compare two multimaps c++

Question

I have two multimaps.i would like to create a new multimap which has the common key-value pairs in the given those two multimaps:

for eg:

#include &l         


        

Answer 1

Use the std::set_intersection function template from <algorithm>

std::multimap<T1, T2> newMap;
std::set_intersection(map1.begin(), map1.end(), 
                      map2.begin(), map2.end(), 
                      std::inserter(newMap, newMap.begin());

Edit Yeah, apparently this doesn't work for a multimap as it would for a map. I suggest the following:

std::multimap<T1, T2> newMap;
std::vector<std::multimap<T1, T2>::value_type> v1(map1.begin(), map1.end());
std::sort(v1.begin(), v1.end());
std::vector<std::multimap<T1, T2>::value_type> v2(map2.begin(), map2.end());
std::sort(v2.begin(), v2.end());
std::set_intersection(v1.begin(), v1.end(), 
                      v2.begin(), v2.end(), 
                      std::inserter(newMap, newMap.begin());

Answer 2

As an alternative solution to Armen's (which is excellent), here's a way to copy and sort at the same time:

typedef std::multimap<std::string, std::string> map_type;
map_type m, n, result;

m.insert(std::make_pair("1-2", "1-1"));
// --------8<--------
n.insert(std::make_pair("1-3", "21-2"));

// --------8<--------    

std::set<map_type::value_type> s1(m.begin(), m.end());
std::set<map_type::value_type> s2(n.begin(), n.end());
std::set_intersection(s1.begin(), s1.end(), 
                      s2.begin(), s2.end(), 
                      std::inserter(result, result.end()));

Output:

intersection
============
1-2 1-1
1-2 1-2
1-3 2-1
1-3 21-2

To get the elements that are only in m:

result.clear();
std::set_difference(s1.begin(), s1.end(), 
                    s2.begin(), s2.end(), 
                    std::inserter(result, result.end()));

And only in n:

result.clear();
std::set_difference(s2.begin(), s2.end(), 
                    s1.begin(), s1.end(), 
                    std::inserter(result, result.end()));

See it run.

Since you've copied m and n (into s1 and s2) at the time of doing the set_difference(), you could clear() these and insert into them instead of result.

Answer 3

For fun, here's the direct algorithm, which moves common elements into a new multimap, changing the original maps. You will recognize the standard "set intersection" algorithm for sorted ranges on the outer loop (concerning first); and since we cannot assume anything about the mapped values, the inner loop (concerning second) is a linear search over the mapped values to find equals.

template <typename T>
T move_common(T & a, T & b)
{
  typename T::const_iterator it1 = a.cbegin(), iend = a.cend(), jt1 = b.cbegin(), jend = b.cend();

  T result;

  while (it1 != iend && jt1 != jend)
  {
    if (it1->first < jt1->first)
    {
      ++it1;
    }
    else if (jt1->first < it1->first)
    {
      ++jt1;
    }
    else
    {
      typename T::const_iterator it2 = it1;

      while (it2 != iend && it2->first == it1->first)
      {
        typename T::const_iterator jt2 = jt1;
        while (jt2 != jend && jt2->first == jt1->first)
        {
          if (jt2->second == it2->second)
          {
            result.insert(*it2);
            if (it2 == it1) { a.erase(it1++); it2 = it1; } else { a.erase(it2++); }
            if (jt2 == jt1) { b.erase(jt1++); } else { b.erase(jt2); }
            break;
          }
          else
          {
            ++jt2;
            if (jt2 == jend || jt2->first != jt1->first) { ++it2; break; }
          }
        }
      }
      it1 = it2;
    }
  }
  return result;
}

Result:

common  = [(1-2, 1-1), (1-2, 1-2), (1-3, 2-1), (1-3, 21-2)]
n after = [(1-2, 1-3), (1-2, 1-4), (1-3, 21-1)]
m after = [(1-2, 1-5), (1-2, 1-7), (1-3, 21-4)]