Question

In my following program I’m currently using unordered_map just because I wanted O(1) search/insert time. But now I wanted the items to be ordered. Sorting it every time is very inefficient. What are my alternatives ? I read that hash_map does the job but the articles i read are very confusing or rather complicated for me to understand. What is the complexity of insert/search for hash_map and is it really ordered ? If so, is it defined in C++0x and how can I implement it ? If not what else can I use ? Thanks.

include <iostream>
#include <iterator>
#include <set>
#include <vector>
#include <unordered_map>

using namespace std;


template <class T>
inline void hash_combine(std::size_t & seed, const T & v)
{
  std::hash<T> hasher;
  seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}

template <typename C> struct ContainerHasher
{
  typedef typename C::value_type value_type;
  inline size_t operator()(const C & c) const
  {
    size_t seed = 0;
    for (typename C::const_iterator it = c.begin(), end = c.end(); it != end; ++it)
    {
      hash_combine<value_type>(seed, *it);
    }
    return seed;
  }
};


typedef std::set<int> my_set;
typedef std::vector<int> my_vector;
typedef std::unordered_map<my_set, my_vector, ContainerHasher<std::set<int>>> my_map;
typedef my_map::iterator m_it;

void print(my_map& data)
{
        for( m_it it(data.begin()) ; it!=data.end(); ++it)
        {
                cout << "Key : ";
                copy(it->first.begin(), it->first.end(), ostream_iterator<int>(cout, " "));
                cout << " => Value: ";
                copy (it->second.begin(),it->second.end(),ostream_iterator<int>(cout," "));
                cout << endl;
        }
        cout << "---------------------------------------------------------------\n";
}

int main()
{
   my_vector v1,v2,v3;
  for(int i = 1; i<=10; ++i)
   {
      v1.push_back(i);
      v2.push_back(i+10);
      v3.push_back(i+20);
   }

   my_set s1(v3.begin(),v3.begin()+3);
   my_set s2(v1.begin(),v1.begin()+3);
   my_set s3(v2.begin(),v2.begin()+3);

   my_map m1;

   m1.insert(make_pair(s1,v1));
   m1.insert(make_pair(s2,v2));
   m1.insert(make_pair(s3,v3));

   print(m1);
   my_set s4(v3.begin(),v3.begin()+3);

   m_it it = m1.find(s4);

   if(it != m1.end())
   {
      cout << endl << "found" << endl;
   }
   else
   {
      cout << endl << "Not found" << endl;
   }
}

EDIT:

I was using std::map before but I have large number of items (in millions). So even if the number of items are so large do you all recommend map if I want it ordered ?

Answer 1

Just use a regular std::map. Note this means you need ordering instead of hashing.

An unordered_map is a hash_map, by the way. “Unordered” just captures the conceptual difference rather than the implementation difference, so it’s a better name.