C++ Unordered Set

C++ Unordered Set

In the Sets chapter, you learned that a set stores unique elements, and keeps them sorted.

An unordered_set also stores unique elements, but it does not sort them. Because it does not have to keep the elements in order, it is usually faster to add and find elements, especially when there are many of them.

To use an unordered set, include the <unordered_set> library:

// Include the unordered_set library
#include <unordered_set>

Create an Unordered Set

You create an unordered set in the same way as a set. Like a set, it only stores unique elements. If you add the same value twice, it is only stored once:

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

cout << cars.size();

"BMW" is only stored once, so the size is 4, not 5.

Add and Remove Elements

Use insert() to add an element, and erase() to remove one:

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

cars.insert("Mazda");  // Add Mazda
cars.erase("Volvo");   // Remove Volvo

cout << cars.size();

Check if an Element Exists

Use count() to check if an element exists. It returns 1 if the element is found, and 0 if it is not:

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

if (cars.count("BMW")) {
  cout << "BMW is in the set";
} else {
  cout << "BMW is not in the set";
}

Loop Through an Unordered Set

You can loop through an unordered set with a for-each loop. But remember that the elements come in no particular order:

unordered_set<int> numbers = {10, 20, 30, 40};
int sum = 0;

for (int num : numbers) {
  sum += num;
}

cout << "Sum: " << sum;

Set vs. Unordered Set