Set in C++ STL

A Set is a container which stores unique elements in some sorted order. It is an implementation of a Self-Balancing Binary Search Tree, specifically a Red-Black Tree which ensures,

• Searching, insertion, and deletion in O(log n) time.
• Automatically avoids duplicates
• Elements are always sorted in ascending order by default. You can also choose your own way of ordering them using a custom rule (comparator).
• Compared to unordered_set, the time taken to search, insert, and delete an higher, but we get elements in sorted order and also support additional functions likeupper_bound() andlower_bound().

Let us take a look at an example to create a set and traverse it.

#include <iostream>
#include <set>
using namespace std;

int main() {

    // Creating an empty set
    set<int> s1;

    // Initialize set with list 
    set<int> s2 = {1, 2, 3, 2, 1}; 

    // Traversing the set
    for (auto& x : s2)
        cout << x << " ";
    cout << endl;

    return 0;
}

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Syntax

The set container is defined as std::set class template inside <set> header file.

set<T> s;

where,

• T: Data type of elements in the set.
• s: Name assigned to the set.

Basic Operations

Basic operations on set containers are shown below:

Inserting Elements

• Theinsert() operation adds a new element to the set only if it not already present.
• If the element already exists, insert() does nothing (since duplicates are not allowed in set).
• Time complexity to insert is O(log n), as the underlying data structure is a Red-Black Tree.

#include <iostream>
#include <set>
using namespace std;

int main() {
    // Initialize set with values
    set<int> s = {2, 3};

    // Inserting an element
    s.insert(1);

    // Traversing the set
    for (auto x : s)
        cout << x << endl;

    return 0;
}

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Searching Elements

• The find() function is used to check whether an element exists. It returns an iterator to the element if found, else returns end() if the element is not found.
• The count() function can also be used to check existence, returns 1 if the element is present, 0 otherwise.
• Time complexity for searching an element is O(log n).

#include <iostream>
#include <set>
using namespace std;

int main() {
    set<int> s = {1, 2, 3};

    // Accessing elements using find()
    auto it = s.find(1);
    if (it != s.end())
        cout << "Element found: " << *it << endl;

    // Accessing elements using count()
    if (s.count(2))
        cout << "2 exists in the set" << endl;

    // Accessing all elements by traversal
    cout << "All elements: ";
    for (auto x : s)
        cout << x << " ";
    cout << endl;

    return 0;
}

Element found: 1
2 exists in the set
All elements: 1 2 3 

Traversing

• Loops (like range-based for loop or iterators) can be used to traverse all elements in a set.
• The traversal visits elements in sorted order (by default ascending, or according to a custom comparator).
• Time complexity to traverse a set is O(n), since each of the n elements is visited exactly once.

#include <iostream>
#include <set>
using namespace std;

int main() {
    set<int> s = {1, 2, 3};

    // Traversing using iterators
    for (auto it = s.begin(); it != s.end(); ++it)
        cout << *it << endl;  

    return 0;
}

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Deleting Elements

• To delete an element from a set, use erase(), it removes the element if it exists, else does nothing.
• Time complexity to delete an element is O(log n), since the set is implemented using a Red-Black Tree.

#include <iostream>
#include <set>
using namespace std;

int main() {
    set<int> s = {1, 2, 3, 4};

    // Deleting by value
    s.erase(2);

    // Deleting by iterator
    s.erase(s.begin());

    // Traversing the set
    for (auto i : s)
        cout << i << " ";
    cout << endl;

    return 0;
}

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Set in C++ STL

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