What is a multimap in C++ STL?

multimap in STL are associative containers like maps where each element consists of a key value and a mapped value, the only difference is multimaps can store duplicate elements

Syntax:

multimap<object_type,object_type> variable_name;

Example:

multimap<int,int> mpp;
multimap<string,int> mpp;

Functions in the multimap:

• insert() – to insert an element in the multimap.

multimap<int,int> mp;
mp.insert({1,10});
mp.insert({2,20});

• begin() – return an iterator pointing to the first element in the multimap.

mp.begin();

• end() – returns an iterator to the theoretical element after the last element.

mp.end();

• clear() – deletes all the elements in the multimap.

mp.clear();

• find() – to search for an element in the map.

multimap<int,int> mp;
mp.insert({1,10});
mp.insert({2,20});
if(mp.find(2)!=mp.end())
cout<<"true"<<endl;

• erase() – to delete a single element or elements between a particular range.

mp.erase(key);
mp.erase(iterator position);
mp.erase(iterator position 1, iterator position 2);

• size() – returns the number of elements on the multimap.

mp.size();

• empty() – to check if the multimap is empty or not.

mp.empty();

Code:

#include<bits/stdc++.h>

using namespace std;

int main() {
  multimap < int, int > mp;
  for (int i = 1; i <= 5; i++) {
    mp.insert({i , i * 10});
  }
  mp.insert({4,40});

  cout << "Elements present in the multimap: " << endl;
  cout << "Key\tElement" << endl;
  for (auto it = mp.begin(); it != mp.end(); it++) {
    cout << it -> first << "\t" << it -> second << endl;
  }

  int n = 2;
  if (mp.find(2) != mp.end())
    cout << n << " is present in multimap" << endl;

  mp.erase(mp.begin());
  cout << "Elements after deleting the first element: " << endl;
  cout << "Key\tElement" << endl;
  for (auto it = mp.begin(); it != mp.end(); it++) {
    cout << it -> first << "\t" << it -> second << endl;
  }

  cout << "The size of the multimap is: " << mp.size() << endl;

  if (mp.empty() == false)
    cout << "The multimap is not empty " << endl;
  else
    cout << "The multimap is empty" << endl;
  mp.clear();
  cout << "Size of the multimap after clearing all the elements: " << mp.size();
}

Other functions:

• cbegin() – it refers to the first element of the multimap.
• cend() – it refers to the theoretical element after the last element of the multimap.
• rbegin() – it points to the last element of the multimap.
• rend() – it points to the theoretical element before the first element of the multimap.
• emplace() – to insert an element in the multimap.
• max_size() – the maximum elements a multimap can hold.

Special thanks to Subhrajit Das for contributing to this article on takeUforward. If you also wish to share your knowledge with the takeUforward fam, please check out this article