What is an unordered set?

An unordered set in STL is a container that stores unique elements in no particular order. Every operation on an unordered set takes O(1) complexity in the average case and takes O(n) in the worst case.

Syntax:

unordered_set<object_type> variable_name;

Example:

unordered_set<int> s;
unordered_set<string> str;

Functions in unordered set:

• insert() – to insert an element in the unordered set.
  

unordered_set<int> s;
s.insert(1);
s.insert(2);

• begin() – return an iterator pointing to the first element in the unordered set.
  

s.begin();

• end() – returns an iterator to the theoretical element after the last element.
  

s.end();

• count() – it returns 1 if the element is present in the container otherwise 0.
  

unordered_set<int> s;
s.insert(1);
s.insert(2);
s.count(2); //returns true

• clear() – deletes all the elements in unordered set.
  

s.clear();

• find() – to search an element in the unordered set.
  

unordered_set<int> s;
s.insert(1);
s.insert(2);
if(s.find(2)!=s.end())
cout<<"true"<<endl;

• erase() – to delete a single element or elements between a particular range.
  

s.erase();

• size() – returns the size of the unordered set.
  

s.size();

• empty() – to check if the unordered set is empty or not.
  

s.empty();

Code:

#include<bits/stdc++.h>

using namespace std;

int main() {
  unordered_set < int > s;
  for (int i = 1; i <= 10; i++) {
    s.insert(i);
  }

  cout << "Elements present in the unordered set: ";
  for (auto it = s.begin(); it != s.end(); it++) {
    cout << * it << " ";
  }
  cout << endl;
  int n = 2;
  if (s.find(2) != s.end())
    cout << n << " is present in unordered set" << endl;

  s.erase(s.begin());
  cout << "Elements after deleting the first element: ";
  for (auto it = s.begin(); it != s.end(); it++) {
    cout << * it << " ";
  }
  cout << endl;

  cout << "The size of the unordered set is: " << s.size() << endl;

  if (s.empty() == false)
    cout << "The unordered set is not empty " << endl;
  else
    cout << "The unordered set is empty" << endl;
  s.clear();
  cout << "Size of the unordered set after clearing all the elements: " << s.size();
}

Other functions:

• cbegin() – it refers to the first element of the unordered set.
• cend() – it refers to the theoretical element after the last element of the unordered set.
• bucket_size() – gives the total number of elements present in a specific bucket in an unordered set.
• emplace() – to insert an element in the unordered set.
• max_size() – the maximum elements an unordered_set can hold.
• max_bucket_count() – to check the maximum number of buckets an unordered set can hold.

Special thanks to Subhrajit Das for contributing to this article on takeUforward. If you also wish to share your knowledge with the takeUforward fam, please check out this article