Store pointer to Eigen Vector 'segment' without copy? - c++

Question

I have an Eigen Vector that I would like to refer to a segment at a later time (e.g. pass between functions) instead of modifying immediately.
Eigen::Matrix<float, Eigen::Dynamic, 1> vec(10);
// initialize
vec << 1, 2, 3, 4, 5, 6, 7, 8, 9, 10;
I would like to create a pointer to a segment that I can refer to later. The following works but it creates a copy so any changes made to the segment are not reflected in the original vector.
const int start = 2;
const int end = 8
Eigen::Matrix<float, Eigen::Dynamic, 1> *block = new Eigen::Matrix<float, Eigen::Dynamic, 1>(end - start + 1, 1);
*block = vec.segment(start-1,end-1);
How can I keep a reference to the segment without copying?

Answer 1

You can use an Eigen::Map to wrap an existing segment of memory without copying. I'm not sure why you're allocating the *block object and not just using block. Using a Map it would look like
Eigen::Map<Eigen::VectorXf> block(&vec(start - 1), end - start + 1);
You then use the Map as you would a normal VectorXd, sans resizing and stuff. Simpler yet (at least according to #ggael), you can use an Eigen:Ref to refer to part of an Eigen object without inducing a copy. For example:
void times2(Eigen::Ref< Eigen::VectorXf> rf)
{
rf *= 2.f;
}
int main()
{
Eigen::Matrix<float, Eigen::Dynamic, 1> vec(10);
// initialize
vec << 1, 2, 3, 4, 5, 6, 7, 8, 9, 10;
const int start = 2;
const int end = 8;
// This would work as well
//Eigen::Map<Eigen::VectorXf> block(&vec(start - 1), end - start + 1);
Eigen::Ref<Eigen::VectorXf> block = vec.segment(start, end - start + 1);
std::cout << block << "\n\n";
times2(block);
std::cout << vec << "\n";
return 0;
}
P.S. I think you're misusing the segment function. It takes a beginning position an the number of elements, i.e. (start, end-start+1).