Eigen: replicate items along one dimension without useless allocations

Question

I have some vector vec and i want to obtain a new \"expression\" vec2 by copying values along dimension of vector

Eigen::VectorXf vec(5);
vec << 1, 2, 3, 4         


        

Answer 1

Using the devel branch you can use LinSpaced to generate the sequence of indices and then index the input vector:

#include <iostream>
#include <Eigen/Dense>
using namespace Eigen;
using namespace std;

int main()
{
  VectorXf vec(5);
  vec << 1, 2, 3, 4, 5;
  auto vecrep = vec(ArrayXi::LinSpaced(5*3,0,4));
  cout << vecrep.transpose() << endl;
}

you can then wrap the key line within a free function returning auto, in c++14:

template<typename XprType>
auto magic_rep(const XprType &xpr, Index K) {
  return xpr(Eigen::ArrayXi::LinSpaced(xpr.size()*K,0,xpr.size()-1));
}

and in main:

cout << magic_rep(vec,3).transpose() << endl;