Find index of the first element larger or equal to a given value in C++

find_index.cc

#include <iterator>
#include <algorithm>

template <typename Iter, typename T>
size_t find_index(Iter begin, Iter  end, T x0)
{
  return std::distance(begin, std::find_if(begin, end, [x0](double x){return x >= x0;}));
}