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Unordered Set in C++

The C++ std::unordered_set is a container in the C++ Standard Library that represents an unordered set of unique elements. It is similar to std::set, but unlike std::set, it does not maintain elements in a sorted order; instead, it uses a hash-based data structure to store elements for faster access times. Here’s how to use std::unordered_set:

C++
#include <iostream>
#include <unordered_set>

int main() {
    // Create an unordered_set of integers
    std::unordered_set<int> mySet;

    // Insert elements into the set
    mySet.insert(42);
    mySet.insert(17);
    mySet.insert(99);

    // Check if an element is in the set
    if (mySet.find(17) != mySet.end()) {
        std::cout << "17 is in the set." << std::endl;
    }

    // Iterating through the elements of the set
    for (const auto& element : mySet) {
        std::cout << element << " ";
    }
    std::cout << std::endl;

    // Erase an element from the set
    mySet.erase(42);

    // Size of the set
    std::cout << "Size of the set: " << mySet.size() << std::endl;

    return 0;
}

In the code above:

  • We include the <unordered_set> header to use std::unordered_set.
  • We create an std::unordered_set<int> called mySet.
  • We insert elements using the insert function.
  • We check if an element is in the set using the find function.
  • We iterate through the elements of the set using a range-based for loop.
  • We erase an element from the set using the erase function.
  • We determine the size of the set using the size function.

Keep in mind that std::unordered_set provides fast average time complexity for insertion, deletion, and search operations, typically O(1). However, the order of elements is not guaranteed, as it depends on the hash values of the elements. If you need elements in a sorted order, you might want to use std::set instead.

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