C++, Sort One Vector Based On Another One

The best example I've got is that I want to sort Names based on their Score.

vector <string> Names {"Karl", "Martin", "Paul", "Jennie"};
vector <int> Score{45, 5, 14, 24};

So if I sort the score to {5, 14, 24, 45}, the names should also be sorted based on their score.

An alternative to consolidating the names and scores into a single structure is to create an index list and sort that:

 std::vector<int> indices(Names.size());
 std::iota(indices.begin(), indices.end(), 0);
 std::sort(indices.begin(), indices.end(),
           [&](int A, int B) -> bool {
                return Score[A] < Score[B];
            });

Now indices can be used to index Names and Scores in the desired sorted order.

As already suggested in other answers: Combining the name and the score of each individual is likely the simplest solution.

Generically, this can be achieved with what is sometimes referred to as a "zip" operation: Combining two vectors into a vector of pairs - along with a corresponding "unzip".

Implemented generically, this may look as follows:

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

// Fill the zipped vector with pairs consisting of the
// corresponding elements of a and b. (This assumes 
// that the vectors have equal length)
template <typename A, typename B>
void zip(
    const std::vector<A> &a, 
    const std::vector<B> &b, 
    std::vector<std::pair<A,B>> &zipped)
{
    for(size_t i=0; i<a.size(); ++i)
    {
        zipped.push_back(std::make_pair(a[i], b[i]));
    }
}

// Write the first and second element of the pairs in 
// the given zipped vector into a and b. (This assumes 
// that the vectors have equal length)
template <typename A, typename B>
void unzip(
    const std::vector<std::pair<A, B>> &zipped, 
    std::vector<A> &a, 
    std::vector<B> &b)
{
    for(size_t i=0; i<a.size(); i++)
    {
        a[i] = zipped[i].first;
        b[i] = zipped[i].second;
    }
}


int main(int argc, char* argv[])
{
    std::vector<std::string> names {"Karl", "Martin", "Paul", "Jennie"};
    std::vector<int> score {45, 5, 14, 24};

    // Zip the vectors together
    std::vector<std::pair<std::string,int>> zipped;
    zip(names, score, zipped);

    // Sort the vector of pairs
    std::sort(std::begin(zipped), std::end(zipped), 
        [&](const auto& a, const auto& b)
        {
            return a.second > b.second;
        });

    // Write the sorted pairs back to the original vectors
    unzip(zipped, names, score);

    for(size_t i=0; i<names.size(); i++)
    {
        std::cout << names[i] << " : " << score[i] << std::endl;
    }
    return 0;
}

Best way to do this would be to have a struct which combines the names with their scores and have one vector.

struct Person
{
    std::string Name;
    int Score;
};

Then you can declare your vector:

std::vector<Person> people{ { "Karl", 45 }, { "Martin", 5 }, { "Paul", 14 } };

And sorting it is easy with std::sort from <algorithm>:

std::sort(people.begin(), people.end(), 
               [](const auto& i, const auto& j) { return i.Score < j.Score; } );

Or you can change the lambda if you want to sort in descending order:

std::sort(people.begin(), people.end(), 
               [](const auto& i, const auto& j) { return i.Score > j.Score; } );

If you cannot merge the data into a vector of pairs or struct with both, you could create a vector of iterators, or the indexes from 0 to size-1. Then sort this using a custom comparator. Finally, create a new vector, populating it using the iterators or indexes.

template<class T1, class A1, class T2, class A2>
std::vector<T1, A1> sort_by(
  std::vector<T1,A1> const& vin, std::vector<T2,A2> const& keys
){
  std::vector<std::size_t> is;
  is.reserve(vin.size());
  for (auto&& unused:keys)
    is.push_back(is.size());
  std::sort(begin(is),end(is),[&](std::size_t l, std::size_t r){
    return keys[l]<keys[r];
  });
  std::vector<T1, A1> r;
  r.reserve(vin.size());
  for(std::size_t i:is)
    r.push_back(vin[i]);
  return r;
}