sort index by array

sort_indexes.cpp

#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>

using namespace std;

template <typename T>
std::vector<size_t> sort_indexes(const vector<T> & v)
{
    // initialize original index locations
    vector<size_t> idx(v.size());
    iota(idx.begin(), idx.end(), 0);

    // sort indexes based on comparing values in v
    // using std::stable_sort instead of std::sort
    // to avoid unnecessary index re-orderings
    // when v contains elements of equal values
    stable_sort(idx.begin(), idx.end(), [&v](size_t i1, size_t i2) { return v[i1] < v[i2]; });

    return idx;
}

int main()
{
    std::vector<int> vec1{9, 5, 7, 4};

    auto vec = sort_indexes(vec1);
    // output 3 1 2 0
    for (auto i : vec)
    {
        std::cout << i << std::endl;
    }
}