Sets

A set is container in C++ which stores values of the same type in a way that:

• Each value appears only once.
• The values are kept sorted (by default, smallest to largest).
• You can quickly check if something exists in it.

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

int main() {
    set<int> numbers = {5, 2, 9, 2, 1};
    for (int x : numbers)
        cout << x << " ";
}

Output:

1 2 5 9

Notice that the second 2 disappeared — because a set doesn’t allow duplicates.

Creating a set

To use a set, include the header:

#include <set>

Then you can create a set like this:

set<int> mySet;              // empty set of integers
set<string> names;           // set of strings
set<char> letters = {'a','b','c'};

Note

You can also initialize it with values using braces { }.

Basic operations

Operation	Example	Explanation
insert(x)	s.insert(5);	Adds 5 to the set. (If 5 already exists, nothing happens.)
erase(x)	s.erase(5);	Removes 5 if it’s there.
find(x)	if (s.find(5) != s.end())	Checks if 5 exists. Returns an iterator (like a pointer).
count(x)	if (s.count(5))	Returns 1 if found, 0 if not. Easier for beginners.
size()	s.size();	Number of elements.
empty()	s.empty();	Checks if the set has no elements.
clear()	s.clear();	Removes everything from the set.

Example: Using a set

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

int main() {
    set<int> s;

    s.insert(3);
    s.insert(1);
    s.insert(4);
    s.insert(3); // duplicate ignored

    cout << "Elements: ";
    for (int x : s) cout << x << " ";
    cout << endl;

    if (s.count(3))
        cout << "3 is in the set" << endl;

    s.erase(1);

    cout << "After removing 1: ";
    for (int x : s) cout << x << " ";
}

Output:

Elements: 1 3 4
3 is in the set
After removing 1: 3 4

Iterating through a set

You can go through a set using a range-based for loop or an iterator.

for (int x : s)
    cout << x << " ";

Or:

for (auto it = s.begin(); it != s.end(); it++)
    cout << *it << " ";

Since sets are sorted, the elements will always appear in order.

Kinds of sets

Type	Keeps order?	Allows duplicates?	Speed (roughly)	Notes
set	Yes	No	O(log n)	Balanced tree inside
multiset	Yes	Yes	O(log n)	Keeps multiple copies of the same value
unordered_set	No	No	O(1) average	Uses hashing, faster but order is random

Tips: If you care about order — use set.
If you need to store duplicates — use multiset.
If you only care about speed — use unordered_set.

Example: removing duplicates

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

int main() {
    vector<int> nums = {1, 2, 2, 3, 4, 3, 1};
    set<int> unique;

    for (int x : nums)
        unique.insert(x);

    for (int x : unique)
        cout << x << " ";
}

Output:

1 2 3 4