C++ STL: unordered_set (Complete Guide)

Unordered_sets in C++ are containers that similar to sets. They store unique elements in any order. The elements inside the unordered_set cannot be changed, once they are added to the set, they can only be inserted or deleted. Insertion in unordered_set is randomized. 

They are implemented as a hash table in memory. The element in the unordered_set acts as both key and value in the hash table. All operations take O(1) time on average and O(N) time in the worst case. Unordered_set is present in #include<unordered_set> header file. The elements inside the unordered_set can be accessed using iterators. 

Unordered_set Declaration

Syntax:

unordered_set <data_type> name = {initial_values};

data_type: data type of the elements to be stored inside the unordered_set

initial_values: optional parameter which initializes the unordered_set with the given values

Example:

unordered_set <int> s; //initializes a unordered_set of size 0 which stores integer values
unordered_set <int> s = {10, 20, 30}; //initializes a unordered_set having initial values as 10,20,30

Functions on unordered_sets

• begin(): Returns an iterator to the first element of the unordered_set.
  Parameters: None
  Return type: iterator
   
• end(): Returns an iterator to the element past the last element of the unordered_set.
  Parameters: None
  Return type: iterator
   
• size(): It tells us the size of the unordered_set.
  Parameters: None
  Return type: integer - total number of elements in the unordered_set
   
• insert(element): Inserts an element in the unordered_set.
  Time Complexity: O(1) in the average case and O(N) in the worst case where N is the size of the unordered_set
  Parameters: the element to be inserted
  Return type: void
   
• erase(value) or erase(start_iterator,end_iterator): Delete elements from the unordered_set.
  Time Complexity: O(1) in the average case and O(N) in the worst case where N is the size of the unordered_set
  Parameters: the value to be removed or iterators pointing to the position between which the value needs to be deleted
  Return type: void
   
• find(element): Returns an iterator pointing to the element, if the element is found else returns an iterator pointing to the end of the unordered_set.
  Parameters: the element which needs to be found
  Return type: iterator
   
• clear(): It deletes all the elements from the unordered_set.
  Parameters: None
  Return type: void
   
• empty(): It tells us whether the unordered_set is empty or not.
  Parameters: None
  Return type: Boolean, true if a unordered_set is empty else false

#include<iostream>
#include<unordered_set>

using namespace std;

int main() {
  unordered_set <int> s;
  for (int i = 0; i < 5; i++) {
    s.insert(i + 1);
  }
  s.insert(0);
  unordered_set <int> ::iterator it;
  cout << "Unordered_set ";
  for (it = s.begin(); it != s.end(); it++)
    cout << * it << " ";
  cout << '\n';

  s.erase(1);
  s.erase(s.find(2));
  cout << "After erasing element, size of unordered_set is " << s.size() << '\n';
  int val = 4;
  if (s.find(val) != s.end())
    cout << "The unordered_set contains " << val << endl;
  else
    cout << "The unordered_set does not contains " << val << endl;
  cout << "Elements of unordered_set ";
  for (it = s.begin(); it != s.end(); it++)
    cout << * it << " ";
  cout << '\n';
  s.clear();
  if (s.empty() == true) {
    cout << "set is empty now!";
  }
  return 0;
}

Output

Unordered_et 0 2 1 3 4 5 
After erasing element, size of unordered_set is 4
The unordered_set contains 4
Elements of unordered_set 0 3 4 5 
set is empty now!