C++ Program to Implement Multimap in STL

A multimap is an associative container that stores elements in a key-value pair format, where multiple values can share the same key. In this article, we will learn how to use a multimap from the Standard Template Library (STL) in C++.

What is Multimap?

Multimap is a sorted associative container that stores data in key-value pairs, where keys can occur multiple times. Keys are stored in sorted order, and you can insert, access, and remove elements based on the key.

For example, in a multimap we can store student names with their scores even if some students have the same score:

multimap<int, string> scores;
scores.insert({90, "Alice"});
scores.insert({85, "Bob"});
scores.insert({90, "Charlie"});

// Display all entries with the same score
auto range = scores.equal_range(90);
for (auto it = range.first; it != range.second; ++it) {
    cout << "Score: " << it->first << ", Name: " << it->second << endl;
}

//Output will be displayed as:
Score: 90, Name: Alice
Score: 90, Name: Charlie

Using multimap Class in STL

The <multimap> class is part of the C++ standard STL library. It stores key-value pairs in sorted order and allows duplicate keys. Below are some functions used in multimap class:

• insert(): Adds a new key-value pair into the multimap, including duplicates.
• erase(): Removes elements with the specified key or at a specific position.
• find(): Searches for the first occurrence of a key and returns an iterator to it.
• count(): Returns the number of elements with the specified key.
• equal_range(): Returns the range of elements with the specified key.
• clear(): Removes all elements from the multimap.

Steps to Implement Multimap in C++ STL

Following are steps/algorithm to implement a multimap using C++ STL:

• Create a multimap using std::multimap.
• Insert key-value pairs using insert().
• Access duplicate keys using equal_range().
• Remove elements using erase().
• Iterate and display multimap elements using a loop.

C++ Program to Implement Multimap using STL

The below code is implemention of the above algorithm in C++ language.

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    multimap<int, string> students;

    // Insert elements with duplicate keys
    students.insert({101, "Rahul"});
    students.insert({102, "Priya"});
    students.insert({101, "Amit"});

    // Display all elements
    cout << "Student Records:" << endl;
    for (auto& entry : students) {
        cout << "Roll No: " << entry.first << ", Name: " << entry.second << endl;
    }

    // Display entries with roll number 101
    cout << "Students with Roll No 101:" << endl;
    auto range = students.equal_range(101);
    for (auto it = range.first; it != range.second; ++it) {
        cout << "Name: " << it->second << endl;
    }

    return 0;
}

The output of above code will be

Student Records:
Roll No: 101, Name: Rahul
Roll No: 101, Name: Amit
Roll No: 102, Name: Priya
Students with Roll No 101:
Name: Rahul
Name: Amit

Time and Space Complexity

Time Complexity:

• Insertion, Deletion, and Search: O(log n) for each operation, using a balanced binary search tree.
• Iteration: O(n) as we visit each element once.

Space Complexity: O(n) where n is the number of key-value pairs in the multimap.