std::set
- C++ course
- Intermediate
- Lesson 57/116
- 5 min
Learn unique ordered collections with std::set.
std::set
std::set stores unique and sorted elements. You add with insert; duplicates are ignored. size() returns the unique count. Its internal structure is a red-black tree.
Example: std::set<int> s; s.insert(1); s.insert(1); s.insert(2);. The second insert(1) is ignored; s.size() returns 2. s.count(1) returns 1 (present). Sorted iteration: for (int x : s) std::cout << x;. This prints in ascending order.
Set ordering
set keeps elements sorted. Iteration is in ascending order. count(key) checks presence; find(key) returns an iterator. lower_bound and upper_bound are used for range queries.
Example: std::set<int> s = {3, 1, 2};. Even though the list order is 3, 1, 2, the set content is 1, 2, 3. s.count(2) returns 1. s.find(2) returns an iterator pointing to 2; if not found, it returns s.end(). This provides fast presence checks.
Practice Work with set
Create std::set<int> s;. Add 1, 1, 2, 3 with insert. Print s.size().
Starter code
#include <iostream>
#include <set>
// create a set and add elements (with duplicates)
int main() {
// print the unique count
return 0;
}Sample solution
#include <iostream>
#include <set>
int main() {
std::set<int> s;
s.insert(1);
s.insert(1);
s.insert(2);
s.insert(3);
std::cout << s.size() << std::endl;
return 0;
}Expected output
3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the insert call
s.insert(____);s.insert(1);Explanation: s.insert(1); adds 1 to the set (unique).
2 Match the property
| Term | Meaning |
|---|---|
unique |
No duplicates |
sorted |
Ordered |
insert |
Add (duplicates ignored) |
size() |
Unique count |
3 If you insert 1, 1, 2 into std::set<int>, what does size() return?
std::set<int> s; s.insert(1); s.insert(1); s.insert(2);- 3
- 1
- 2 Correct answer
- 0
Explanation: Duplicates are ignored; unique elements are 1 and 2 → size 2.