How to Store Vectors as Keys in a Multimap in C++?

In C++, the std::multimap is a container that stores elements in a key-value pair, whereas std::vector is a sequence container that stores elements in contiguous memory. In this article, we will learn how to store vectors as keys in a multimap in C++.

Example:

Input:
myVector ={1,2,3};
myVector ={4,5,6};

Output:
myMultimapOfVectors = { {{1, 2, 3}, “first”},
                         {{4, 5, 6}, “second”},
                         {{7, 8, 9}, “third”} }

Store Vectors as Keys in a Multimap in C++

To store vectors as keys in a multimap in C++, we can define the type of std::multimap container keys as vector type during the declaration of the multimap. The below syntax shows how to do it.

Syntax to Declare Multimap with Vectors as Keys in C++

multimap< vector<dataType>, valueType> multimap_name;

Here,

• datatype denotes the type of data you want to store in the vector.
• mapped_datatype denotes the type of data you want to store as the mapped value in the multimap.
• multimap_name is the name of the multimap.

We can then use the multimap::insert or array subscript operator to insert data into the multimap.

Note: The multimap in C++ is implemented as Red-Black Trees and can work on data types that have valid <, >, == operators. So vectors can be stored as keys in it. While unordered_maps may requre a custom hash function to work on vectors.

C++ Program to Store Vectors as Keys in a Multimap

The below program demonstrates how we can store vectors as keys in a multimap in C++ STL.

// C++ Program to illustrate how to store vectors as keys in
// a multimap
#include <iostream>
#include <map>
#include <vector>
using namespace std;

int main()
{
    // Define the type of vector
    typedef vector<int> VectorType;

    // Initialize two vectors
    VectorType vec1 = { 1, 2, 3 };
    VectorType vec2 = { 4, 5, 6 };

    // Create a multimap with vectors as keys
    multimap<VectorType, char> myMultimap;
    myMultimap.insert(make_pair(vec1, 'a'));
    myMultimap.insert(make_pair(vec2, 'b'));

    // Print the multimap with vectors as keys
    for (auto& pair : myMultimap) {
        cout << "{ ";
        for (auto& element : pair.first) {
            cout << element << ", ";
        }
        cout << "}, " << pair.second << endl;
    }

    return 0;
}

{ 1, 2, 3, }, a
{ 4, 5, 6, }, b

Time Complexity O(N), here N is the number of elements in the multimap.
Auxiliary Space: O(N * M) where M is the size of the vectors.