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How to swap Keys with Values of a Map in C++?

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  • Last Updated : 19 Nov, 2019
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Given a map, the task is to swap the Keys of this map with its values, in C++.

Examples:

Input: map =
    {'e', 1 },
    {'o', 1 },
    {'r', 3 },
    
Output:
    {1, 'e' },
    {1, 'o' },
    {3, 'r' },
  • Method 1:
    1. By using vector pair, traverse through the given map
    2. push_back the swapped values in the vector pair
    3. sort the vector pair in ascending order
    4. in this case, we won’t get STL maps functionality since we are storing are result in a vector pair

    Below is the implementation of the above approach:




    // C++ program to swap keys
    // and values of a map
    // using Vector pair
      
    #include <bits/stdc++.h>
    using namespace std;
      
    // Function to swap keys and values of a map
    // and return a vector pair of the swapped values
    template <typename K, typename V>
    vector<pair<V, K> >
    invertMap2(map<K, V> const& myMap)
    {
      
        vector<pair<V, K> > myvec;
      
        // traverse the map
        // and pushback the values -> keys
        for (auto const& pair : myMap) {
            myvec.push_back(
                make_pair(pair.second,
                          pair.first));
        }
      
        return myvec;
    }
      
    // Function to call the swap operation
    void swap(map<char, int> char_frequency)
    {
      
        cout << "Original map:\n\n"
             << "KEY   Value\n";
        for (auto it
             = char_frequency.begin();
             it != char_frequency.end();
             ++it)
            cout << it->first << "   "
                 << it->second << "\n";
      
        // create a vector pair of int, char
        // which store the swaped values
        vector<pair<int, char> > freq_char
            = invertMap2(char_frequency);
      
        // sort the vector pair to get
        // results similar to a map
        sort(freq_char.begin(),
             freq_char.end(),
             [](pair<int, char>& a,
                pair<int, char>& b) {
                 if (a.first != b.first)
                     return a.first < b.first;
                 return a.second < b.second;
             });
      
        cout << "\nMap with Keys and"
             << " Values swapped:\n\n"
             << "KEY   Value\n";
        for (auto it
             = freq_char.begin();
             it != freq_char.end();
             ++it)
            cout << it->first << "   "
                 << it->second << "\n";
    }
      
    // Driver code
    int main()
    {
      
        // Taking the frequency map of
        // GeeksforGeeks as the input map here
        string input = "geeksforgeeks";
        map<char, int> char_frequency;
        for (int i = 0; i < input.length(); ++i) {
            char_frequency[input[i]]++;
        }
      
        // Swap the keys with values of this map
        swap(char_frequency);
      
        return 0;
    }

    
    

    Output:

    Original map:
    
    KEY   Value
    e   4
    f   1
    g   2
    k   2
    o   1
    r   1
    s   2
    
    Map with Keys and Values swapped:
    
    KEY   Value
    1   f
    1   o
    1   r
    2   g
    2   k
    2   s
    4   e
    
  • Method 2: Another approach is to use STL MultiMap to store swapped map because Maps can’t have duplicate keys. We will traverse through the input map and insert the swapped values into the Multi-Map.

    Below is the implementation of the above approach:




    // C++ program to swap keys
    // and values of a map
    // using STL MultiMap
      
    #include <bits/stdc++.h>
      
    using namespace std;
      
    // Function which swaps keys and
    // values of a map and
    // returns a multimap.
    template <typename K, typename V>
    multimap<V, K>
    invertMap(map<K, V> const& myMap)
    {
      
        multimap<V, K> myMultiMap;
      
        // Traverse the map and
        // pushback the values -> keys
        for (auto const& pair : myMap) {
      
            myMultiMap.insert(
                make_pair(pair.second,
                          pair.first));
        }
        return myMultiMap;
    }
      
    // Function to call the swap operation
    void swap(map<char, int> char_frequency)
    {
      
        cout << "Original map:\n\n"
             << "KEY   Value\n";
        for (auto it
             = char_frequency.begin();
             it != char_frequency.end();
             ++it)
            cout << it->first << "   "
                 << it->second << "\n";
      
        // create a multimap of int, char
        multimap<int, char> freq_char
            = invertMap(char_frequency);
      
        cout << "\nMap with Keys and"
             << " Values swapped:\n\n"
             << "KEY   Value\n";
        for (auto it
             = freq_char.begin();
             it != freq_char.end();
             ++it)
            cout << it->first << "   "
                 << it->second << "\n";
    }
      
    // Driver code
    int main()
    {
      
        // Taking the frequency map of
        // GeeksforGeeks as the input map here
        string input = "geeksforgeeks";
        map<char, int> char_frequency;
        for (int i = 0; i < input.length(); ++i) {
            char_frequency[input[i]]++;
        }
      
        // Swap the keys with values of this map
        swap(char_frequency);
      
        return 0;
    }

    
    

    Output:

    Original map:
    
    KEY   Value
    e   4
    f   1
    g   2
    k   2
    o   1
    r   1
    s   2
    
    Map with Keys and Values swapped:
    
    KEY   Value
    1   f
    1   o
    1   r
    2   g
    2   k
    2   s
    4   e
    

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