Multimap in C++ Standard Template Library (STL)

Last Updated : 17 Jan, 2022

Multimap is similar to a map with the addition that multiple elements can have the same keys. Also, it is NOT required that the key-value and mapped value pair have to be unique in this case. One important thing to note about multimap is that multimap keeps all the keys in sorted order always. These properties of multimap make it very much useful in competitive programming.

Some Basic Functions associated with multimap:

begin() – Returns an iterator to the first element in the multimap
end() – Returns an iterator to the theoretical element that follows last element in the multimap
size() – Returns the number of elements in the multimap
max_size() – Returns the maximum number of elements that the multimap can hold
empty() – Returns whether the multimap is empty
pair<int,int> insert(keyvalue,multimapvalue) – Adds a new element to the multimap

C++ implementation to illustrate above functions:

CPP

// CPP Program to demonstrate the implementation of multimap
#include <iostream>
#include <iterator>
#include <map>
using namespace std;
 
// Driver Code
int main()
{
    multimap<int, int> gquiz1; // empty multimap container
 
    // insert elements in random order
    gquiz1.insert(pair<int, int>(1, 40));
    gquiz1.insert(pair<int, int>(2, 30));
    gquiz1.insert(pair<int, int>(3, 60));
    gquiz1.insert(pair<int, int>(6, 50));
    gquiz1.insert(pair<int, int>(6, 10));
 
    // printing multimap gquiz1
    multimap<int, int>::iterator itr;
    cout << "\nThe multimap gquiz1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;
 
    // adding elements randomly,
    // to check the sorted keys property
    gquiz1.insert(pair<int, int>(4, 50));
    gquiz1.insert(pair<int, int>(5, 10));
 
    // printing multimap gquiz1 again
 
    cout << "\nThe multimap gquiz1 after adding extra "
            "elements is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;
 
    // assigning the elements from gquiz1 to gquiz2
    multimap<int, int> gquiz2(gquiz1.begin(), gquiz1.end());
 
    // print all elements of the multimap gquiz2
    cout << "\nThe multimap gquiz2 after assign from "
            "gquiz1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;
 
    // remove all elements up to
    // key with value 3 in gquiz2
    cout << "\ngquiz2 after removal of elements less than "
            "key=3 : \n";
    cout << "\tKEY\tELEMENT\n";
    gquiz2.erase(gquiz2.begin(), gquiz2.find(3));
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
 
    // remove all elements with key = 4
    int num;
    num = gquiz2.erase(4);
    cout << "\ngquiz2.erase(4) : ";
    cout << num << " removed \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
 
    cout << endl;
 
    // lower bound and upper bound for multimap gquiz1 key =
    // 5
    cout << "gquiz1.lower_bound(5) : "
         << "\tKEY = ";
    cout << gquiz1.lower_bound(5)->first << '\t';
    cout << "\tELEMENT = " << gquiz1.lower_bound(5)->second
         << endl;
    cout << "gquiz1.upper_bound(5) : "
         << "\tKEY = ";
    cout << gquiz1.upper_bound(5)->first << '\t';
    cout << "\tELEMENT = " << gquiz1.upper_bound(5)->second
         << endl;
 
    return 0;
}

Output

The multimap gquiz1 is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    6    50
    6    10


The multimap gquiz1 after adding extra elements is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    4    50
    5    10
    6    50
    6    10


The multimap gquiz2 after assign from gquiz1 is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    4    50
    5    10
    6    50
    6    10


gquiz2 after removal of elements less than key=3 : 
    KEY    ELEMENT
    3    60
    4    50
    5    10
    6    50
    6    10

gquiz2.erase(4) : 1 removed 
    KEY    ELEMENT
    3    60
    5    10
    6    50
    6    10

gquiz1.lower_bound(5) :     KEY = 5        ELEMENT = 10
gquiz1.upper_bound(5) :     KEY = 6        ELEMENT = 50

List of Functions of Multimap

Function	Definition
multimap::operator=	It is used to assign new contents to the container by replacing the existing contents.
multimap::crbegin() and multimap::crend()	crbegin() returns a constant reverse iterator referring to the last element in the multimap container. crend() returns a constant reverse iterator pointing to the theoretical element before the first element in the multimap.
multimap::emplace_hint()	Insert the key and its element in the multimap container with a given hint.
multimap clear()	Removes all the elements from the multimap.
multimap empty()	Returns whether the multimap is empty.
multimap maxsize()	Returns the maximum number of elements a multimap container can hold.
multimap value_comp()	Returns the object that determines how the elements in the multimap are ordered (‘<‘ by default).
multimap rend	Returns a reverse iterator pointing to the theoretical element preceding to the first element of the multimap container.
multimap::cbegin() and multimap::cend()	cbegin() returns a constant iterator referring to the first element in the multimap container. cend() returns a constant iterator pointing to the theoretical element that follows the last element in the multimap.
multimap::swap()	Swap the contents of one multimap with another multimap of same type and size.
multimap rbegin	Returns an iterator pointing to the last element of the container.
multimap size()	Returns the number of elements in the multimap container.
multimap::emplace()	Inserts the key and its element in the multimap container.
multimap::begin() and multimap::end()	begin() returns an iterator referring to the first element in the multimap container. end() returns an iterator to the theoretical element that follows the last element in the multimap.
multimap upper_bound()	Returns an iterator to the first element that is equivalent to multimapped value with key-value ‘g’ or definitely will go after the element with key-value ‘g’ in the multimap.
multimap::count()	Returns the number of matches to element with key-value ‘g’ in the multimap.
multimap::erase()	Removes the key value from the multimap.
multimap::find()	Returns an iterator to the element with key-value ‘g’ in the multimap if found, else returns the iterator to end.
multimap equal_range()	Returns an iterator of pairs. The pair refers to the bounds of a range that includes all the elements in the container which have a key equivalent to k.
multimap insert()	Used to insert elements in the multimap container.
multimap lower_bound()	Returns an iterator to the first element that is equivalent to multimapped value with key-value ‘g’ or definitely will not go before the element with key-value ‘g’ in the multimap.
multimap key_comp()	Returns the object that determines how the elements in the multimap are ordered (‘<‘ by default).