What is an unordered_multimap in C++ STL?

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

Syntax:

unordered_multimap<object_type,object_type> variable_name;

Example:

unordered_multimap<int,int> mpp;
unordered_multimap<string,int> mpp;

Functions in the multimap:

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

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

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

mp.begin();

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

mp.end();

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

mp.clear();

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

unordered_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 unordered multimap.

mp.size();

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

mp.empty();

Code:

#include<bits/stdc++.h>

using namespace std;

int main() {
  unordered_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 unordered 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 unordered 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 unordered multimap is: " << mp.size() << endl;

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

Other functions:

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