Multiset in C++ STL

In C++, multiset is an associative container similar to the set, but it can store multiple elements with same value. It is sorted in increasing order by default, but it can be changed to any desired order. It provides fast insertion, deletion and search operations.

Example:

#include <iostream>
#include <set>
using namespace std;

int main() {
    
    // Creating a set of integers
    multiset<int> ms = {5, 1, 3, 3};

    for (auto i : ms)
        cout << i << " ";
    return 0;
}

1 3 3 5 

Example: In this program, we created a multiset of integers ms and inserted four elements {5, 1, 3, 3} into it. The multiset allows duplicate elements, so the value 3 can appears twice.

Syntax

Multiset is defined as std::multiset class template inside <set> header file.

multiset<T, comp> ms;

where,

• T: Datatype of elements in the multiset.
• ms: Name assigned to the multiset.
• comp: It is a binary predicate function that tells multiset how to compare two elements in sorting. It is optional and if not provided, multiset is sorted in increasing order.

Declaration and Initialization

We can declare and initialize a multiset in multiple ways as shown in the below example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    
    // Creating an empty set of integers
    multiset<int> ms1;
    
    // Initialize with initializer list
    multiset<int> ms2 = {5, 3, 3, 1};

    for (auto i : ms2)
        cout << i << " ";
    return 0;
}

1 3 3 5 

Explanation: In the above program,

• Statement multiset<int> ms1 creates an empty multiset of integers.
• Statement multiset<int> ms2 = {5, 3, 3, 1} initializes the multiset with values from an initializer list.

Basic Operations

Here are the basic operations that can be performed on a multiset:

1. Inserting Elements

We can insert elements into a multiset by using insert() method. The multiset will automatically keep the elements sorted.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    multiset<int> ms;

    // Inserting elements
    ms.insert(5);
    ms.insert(3);
    ms.insert(3);
    ms.insert(1);
    
    for(auto i : ms)
        cout << i << " ";
    return 0;
}

1 3 3 5 

To know more methods of insertion, refer to the article - Different Ways to Insert Elements into Multiset

2. Accessing Elements

In multiset, we have to use iterator to access the elements by their position. For example, to access 3rd element, we have to move the begin() iterator two positions by incrementing. We can also use next() or advance() function instead of incrementing or decrementing.

However, we can quickly access the first and last element using begin() and end() iterators.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    multiset<int> ms = {5, 3, 3, 1};

    // Access first element
    auto it1 = ms.begin();
    cout << *it1 << " ";
    
    // Access third element
    auto it2 = next(it1, 2);
    cout << *it2;
    return 0;
}

1 3

Explanation: In the above program, s.begin() returns an iterator to the first element of the multiset which is stored in it1. To access the third element, next() is used to move the iterator it1 by 2 positions and store it in it2. The *it1 and *it2 dereference the iterators to access the values at those positions.

3. Updating Elements

We cannot change the value of elements once they are stored in the set.

4. Finding Elements

Multiset provides fast search by value operation using the find() member function. This function returns iterator the element if found, otherwise returns end() iterator.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    multiset<int> ms = {5, 3, 3, 1};

    // Finding 3
    auto it = ms.find(3);
    
    if (it != ms.end()) cout << *it;
    else cout << "Not Found!";

    return 0;
}

3

5. Traversing

Just like other containers, multisets can be easily traversed using range-based for loop or using begin() and end() iterators. To traverse all the elements with same key, se the range returned by equal_range() function instead of begin() and end().

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    multiset<int> ms = {5, 3, 3, 1};
    
    // Traversing using range-based loop
    for(auto i : ms)
        cout << i << " ";
    return 0;
}

1 3 3 5 

6. Deleting Elements

To delete elements from a multiset, use the erase() method. This method can be used either by passing the value or an iterator. But if the value is passed, it deletes all the occurrences of that value.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    multiset<int> ms = {5, 3, 3, 1};
    
    // Delete first element
    ms.erase(ms.begin());
    
    // Deleting all 3s
    ms.erase(3);
    
    for (auto x: ms) cout << x << " ";
    return 0;
}

5 

To know more methods of deletion, refer to the article - Different Ways to Remove Elements from Multiset

Other Common Operations

The following examples aim to help you master multiset operations beyond the basic operations:

• Check if the Multiset is Empty
• Find the Size of Multiset
• Find Frequency of Element in Multiset
• Find All Occurrences of an Element in Multiset
• Custom Sorting Order in Multiset
• Multiset of Pairs
• Create Multiset of User Defined Datatype
• Swap Two Multiset

Internal Working

In C++, multiset is an associated container that provides the built-in implementation of Red-Black Tree. Similar to set, it allows for fast insertion, deletion, and searching of elements in logarithmic time, O(log (n)). But it also allows duplicate elements i.e. multiple copies of the same element are stored in the tree structure. The same values are sorted on the basis of their insertion order.

Multiset vs Set

Following is the primary differences between set and unordered_set in C++:

• Set stores unique elements in sorted order.
• Multiset allows duplicate elements to be stored in sorted order.

All Member Functions

Following is the list of all member functions of std::multiset class in C++:

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Article Tags :

• C++
• STL
• cpp-containers-library
• cpp-multiset

Practice Tags :

• CPP
• STL