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.

Example

// 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 {}:

Example

// 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";
}

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:

Example

// 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";
}

Note: The type of the set (e.g. string and int in the examples above) cannot be changed after its been declared.

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:

Example

// 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";
}

Note: The type specified in greater must match the type of elements in the set (int in our example).

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:

Example

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

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

Add Elements

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

Example

  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:

Example

  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:

Example

  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:

Example

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:

Example

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

Example

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:

Example

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

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

Tip: It is also possible to loop through sets with an iterator, which you will learn more about in a later chapter.