see https://gist.github.com/mtao/172c73a4a7abdf7a5724b71523a5fbe8, but for matlab; mtao imports are findable here: https://github.com/mtao/core/blob/master/include/mtao/eigen/stack.h

enumerate_in_eigen.cpp

#include <Eigen/Dense>
#include <mtao/types.h>
#include <mtao/eigen/stack.h>
#include <iostream>
#include <numeric>

mtao::RowVectorX<int> range(int n) {
    mtao::RowVectorX<int> ret(n);
    std::iota(ret.data(),ret.data()+n,0);
    return ret;
}
template <typename D>
mtao::RowVectorX<int> range(const Eigen::MatrixBase<D>& n) {
    return range(n.size());
}
int main() {
    mtao::RowVector<int,5> list; list << 1,5,4,3,2;
    auto pairs = mtao::eigen::vstack(range(list),list);
    std::cout << pairs << std::endl;
    return 0;
}