Everything about Unordered_Set in C++
Published: 2023-06-19 14:48:05

In C++, std::unordered_set is an associative container that stores a collection of unique elements. It is an implementation of a hash table that provides fast average constant-time search, insertion, and deletion operations on its elements.

Unlike std::set, which stores elements in a sorted order, std::unordered_set does not impose any specific order on its elements. Instead, it uses a hash function to map each element to a unique bucket, which is a slot in the hash table where the element is stored. When two or more elements are mapped to the same bucket, they are stored as a linked list in that bucket. This makes std::unordered_set a good choice when you need to store a large collection of elements and perform frequent lookups, insertions, and deletions on them.

One of the main advantages of C++ Unordered_Set is its constant time complexity for insertions, deletions, and searches. This makes it a great choice for applications that involve frequent operations on a large dataset. Additionally, C++ Unordered_Set does not allow duplicate elements, which helps to avoid errors that can arise from accidentally inserting a duplicate element.

Unordered_Set also supports iterators, which makes it easy to traverse the elements in the data structure. Another advantage of Unordered_Set is that it maintains the order of elements in which they were inserted, unlike other data structures such as Set.

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## Syntax and initialization of Unordered_Set
The syntax of an unordered_set is quite similar to that of other C++ Standard Template Library containers. An unordered_set in C++ can be initialized using several methods, including copy initialization, direct initialization, and uniform initialization.

```cpp
// Copy initialization
std::unordered_set<int> setOne = {1, 2, 3, 4, 5};

// Direct initialization
std::unordered_set<int> setTwo({6, 7, 8, 9, 10});

// Uniform initialization
std::unordered_set<int> setThree{11, 12, 13, 14, 15};
```

The elements of an unordered_set are stored as keys, and thus, each element should be unique. Additionally, the elements are stored in an unordered manner, which means that the order in which they are inserted is not necessarily the order in which they will be stored.

## Insertion and Deletion of Elements in Unordered_Set
Unordered_Set in C++ is a container that holds a unique set of values in no particular order. It is implemented using a Hash Table. Insertion and deletion of elements in Unordered_Set is much faster than in other containers like a Vector or List. The insert() function is used to insert an element in an Unordered_Set. It returns a pair of an iterator pointing to the inserted element and a Boolean value indicating whether the insertion is successful or not.

Here's an example of how to use it:
```cpp
#include <unordered_set>

int main() {
    std::unordered_set<int> mySet;

    // Inserting elements using insert()
    mySet.insert(10);
    mySet.insert(20);
    mySet.insert(30);

    return 0;
}
```

In this example, we create an unordered_set called mySet. We then use the insert() function to insert elements 10, 20, and 30 into the set. After executing the code, the unordered_set will contain these three elements.

The erase() function is used to delete an element from an Unordered_Set. It accepts either an iterator pointing to the element to be deleted or the element itself. When an element is deleted, all the iterators and references to it become invalid.

Here's an example of how to use it:
```cpp
#include <unordered_set>
#include <iostream>

int main() {
    std::unordered_set<int> mySet = {10, 20, 30};

    // Erasing an element using erase()
    mySet.erase(20);

    // Printing the remaining elements
    for (const auto& element : mySet) {
        std::cout << element << " ";
    }
    std::cout << std::endl;

    return 0;
}
```

In this example, we have an unordered_set called mySet with elements 10, 20, and 30. We use the erase() function to remove the element 20 from the set. After erasing the element, the loop prints the remaining elements of the set, which in this case are 10 and 30.

## Traversal of Unordered_Set using iterators
Iterators are used to traverse through the elements of a container in C++, and they are supported by almost all the STL containers, including the unordered_set. The unordered_set class provides four types of iterators- begin (), end (), cbegin (), and cend (), which can be used to traverse through the elements of the unordered_set.

The begin () function returns an iterator to the first element of the unordered_set, while the end () function returns an iterator to the element next to the last element of the unordered_set. The cbegin () and cend () functions are similar to begin () and end (), respectively, but they return a constant iterator that cannot be used to modify the elements of the unordered_set.

Here are examples of how to use begin(), end(), cbegin(), and cend() with an unordered_set in C++:
```cpp
#include <unordered_set>
#include <iostream>

int main() {
    std::unordered_set<int> mySet = {10, 20, 30, 40, 50};

    // Using begin() and end() to iterate over the elements
    std::cout << "Iterating using begin() and end(): ";
    for (auto it = mySet.begin(); it != mySet.end(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    // Using cbegin() and cend() to iterate over the elements (const iterator)
    std::cout << "Iterating using cbegin() and cend(): ";
    for (auto it = mySet.cbegin(); it != mySet.cend(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

    return 0;
}
```

In this example, we have an unordered_set called mySet with elements 10, 20, 30, 40, and 50. 

- Using begin() and end(), we iterate over the elements of the set and print them out.
- Using cbegin() and cend(), we iterate over the elements again, but this time using const iterators. This ensures that the elements cannot be modified within the loop.

Both iterations output the elements in an arbitrary order, as unordered_set does not maintain a specific order.

To traverse through the elements of an unordered_set, we can use a range-based for loop or a while loop with iterators. In the range-based for loop, we can simply iterate through the elements of the unordered_set using the auto keyword. In contrast, using while loop with iterators, we can iterate through the unordered_set by incrementing the iterator until it reaches the end of the unordered_set.

In C++, you can use the auto keyword to simplify the iteration process through the elements of an unordered_set. Here's an example:

#include <unordered_set>
#include <iostream>

int main() {
    std::unordered_set<int> mySet = {10, 20, 30, 40, 50};

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

    return 0;
}

In this example, we have an unordered_set called mySet with elements 10, 20, 30, 40, and 50. By using the auto keyword in the range-based for loop, the type of element is automatically deduced based on the type of elements in the unordered_set. The loop iterates through each element in the set, and element represents the current element being processed. In this case, we print out the elements, resulting in the output: 10 20 30 40 50.

## Comparison of C++ Unordered_Set with other data structures
The unordered_set data structure in C++ is a powerful tool for fast and efficient lookups. Compared to some other data structures like arrays, linked lists, and even set, unordered_set provides a more efficient way to store and retrieve data.

While arrays and linked lists have a time complexity of O(n) for searching, unordered_set can achieve a time complexity of O(1) for average cases and O(n) for worst cases. set has a better worst-case time complexity of O(log n), but is slower in practice than unordered_set for most cases due to its additional overhead in maintaining a balanced tree. Overall, unordered_set is a great choice for any application that requires fast and efficient lookups.

## Suggestions for optimal usage of Unordered_Set
Unordered_Set is a powerful container in C++ that can boost your programming efficiency. However, to make the most of it, you need to use it optimally. Here are some suggestions for doing so:

1. Choose the right hash function: The efficiency of Unordered_Set largely depends on the quality of the hash function you choose. Make sure you pick a function that minimizes collisions and distributes the elements uniformly across the hash table.
2. Avoid frequent rehashing: Rehashing can be an expensive operation, so try to avoid it as much as possible. You can do this by setting an appropriate load factor or reserving enough space in advance.
3. Be mindful of iterator invalidation: Just like other containers, Unordered_Set can invalidate its iterators when elements are added, removed, or rehashed. Be careful not to use invalidated iterators, as this can cause unexpected behavior.
4. Consider using emplace instead of insert: If you're creating new elements on the fly, using emplace can be more efficient than insert, as it avoids the cost of creating temporary objects.

There are also some common pitfalls that you should be aware of to avoid unexpected behavior and bugs. One common mistake is assuming that the order of elements in an Unordered_Set is always the same. Since Unordered_Set is unordered, the order of elements can vary each time you iterate through the set.

Another pitfall is using invalid iterators, which can lead to undefined behavior. Always make sure to check if an iterator is valid before dereferencing it. Finally, be careful with hash collisions, as they can cause performance issues and degrade the performance benefits of Unordered_Set.

## Limitations of Unordered_Set and alternative data structures for certain use cases
While unordered_set is a powerful data structure that provides constant time complexity for insertion, deletion, and search operations, it has some limitations. One of the primary limitations is that it does not maintain the order of elements. If you need to iterate through the elements of the set in a specific order, you will need to use a different data structure, such as std::set.

Additionally, unordered_set requires a hash function for its type, and you may need to provide a custom hash function if one is not available for your specific type.

Finally, unordered_set may have a higher memory overhead than other data structures, such as std::vector or std::list, which may be more appropriate for certain use cases. It is important to carefully consider the requirements of your specific use case when choosing a data structure to ensure optimal performance.

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[Everything about Unordered_Set in C++](https://www.lightly-dev.com/blog/what-is-cpp-unordered-set/)