C++ Library - <unordered_multiset>

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Introduction

An unordered_multiset is an associative container for storing elements and has no particular order unlike multiset that stores elements in ascending order. It uses hashing for fast access. It accepts duplicate elements just like multiset.

The elements can not be modified once added to the unordered_multiset. The keys are treated as const and the elements can be inserted and removed from the container. Since hash tables are used, an unordered_multiset provides faster average-time access, insertion, and deletion compared to multiset.

Characteristics of Unordered Multiset

The key characteristics of an unordered_multiset are given below −

• An unordered_multiset is implemented using hash tables.
• In unordered multiset, there is no specific order to store elements unlike a multiset.
• Duplicate elements are allowed in an unordered_multiset.
• It has constant (O(1)) average time for inserting, deleting, and searching operations and O(n) in worst case when many hash collisions occur.
• Elements in an unordered_multiset are immutable.
• It maps keys to buckets using a hash function for efficient access.

Definition

Below is definition of std::unordered_multiset −

 template< class Key, class Hash = std::hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<Key> > class unordered_multiset; 

Parameters

Below is the description of parameters used in the unordered multiset −

• Key − It defines the type of element.
• Hash − It is a unary function object.
• Pred − It is a binary predicate that takes two arguments of the same type as the elements and returns a bool.
• Alloc − It defines the type of allocator.

Member Types

Following member types can be used as parameters or return type by member functions.

Member functions

Below is list of member functions −

Capacity

Iterators

Lookup

Modifiers

Buckets

Hash policy

Observers

Non-member overloaded functions

Examples of Unordered Multiset

The following example demonstrates the usage of unordered_multiset in C++ STL:

Inserting and Printing Elements in Unordered Multiset

In this example, we have created an unordered_multiset of integers and performed various operations like insertion, counting elements, and displaying the contents.

 #include <iostream> #include <unordered_set> using namespace std; int main(){ unordered_multiset<int> nums; nums.insert(5); nums.insert(2); nums.insert(5); nums.insert(3); cout << "Elements in unordered multiset: "; for (int x : nums) cout << x << " "; } 

The output of the above code is given below −

 Elements in unordered multiset: 3 5 5 2 

Checking Occurrence of an Element

Here is an example of checking Occurrence of duplicate elements −

 #include <iostream> #include <unordered_set> using namespace std; int main(){ unordered_multiset<int> nums = {10, 20, 10, 30, 10}; cout << "Elements in unordered multiset: "; for (int x : nums) cout << x << " "; cout << endl; cout << "Count of 10:" << nums.count(10) << endl; } 

The output of the above code is given below −

 Elements in unordered multiset: 30 20 10 10 10 Count of 10:3 

Erasing All Occurrence of Element

In this example, we are erasing all the occurrences of specified element −

 #include <iostream> #include <unordered_set> using namespace std; int main() { unordered_multiset<int> nums = {5, 5, 2, 3, 5}; cout << "Original elements: "; for (int x : nums) cout << x << " "; cout << endl; nums.erase(5); cout << "Elements after erasing 5: "; for (int x : nums) cout << x << " "; } 

The output of the above code is given below −

 Original elements: 3 2 5 5 5 Elements after erasing 5: 3 2