mapped.

Answer 1

Since you did not clarify, I'm speculating three possible errors you could have made:

• Either your mesh is actually a VectorXd , but then it will always have exactly one column, but potentially multiple rows, ie, you need to write:

   Eigen::VectorXd::Map(r, mesh.rows()) = mesh; 

• Or your mesh is a RowVectorXd (ie, having one row and multiple columns). Then you need to write:

   Eigen::RowVectorXd::Map(r, mesh.cols()) = mesh; 

• If mesh actually is a matrix, you need to decide how to map it to linear memory (ie row-major or column-major). This is also possible with Map :

   Eigen::MatrixXd::Map(r, mesh.rows(), mesh.cols()) = mesh; 

Answer 2

You don't have to copy anything actually. You can access the raw data using the .data() member function.

#include <Eigen/Core>

int main()
{
  Eigen::VectorXd mesh = Eigen::VectorXd::Random(10);
  double * r = mesh.data();

  r[5] = 0; // writes to mesh directly
  assert(mesh(5) == 0);
}

If you want to copy the data to the pointer, you have to allocate memory, perform the copy and deallocate after use.

#include <algorithm>
#include <Eigen/Core>

int main()
{
  Eigen::VectorXd mesh = Eigen::VectorXd::Random(10);
  double * r = new double[mesh.size()];

  std::copy(mesh.data(), mesh.data() + mesh.size(), r);
  assert(r[5] == mesh(5));

  delete[] r;
}