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,

• Search, insert, and delete in O(log n) time.
• Does not allow 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 like upper_bound() and lower_bound().

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

1 2 3 

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:

1. Inserting Elements

• The insert() 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).

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2. 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.

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

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).

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4. Deleting Elements

To delete an element from a set, use erase(), it removes the element if it exists, else does nothing.