C++ STL Set Container – std::set

In this tutorial you will learn about STL Set container in C++ i.e. std::set and all functions applicable on it.

Set is a associative container. We know that in associative containers each element is unique. So sets are also containers that stores unique elements following in a specific order. The word associative means each value associated with a key value. For any kind of operation key will be more preferred than actual value. Here sets are special type of associative containers where value itself is key value.

Some more facts about set are, elements in set are constant. It means that we are unable to modify once we insert the element. If we want to update element then we should delete that element and again insert with updated element. The elements in the set are always sorted.

C++ STL Set

Let see some functions associated with sets:

Before working with functions let see iterators that can apply on list to manipulate the data in list.

begin(): returns iterator to the beginning

end(): returns iterator to the end of the list

rbegin(): returns reverse iterator to reverse beginning

rend(): returns reverse iterator to reverse end.

These iterators we can use in our programs.

First thing we need to include is set header file. Which is #include<set>

Inserting element into set:

There are different ways we can insert elements into set.

Note: In any method below when we insert an element into set it automatically inserted at proper position based on ascending sorted order.

Method 1: Insert directly by passing element.  setName.insert(element);

Method 2: Using iterator. This returns iterator at inserted position. setName.insert (iterator,value)

Method 3: Copying from another container.

Example program for inserting into set:

#include<iostream>
#include<set>

using namespace std;

int main(){
	set<int> s1; // declaring a set
	set<int> :: iterator it; // iterator for set
	
	for(int i=0;i<5;i++){
		s1.insert(i*10); // inserting using Method1
	}
	
	it= s1.begin();
	s1.insert(it,99); // inserting using Method2
	
	int ary[]= { 23, 34, 45, 56};
	s1.insert(ary, ary+4); // inserting using Method3
	
	//checking by printing
	for(it= s1.begin(); it!=s1.end(); it++)
		cout << *it << " ";
	
	// We can observe that output will be print in sorted order. That is the property of set
	
	return 0;
}

Output

0 10 20 23 30 34 40 45 56 99

Some more functions applicable on set are:

erase(): We can erase an element by specifying value or pointing to iterator.

swap(): swaps elements of set1 to set2 and set2 to set1.

clear(): removes all elements in the list. It results list of size 0.

Example program to show usage of above functions:

#include<iostream>
#include<set>

using namespace std;

int main(){
	set<int> s1;
	set<int> :: iterator it;
	
	for(int i=0; i<5; i++)
		s1.insert(i+10);
	
	s1.erase(12); // deleting element 12
	cout << "deleting  element 12 --> ";
	
	for(it= s1.begin(); it!=s1.end(); it++)
		cout << *it << " ";
	
	cout << endl;
	
	set<int> s2;
	for(int i=0;i<4;i++)
		s2.insert(i);
	
	cout << "set1 elements before swapping --> ";
	for(it= s1.begin(); it!= s1.end(); it++)
		cout<< *it << " ";
	
	cout << endl;
	
	cout << "set2 elements before swapping --> ";
	for(it= s2.begin(); it!= s2.end(); it++)
		cout<< *it << " ";
	
	cout << endl;
	
	s1.swap(s2); // swapping operation

	cout << "set1 elements after swapping --> ";
	for(it= s1.begin(); it!= s1.end(); it++)
		cout<< *it << " ";
	
	cout << endl;
	
	cout << "set2 elements after swapping --> ";
	for(it= s2.begin(); it!= s2.end(); it++)
		cout<< *it << " ";
	
	cout << endl;

	s1.clear(); // clearing list1
	s1.empty() ? cout <<"list is empty" << endl: cout << "list is not empty" << endl;
	// ternary operation which resutls list is empty or not

	return 0;
}

Output

deleting element 12 –> 10 11 13 14
set1 elements before swapping –> 10 11 13 14
set2 elements before swapping –> 0 1 2 3
set1 elements after swapping –> 0 1 2 3
set2 elements after swapping –> 10 11 13 14
list is empty 

Some more functions are:

empty(): returns a Boolean value whether set is empty or not.

size(): returns the size of the list.

max_size(): returns the maximum size a set can have.

find(): It returns iterator to the element.

count(x):  Returns how many times elements “x” present in set.

Example program to show usage of above functions:

#include<iostream>
#include<set>

using namespace std;

int main(){
	set<int> s1;
	set<int> :: iterator it;
	
	for(int i=0; i<5; i++)
		s1.insert(i+10);
	
	s1.empty() ? cout <<"list is empty" << endl: cout << "list is not empty" << endl;
	cout << "size of the list is " << s1.size() << endl;
	cout << "maximum size of the list is " << s1.max_size() << endl;
	cout << "finding elemnt 12 in list" << endl;
	it= s1.find(12);
	cout << *it << endl;
	
	s1.insert(12);
	if(s1.count(22))
		cout << "number 22 is in the list " << endl;
	else
		cout << "22 is not in the list";
		
	return 0;
}

Output

list is not empty
size of the list is 5
maximum size of the list is 461168601842738790
finding elemnt 12 in list
12
22 is not in the list

Comment below if you have any queries or found any information incorrect in above tutorial for STL Set container in C++.