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LAST UPDATED: NOVEMBER 1, 2020

# C++ Program to Sort an Unordered Set in STL

Hello Everyone!

In this tutorial, we will learn about the working of an Unordered Set and its implementation in the C++ programming language.

## What is an Unordered Set?

An Unordered Set also stores only the single copy of the elements by removing the duplicates but does not sort the elements automatically as in Set.

An Unordered Set internally uses a Hash Table and hence the ordering is random depending upon the internally used Hash function.

### Sorting an Unordered Set:

An Unordered Set can be sorted by copying its elements to a Vector and then using the `sort()` method of the STL.

For a better understanding of its implementation, refer to the well-commented C++ code given below.

Code:

``````#include <iostream>
#include <bits/stdc++.h>

using namespace std;

bool cmp(int x, int y)
{
if (x > y)
return true;
else
return false;
}

//Function to print the elements of the unordered set using an iterator
void show(unordered_set<int> s)
{
//declaring an iterator to iterate through the unordered set
unordered_set<int>::iterator i;

for (i = s.begin(); i != s.end(); i++)
{
cout << *i << "  "; //accessing the elements of the unordered set using * as i stores the address to each element
}

cout << endl;
}

int main()
{
cout << "\n\nWelcome to Studytonight :-)\n\n\n";
cout << " =====  Program to demonstrate the Sorting of an Unordered Set, in CPP  ===== \n\n\n\n";

cout << " *** Unordered Set automatically removes the duplicate elements and maintains a random ordering. *** \n\n";

cout << " *** This random ordering depends on the hash function that is used internally. *** \n\n";

cout << " *** Unordered set can be sorted by copying its elements to a Vector. *** \n\n";

//Unordered Set declaration (Unordered Set of integers)
unordered_set<int> s;

//Filling the elements by using the insert() method.
cout << "\n\nFilling the Unordered Set with integers in random order."; //Unlike Set, this is not automatically sorted

s.insert(5);
s.insert(39);
s.insert(64);
s.insert(82);
s.insert(35);
s.insert(54);

cout << "\n\nThe elements of the Unordered Set before sorting are: ";
show(s);

//Declaring a vector and initializing it with the elements of the unordered set
vector<int> v(s.begin(), s.end());

//Sorting the vector elements in descending order using a custom comparator
sort(v.begin(), v.end(), cmp);

cout << "\n\nThe elements of the Unordered Set after sorting in descending Order using a Custom sort method are: \n";

//declaring an iterator to iterate through the unordered set
vector<int>::iterator it;

for (it = v.begin(); it != v.end(); it++)
{
cout << *it << "  "; //accessing the elements of the vector using * as i stores the address to each element
}

cout << "\n\n\n";

return 0;
}
``````

Output:

We hope that this post helped you develop a better understanding of the concept of Sorting an Unordered Set and its implementation in CPP. For any query, feel free to reach out to us via the comments section down below.

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