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Unordered Sets in C++ STL

Last Updated : 23 Jul, 2025

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In C++, unordered_set is an unordered associative container that stores unique elements. Unlike set, it stores its elements using hashing. This provides average constant-time O(1) search, insert, and delete operations but the elements are not sorted in any particular order.

Example:

C++


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

int main() {
  
    // Creating an unordered_set of integers
    unordered_set<int> us = {1, 2, 3, 4, 5};

    for (auto x : us) 
        cout << x << " ";
    return 0;
}

Explanation: In the above program, we created an unordered set us with values: {1, 2, 3, 4, 5}

Syntax

Unordered set is defined as std::unordered_set class template inside the <unordered_set> header file.

unordered_set<T> us;

where,

T: Type of the key.
us: Name assigned to the unordered set.

Declaration and Initialization

The unordered set can be declared and initialized in several ways as shown in the below example:

C++


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

int main() {
    
    // Create an empty unordered_set
    unordered_set<int> us1;
  
    // Initialize an unordered_set using
    // using intializer list
    unordered_set<int> us2 = {1, 2, 3, 4, 5};
    
    for(auto x : us2)
        cout << x << " ";
    return 0;
}

Explanation: In this program:

Statement unordered_set<int> us1 creates an empty unordered set.
Statement unordered_set<int> us2 = {1, 2, 3, 4, 5} creates the unordered set using the initialization list.

Basic Operations

Here are some common operations performed on an unordered set:

1. Inserting Elements

New elements can be inserted into unordered set using insert() method. We cannot specify the position to insert the element as it is automatically decided by its hashed value.

Example:

C++


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

int main() {
    unordered_set<int> us;
    
    // Insert elments using insert()
    us.insert(3);
    us.insert(1);
    us.insert(2);
    
    for(auto x : us)
        cout << x << " ";
    return 0;
}

2. Accessing Elements

We can’t access elements of an unordered set by index like in an array or vector. We have to increment or decrement iterator obtained from begin() or end() methods respectively to access the element by position. This can also be done with the help of next() or advance() function.

Example:

C++


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

int main() {
    unordered_set<int> us = {1, 2, 3, 4, 5};
    
    // Accessing third element
    auto it = next(us.begin(), 2);
    cout << *it;
    return 0;
}

Explanation: In the above code, iterator it accesses the third element by advancing begin() by 2 positions using next().

This operation is not very useful in unordered containers.

3. Updating Elements

In unordered set, the value of the elements cannot be changed after it is inserted.

4. Finding Elements

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

Example:

C++


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

int main() {
    unordered_set<int> us = {1, 2, 3, 4, 5};

    // Finding 4
    auto it = us.find(4);
    
    if (it != us.end()) cout << *it;
    else cout << "Element not Found!";
    return 0;
}

5. Traversing

Unordered set can be traverse either range-based for loop or using begin() and end() iterator in a loop.

Example:

C++


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

int main() {
    unordered_set<int> us = {1, 2, 3, 4, 5};
    
    // Using iterator in loop
    for(auto it = us.begin(); it != us.end(); it++)
        cout << *it << " ";
    return 0;
}

Output

5 4 3 2 1 
5 4 3 2 1

Other method is to use the range based for loop that we have used in previous examples.

6. Deleting Elements

Elements can be removed from the unordered set using erase() method. We can erase elements either by value or by position.

C++


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

int main() {
    unordered_set<int> us = {1, 2, 3, 4, 5};

    // Delete element by value
    us.erase(5);
    
    // Delete element by position
    us.erase(us.begin());
    
    for(auto x : us)
        cout << x << " ";
    return 0;
}

Explanation: In the above example, the element with value 5 is deleted using us.erase(5). Then, the first element is deleted by position using us.erase(us.begin()).

Time Complexity

The below table lists the time complexity of above operations on unordered set:

Operation Time Complexity Insert an element O(1) (average) Delete an element O(1) (average)

Access element by position

O(n)

Find element by value O(1) (average) Traverse the set O(n) Other Basic Operations

Unordered set is used in many situations for different purposes. The following examples’ aim is to help you master unordered set beyond the basics:

Check if Unordered Set is Empty

Find the Size of Unordered Set

Check if an Element is Present

Swap Two Unordered Set

Create Unordered Set for User Defined Data type

Internal Working

In C++, unordered_set provides the built-in implementation of hash table data structure. Each element is hashed on the basis of its value. This hash value determines where to store it in hash table. Each key in an unordered set is unique, and if an attempt is made to insert a duplicate element, it will be ignored. As it uses hashing, insertion, deletion and search operations take O(1) amortized time.

Unordered Sets vs Set

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

Unordered set stores elements in any order and insertion, deletion, and access operations are O(1) time due to the use of hashing.
Set stores elements in a sorted order and operations such as insertions, deletions, and accessing operations are takes logarithmic O(log n) in time complexity.

All Member Functions

Here’s the list of all member functions of std::unordered_set:

Function

Description

insert()

Insert a new element in the unordered set.

begin()

Return an iterator pointing to the first element in the unordered set.

end()

Return an iterator pointing to beyond the element in the unordered set

count()

Count occurrences of a particular element in an unordered set.