Question

Suppose a vector with values [0,1,2,3,4,5,6,7,8,9]. How can I create a vector that refers to not necessarily contiguous values, e.g. [3,4,7,9], i.e. given by some index, by using STL.

Answer 1

You can express this as a transformation, e.g.:

#include <valarray>
#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>

template <typename T>
void pick(std::vector<T>& result, const std::vector<T>& in, const std::vector<typename std::vector<T>::size_type>& s) {
  result.reserve(s.size());
  std::transform(s.begin(), s.end(), std::back_inserter(result),
                 [&in](typename std::vector<T>::size_type idx) {
                   return in.at(idx);
                 });
}

int main() {
  const std::vector<int> arr={0,1,2,3,4,5,6,7,8,9,10};
  std::vector<int> result;
  pick(result, arr, {3,4,7,9});
}

I used a lambda, but you can also use std::bind or the (now deprecated) std::bind2nd for this.

The example with C++11’s std::bind makes pick:

template <typename T>
void pick(std::vector<T>& result, const std::vector<T>& in, const std::vector<typename std::vector<T>::size_type>& s) {
  result.reserve(s.size());
  std::transform(s.begin(), s.end(), std::back_inserter(result),
                 std::bind(static_cast<const T& (std::vector<T>::*)(typename std::vector<T>::size_type) const>(&std::vector<T>::at),  in, std::placeholders::_1));
}

It’s seriously ugly though because of the need to cast the member function pointer to resolve the overload of at (const vs non-const versions).