Question

In C++ would like to sort a lengthy (2^20) vector of reals, obviously sort() does the trick. Having used R before I was used to the nice order() function which yields the permutation that leads to the sorted vector.

Example:

x = {24, 55, 22, 1}

Then the permutation

perm = {3, 2, 0, 1}

Maps the original x to the sorted x in ascending order.

I can probably implement some bubble sort which does not only sort x but performs the same transpositions on the vector {0,1,2,...} and outputs both, but I believe someone must have thought about it and especially have done it efficiently.

Answer 1

Edit

Better than before approach without using helper vectors: (source on ideone):

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

template<class Vals>
void sortingPermutation(const Vals& values, std::vector<int>& v){
  int size = values.size(); 
  v.clear(); v.reserve(size);
  for(int i=0; i < size; ++i)
    v.push_back(i);

  std::sort(v.begin(), v.end(), [&values](int a, int b) -> bool { 
    return values[a] < values[b];
  });
}

int main()
{
    std::vector<double> values;
    values.push_back(24);
    values.push_back(55);
    values.push_back(22);
    values.push_back(1);

    std::vector<int> permutation;
    sortingPermutation(values, permutation);

    typedef std::vector<int>::const_iterator I;
    for (I p = permutation.begin(); p != permutation.end(); ++p)
        std::cout << *p << " ";
    std::cout << "\n";
}

I am using lambda from C++0x, but it can be replaced with simple functor object:

template<class T>
struct CmpPairs{
  CmpPairs(const std::vector<T> &v): v_(v) {}
  std::vector<T> v_;
  bool operator()(int a, int b){ return v_[a] < v_[b]; }
};

template<class T>
CmpPairs<T> CreateCmpPairs(const std::vector<T> & v) { return CmpPairs<T>(v); }
//in sortingPermutation:
std::sort(v.begin(), v.end(), CreateCmpPairs(values));

Source of old solution with std::map: ideone