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Unordered Sets in C++ Standard Template Library

Last Updated : 12 Jun, 2023
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An unordered_set is an unordered associative container implemented using a hash table where keys are hashed into indices of a hash table so that the insertion is always randomized. All operations on the unordered_set take constant time O(1) on an average which can go up to linear time O(n) in the worst case which depends on the internally used hash function, but practically they perform very well and generally provide a constant time lookup operation. 
The unordered_set can contain a key of any type – predefined or user-defined data structure but all the keys must be unique. It is defined inside <unordered_set> header file as std::unordered_set class.


std::unordered_set<data_type> name;

In std::unordered_set, many functions are defined among which the most used functions are:

  • The size() and empty() for capacity.
  • The find() for searching a key.
  • The insert () and erase() for modification.



// C++ program to demonstrate various function of
// unordered_set
#include <bits/stdc++.h>
using namespace std;
int main()
    // declaring set for storing string data-type
    unordered_set<string> stringSet;
    // inserting various string, same string will be stored
    // once in set
    string key = "slow";
    // find returns end iterator if key is not found,
    // else it returns iterator to that key
    if (stringSet.find(key) == stringSet.end())
        cout << key << " not found" << endl << endl;
        cout << "Found " << key << endl << endl;
    key = "c++";
    if (stringSet.find(key) == stringSet.end())
        cout << key << " not found\n";
        cout << "Found " << key << endl;
    // now iterating over whole set and printing its
    // content
    cout << "\nAll elements : ";
    unordered_set<string>::iterator itr;
    for (itr = stringSet.begin(); itr != stringSet.end();
        cout << (*itr) << endl;


slow not found

Found c++

All elements : is

The functions find(), insert() and erase() take a constant amount of time on average. The find() function returns an iterator to end() if a key is not there in the set, otherwise, an iterator to the key position is returned. The iterator works as a pointer to the key values so that we can get the key by dereferencing them using the * operator.

We can also see that the order of output is messed up because there is no particular order to store data in unordered_set.

Note: The Unordered_set allows only unique keys, for duplicate keys unordered_multiset should be used.

set vs unordered_set in C++ STL


Unordered Set

A Set is an ordered sequence of unique keys. An unordered_set is a set in which a key can be stored in any order, so unordered.
Set is implemented as a balanced tree structure making it possible to maintain order between the elements (by specific tree traversal). The unordered_set is implemented as hash tables as we don’t have to worry about any order.
The time complexity of set operations is O(log n). The time complexity of basic set operations is O(1).

A practical problem based on unordered_set

Given an array(list) of integers, find all the duplicates among them(suppose the duplicates are the same number guessed by random people in an experiment).


arr[] = {1, 5, 2, 1, 4, 3, 1, 7, 2, 8, 9, 5}


Duplicate items are: 5 2 1 



// C++ program to find duplicate from an array using
// unordered_set
#include <bits/stdc++.h>
using namespace std;
// Print duplicates in arr[0..n-1] using unordered_set
void printDuplicates(int arr[], int n)
    // declaring unordered sets for checking and storing
    // duplicates
    unordered_set<int> intSet;
    unordered_set<int> duplicate;
    // looping through array elements
    for (int i = 0; i < n; i++) {
        // if element is not there then insert that
        if (intSet.find(arr[i]) == intSet.end())
        // if element is already there then insert into
        // duplicate set
    // printing the result
    cout << "Duplicate item are : ";
    unordered_set<int>::iterator itr;
    // iterator itr loops from begin() till end()
    for (itr = duplicate.begin(); itr != duplicate.end();
        cout << *itr << " ";
// Driver code
int main()
    int arr[] = { 1, 5, 2, 1, 4, 3, 1, 7, 2, 8, 9, 5 };
    int n = sizeof(arr) / sizeof(int);
    printDuplicates(arr, n);
    return 0;


Duplicate item are : 5 2 1 

Time Complexity: O(n log n)
Auxiliary Space: O(n)

std::unordered_set Member Methods

The following table contains all the member functions of std::unordered_set class:

Function Name

Function Description


Insert a new {element} in the unordered_set container.


Return an iterator pointing to the first element in the unordered_set container.


Returns an iterator pointing to the past-the-end-element.


Count occurrences of a particular element in an unordered_set container.


Search for an element in the container.


Removes all of the elements from an unordered_set and empties it.


Return a const_iterator pointing to the first element in the unordered_set container.


Return a const_iterator pointing to a past-the-end element in the unordered_set container or in one of its buckets.


Returns the total number of elements present in a specific bucket in an unordered_set container.


Remove either a single element or a range of elements ranging from start(inclusive) to end(exclusive).


Return the number of elements in the unordered_set container.


Exchange values of two unordered_set containers.


Insert an element in an unordered_set container.


Returns maximum number of elements that an unordered_set container can hold.


Check if an unordered_set container is empty or not.


Returns range that includes all elements equal to a given value.


Copies (or moves) an unordered_set to another unordered_set and unordered_set::operator= is the corresponding operator function.


This hash function is a unary function that takes a single argument only and returns a unique value of type size_t based on it.


Used to request a capacity change of unordered_set.


Returns the bucket number of a specific element.


Returns the total number of buckets present in an unordered_set container.


Returns the current load factor in the unordered_set container.


Set the number of buckets in the container of unordered_set to a given size or more.


Returns(Or sets) the current maximum load factor of the unordered set container.


Inserts a new element in the unordered_set only if the value to be inserted is unique, with a given hint.

== operator 

The ‘==’ is an operator in C++ STL that performs an equality comparison operation between two unordered sets and unordered_set::operator== is the corresponding operator function for the same.


Returns a boolean value according to the comparison. It returns the key equivalence comparison predicate used by the unordered_set.


The != is a relational operator in C++ STL which compares the equality and inequality between unordered_set containers.


Find the maximum number of buckets that unordered_set can have.

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