boost zip_iterator and std::sort

Question

I have two arrays values and keys both of the same length. I want to sort-by-key the values array using the keys array a

Answer 1

After seeing another of your comments in another answer.

I though I would enlighten you to the std::map. This is a key value container, that preserves key order. (it is basically a binary tree, usually red black tree, but that isn't important).

size_t elements=10;
std::map<int, double> map_;
for (size_t i = 0; i < 10; ++i)
{
    map_[rand()%M]=1.0*rand()/RAND_MAX;
}
//for every element in map, if you have C++11 this can be much cleaner
for (std::map<int,double>::const_iterator it=map_.begin(); 
         it!=map_.end(); ++it) 
{
    std::cout << it->first   <<  "\t"  << it->second  << std::endl;  
}

untested, but any error should be simple syntax errors

Answer 2

You can't sort a pair of zip_iterators.

Firstly, make_zip_iterator takes a tuple of iterators as input, so you could call:

boost::make_zip_iterator(boost::make_tuple( ... ))

but that won't compile either, because keys and keys+N doesn't have the same type. We need to force keys to become a pointer:

std::sort(boost::make_zip_iterator(boost::make_tuple(+keys, +values)),
          boost::make_zip_iterator(boost::make_tuple(keys+N, values+N)));

this will compile, but the sorted result is still wrong, because a zip_iterator only models a Readable iterator, but std::sort also needs the input to be Writable as described here, so you can't sort using zip_iterator.

Answer 3

A very good discussion of this problem can be found here: https://web.archive.org/web/20120422174751/http://www.stanford.edu/~dgleich/notebook/2006/03/sorting_two_arrays_simultaneou.html

Here's a possible duplicate of this question: Sorting zipped (locked) containers in C++ using boost or the STL

The approach in the link above uses std::sort, and no extra space. It doesn't employ boost::zip_iterator, just boost tuples and the boost iterator facade. Std::tuples should also work if you have an up to date compiler.

If you are happy to have one extra vector (of size_t elements), then the following approach will work in ~ o(n log n) time average case. It's fairly simple, but there will be better approaches out there if you search for them.

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

using namespace std;

template <typename T1, typename T2>
void sortByPerm(vector<T1>& list1, vector<T2>& list2) {
  const auto len = list1.size();
  if (!len || len != list2.size()) throw;

  // create permutation vector
  vector<size_t> perms;
  for (size_t i = 0; i < len; i++) perms.push_back(i);
  sort(perms.begin(), perms.end(), [&](T1 a, T1 b){ return list1[a] < list1[b]; });

  // order input vectors by permutation
  for (size_t i = 0; i < len - 1; i++) {
    swap(list1[i], list1[perms[i]]);
    swap(list2[i], list2[perms[i]]);

    // adjust permutation vector if required
    if (i < perms[i]) {
      auto d = distance(perms.begin(), find(perms.begin() + i, perms.end(), i));
      swap(perms[i], perms[d]);
    }
  }
}

int main() {
  vector<int> ints = {32, 12, 40, 8, 9, 15};
  vector<double> doubles = {55.1, 33.3, 66.1, 11.1, 22.1, 44.1};

  sortByPerm(ints, doubles);   

  copy(ints.begin(), ints.end(), ostream_iterator<int>(cout, " ")); cout << endl;
  copy(doubles.begin(), doubles.end(), ostream_iterator<double>(cout, " ")); cout << endl;
}

Answer 4

boost::make_zip_iterator take a boost::tuple.

#include <boost/iterator/zip_iterator.hpp>
#include <boost/tuple/tuple.hpp>
#include <boost/tuple/tuple_comparison.hpp>

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <vector>
#include <algorithm>

int main(int argc, char *argv[])
{
  std::vector<int> keys(10);      //lets not waste time with arrays
  std::vector<double> values(10);
  const int M=100;

  //Create the vectors.
  for (size_t i = 0; i < values.size(); ++i)
   {
     keys[i]   = rand()%M;
     values[i] = 1.0*rand()/RAND_MAX;
   }


  //Now we use the boost zip iterator to zip the two vectors and sort them "simulatneously"
  //I want to sort-by-key the keys and values arrays
   std::sort ( boost::make_zip_iterator(
                    boost::make_tuple(keys.begin(), values.begin())), 
               boost::make_zip_iterator(
                     boost::make_tuple(keys.end(), values.end()))
             );
    //The values array and the corresponding keys in ascending order. 
   for (size_t i = 0; i < values.size(); ++i)
    {
      std::cout << keys[i]   <<  "\t"  << values[i]    << std::endl;  
     }
  return 0;
}