C++ Sets

C++ Set

A set stores unique elements where they:

• Are sorted automatically in ascending order.
• Are unique, meaning equal or duplicate values are ignored.
• Can be added or removed, but the value of an existing element cannot be changed.
• Cannot be accessed by index numbers, because the order is based on sorting and not indexing.

To use a set, you have to include the <set> header file:

// Include the set library
#include <set>

Create a Set

To create a set, use the set keyword, and specify the type of values it should store within angle brackets <> and then the name of the set, like: set<type>
setName.

// Create a set called cars that will store strings
set<string> cars;

If you want to add elements at the time of declaration, place them in a comma-separated list, inside curly braces {}:

// Create a set called cars that will store strings
set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Print set elements
for (string car : cars) {
  cout << car << "\n";
}

BMW
Ford
Mazda
Volvo

As you can see from the result above, the elements in the set are sorted automatically. In this case, alphabetically, as we are working with strings.

If you store integers in the set, the returned values are sorted numerically:

// Create a set called numbers that will store integers
set<int> numbers = {1, 7, 3, 2, 5, 9};

// Print set elements
for (int num : numbers) {
  cout << num << "\n";
}

1
2
3
5
7
9

Sort a Set in Descending Order

By default, the elements in a set are sorted in ascending order. If you want to reverse the order, you can use the greater<type> functor inside the angle brackets, like this:

// Sort elements in a set in descending order
set<int, greater<int>> numbers = {1, 7, 3, 2, 5, 9};
// Print the elements
for (int num : numbers) {
  cout << num << "\n";
}

9
7
5
3
2
1

Unique Elements

Elements in a set are unique, which means they cannot be duplicated or equal.

For example, if we try to add “BMW” two times in the set, the duplicate element is ignored:

set<string> cars = {"Volvo", "BMW", "Ford", "BMW", "Mazda"};

// Print set elements
for (string car : cars) {
  cout << car << "\n";
}

BMW
Ford
Mazda
Volvo

Add Elements

To add elements to a set, you can use the .insert() function:

set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Add new elements
cars.insert("Tesla");
cars.insert("VW");
cars.insert("Toyota");
cars.insert("Audi");

Remove Elements

To remove specific elements from a set, you can use the .erase() function:

set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Remove elements
cars.erase("Volvo");
cars.erase("Mazda");

To remove all elements from a set, you can use the .clear() function:

set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

// Remove all elements
cars.clear();

Find the Size of a Set

To find out how many elements a set has, use the .size() function:

set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.size();  // Outputs 4

Check if a Set is Empty

Use the .empty() function to find out if a set is empty or not.

The .empty() function returns
1 (true) if the set is empty and 0 (false) otherwise:

set<string> cars;
cout << cars.empty();  // Outputs 1 (The set is empty)

set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};
cout << cars.empty();  // Outputs 0 (not empty)

Loop Through a Set

You can loop through a set with the for-each loop:

set<string> cars = {"Volvo", "BMW", "Ford", "Mazda"};

for (string car : cars) {
  cout << car << "\n";
}