C++ Sets

What is a Set?

A set stores unique, sorted elements. Duplicate insertions are silently ignored. Include <set>.

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

int main() {
    set<int> primes = {2, 3, 5, 7, 11, 13};
    primes.insert(5);    // duplicate — ignored
    primes.insert(17);   // added

    cout << "Size: " << primes.size() << endl;
    for (int p : primes) cout << p << " ";
    cout << endl;

    cout << "Contains 7:  " << primes.count(7)  << endl;
    cout << "Contains 10: " << primes.count(10) << endl;
    primes.erase(3);
    cout << "After erase 3, size: " << primes.size() << endl;
    return 0;
}

Output:

Size: 7
2 3 5 7 11 13 17
Contains 7:  1
Contains 10: 0
After erase 3, size: 6

Common Use Cases

• Removing duplicates from a list
• Fast membership testing
• Maintaining a sorted unique collection

// Deduplicate a vector using a set
vector<int> nums = {5, 3, 8, 3, 5, 9, 1, 8};
set<int> unique(nums.begin(), nums.end());
for (int n : unique) cout << n << " ";
// 1 3 5 8 9

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

int main() {
    set<int> unique;
    for (int i = 0; i < 10; i++) {
        int n; cin >> n;
        unique.insert(n);
    }
    cout << "Unique values: ";
    for (int n : unique) cout << n << " ";
    cout << endl;
    return 0;
}

Summary

• set<T> stores unique, automatically sorted elements.
• Use insert(), erase(), count(), find().
• Great for deduplication and fast membership checks.