c++ sort keeping track of indices

Question

Do you have some efficient routine for returning array with indices for sorted elements in a array? I think that some convenient way exists using stl vector. Do you have already implemented an efficient algo without stl, or do you have a reference to pseudo code or C++ code?

So you are returning an array containing indices to another array? And those indices will be ordered in your new array in a way it represents the older array sorted but without actually sorting it? — Artie, May 14 '12 at 10:26

score 43 · Accepted Answer · answered May 14 '12 at 09:55

43

Using C++11, the following should work just fine:

template <typename T>
std::vector<size_t> ordered(std::vector<T> const& values) {
    std::vector<size_t> indices(values.size());
    std::iota(begin(indices), end(indices), static_cast<size_t>(0));

    std::sort(
        begin(indices), end(indices),
        [&](size_t a, size_t b) { return values[a] < values[b]; }
    );
    return indices;
}

answered May 14 '12 at 09:55

Konrad Rudolph

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1

Shouldn't that be `size_t` rather than `unsigned`? – Fred Foo May 14 '12 at 09:55
5

In C++11 you can use `std::iota` to fill the vector with increasing values. – Maxim Egorushkin May 14 '12 at 09:56

score 7 · Answer 2 · answered May 14 '12 at 10:04

You could try something like this:

template<typename C>
class index_sorter {
  public:
    compare(C const& c) : c(c) {}
    bool operator()(std::size_t const& lhs, std::size_t const& rhs) const {
      return c[lhs] < c[rhs];
    }
  private:
    C const& c;
};

std::sort(index_vector.begin(), index_vector.end(), index_sorter(vector));

score 4 · Answer 3 · answered May 14 '12 at 14:45

Adding to @Konrad answer:

If for some reason you are not able to use C++11, then you can use boost::phoenix to simulate it like

    #include <vector>
    #include <algorithm>

    #include <boost/spirit/include/phoenix_core.hpp>
    #include <boost/spirit/include/phoenix_operator.hpp>

    template <typename T>
    std::vector<size_t> ordered(std::vector<T> const& values)
    {
        using namespace boost::phoenix;
        using namespace boost::phoenix::arg_names;

        std::vector<size_t> indices(values.size());
        int i = 0;
        std::transform(values.begin(), values.end(), indices.begin(), ref(i)++);
        std::sort(indices.begin(), indices.end(), ref(values)[arg1] < ref(values)[arg2]);
        return indices;
    }

score 2 · Answer 4 · answered May 14 '12 at 09:58

For C++03, I think this guru of the week can help you :

namespace Solution3
{
  template<class T>
  struct CompareDeref
  {
    bool operator()( const T& a, const T& b ) const
      { return *a < *b; }
  };


  template<class T, class U>
  struct Pair2nd
  {
    const U& operator()( const std::pair<T,U>& a ) const
      { return a.second; }
  };


  template<class IterIn, class IterOut>
  void sort_idxtbl( IterIn first, IterIn last, IterOut out )
  {
    std::multimap<IterIn, int, CompareDeref<IterIn> > v;
    for( int i=0; first != last; ++i, ++first )
      v.insert( std::make_pair( first, i ) );
    std::transform( v.begin(), v.end(), out,
                    Pair2nd<IterIn const,int>() );
  }
}

#include <iostream>

int main()
{
  int ai[10] = { 15,12,13,14,18,11,10,17,16,19 };

  std::cout << "#################" << std::endl;
  std::vector<int> aidxtbl( 10 );


  // use another namespace name to test a different solution
  Solution3::sort_idxtbl( ai, ai+10, aidxtbl.begin() );


  for( int i=0; i<10; ++i )
  std::cout << "i=" << i
            << ", aidxtbl[i]=" << aidxtbl[i]
            << ", ai[aidxtbl[i]]=" << ai[aidxtbl[i]]
            << std::endl;
  std::cout << "#################" << std::endl;
}

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c++ sort keeping track of indices

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