C++ STL: set (Complete Guide)

Sets in C++ are containers that store unique elements in a sorted manner. The elements inside the set cannot be changed, once they are added to the set, they can only be inserted or deleted. They are implemented as BST (Binary Search Trees) in memory. Set is present in #include<set> header file. The elements inside the set can be accessed using iterators. 

Set Declaration

Syntax:

set <data_type> name = {initial_values};

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

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

Note: By default the set stores values in non-decreasing order. To store the values in non-increasing order, we use an inbuilt comparator function

set <data_type, greater <data_type>> name;

Example:

set <int> s; //initializes a set of size 0 which stores integer values arranged in non-decreasing order
set <int> s = {10, 20, 30}; //initializes a set having initial values as 10,20,30
set <int, greater <int>> s; //initializes a set of size 0 which stores integer values arranged in non-increasing order

Functions on sets

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

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

int main() {
  set <int> s1;
  set <int, greater <int>> s2;
  for (int i = 0; i < 5; i++) {
    s1.insert(i + 1);
  }
  for (int i = 0; i < 5; i++) {
    s2.insert((i + 1) * 10);
  }
  set <int> ::iterator it;
  cout << "Set1 ";
  for (it = s1.begin(); it != s1.end(); it++)
    cout << * it << " ";
  cout << '\n';
  cout << "Set2 ";
  for (it = s2.begin(); it != s2.end(); it++)
    cout << * it << " ";
  cout << '\n';

  s1.erase(1);
  s2.erase(s2.begin(), s2.find(10));
  cout << "After erasing element, size of set1 is " << s1.size() << '\n';
  int val = 4;
  if (s1.find(val) != s1.end())
    cout << "The set1 contains " << val << endl;
  else
    cout << "The set1 does not contains " << val << endl;
  cout << "Elements of set1 ";
  for (it = s1.begin(); it != s1.end(); it++)
    cout << * it << " ";
  cout << '\n';
  cout << "Elements of set2 ";
  for (it = s2.begin(); it != s2.end(); it++)
    cout << * it << " ";
  cout << '\n';

  s1.clear();
  if (s1.empty() == true) {
    cout << "set1 is empty now!";
  }
  return 0;
}

Output

Set1 1 2 3 4 5 
Set2 50 40 30 20 10 
After erasing element, size of set1 is 4
The set1 contains 4
Elements of set1 2 3 4 5 
Elements of set2 10 
set1 is empty now!