Multiset in C++
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C++ STL, which stands for Standard Template Library, includes multisets in the programming language. Multiset in C++ is defined as associative containers that contain ordered values. The value of a multiset's items can be added or removed but not changed (The items are also known as the values and are always constant).
Scope of the Article
- In this article, we will learn what multisets are in C++.
- The article will discuss the syntax, member functions, and examples of Multiset in C++.
What is Multiset in C++?
A Multiset in C++ is a container storing elements in a predefined order and allowing multiple elements to have the same value. The value of an element also identifies it in a multiset (the value is the key of type T). Multisets allow you to store elements in a specified sequence, with more than two items or many elements having comparable values. These values in the multiset identify the elements with their key value and pair value, and they cannot be changed once they have been placed into the multiset. However, one requirement can be met: more items can be inserted and retrieved at any time.
Syntax
Where T = multiset:: which defines the type of value or the key type
When working with multiset in C++, it's critical to include the set's header file.
#include <set> to make all multiset methods work.
Creating a Multiset
If you need the sorting features of a set but also want to allow repetitions in the container, utilize the multiset class in C++. For multisets, we can use the same functions we used for sets.
A multiset in C++ is used when you don't need a key/value pair but still need the search features of a set with several elements per key.
Here's an example demonstrating the same:
Output:
Member Functions
Let's go through the various member functions in multiset in C++, along with their uses.
| Member Functions | Uses of Functions |
|---|---|
| begin() | Responsible for returning an iterator to the multiset's initial item. |
| end() | Responsible for returning an iterator to the theoretical item that comes after the multiset's final item. |
| size() | Responsible for returning the total number of elements in the multiset. |
| max_size() | The highest number of items the multiset can contain is returned. |
| empty() | The function returns if the multiset is empty or not. |
| pair insert(const x) | The multiset now has a new element called 'x.' |
| iterator insert (iterator pos, const f) | At the index pos indicated by the iterator, a new item 'f' is added. |
| erase(iterator pos) | Eliminates the item at the iterator's current position. |
| clear() | All items in the multiset are removed using the clear function. |
| value_comp()/key_comp() | Gives the object ( by default) that controls how the items in the multiset are arranged. |
| find(const x) | If the item 'x' in the multiset is located, it returns an iterator to that item; otherwise, it returns the iterator to the end. |
| count(const x) | Returns the frequency of element 'x' in the multiset that would be matched. |
| lower_bound(const y) | Returns an iterator to the 1st element in the multiset that is greater or equal to 'x' or would not go before the element 'x' if discovered; otherwise, it returns the iterator to the end. |
| multiset::swap() | This method swaps the information of 2 multisets; however, the sets must be of a similar type, even if their sizes differ. |
| upper_bound(const y) | Returns an iterator to the 1st element in the multiset that is identical to 'y' or will surely go after the element 'y' if found; otherwise, it returns the iterator to the end. |
| multiset::operator() | Operator function is used to replace the existing contents in the container with new ones. |
| multiset::emplace() | A new element is inserted into the multiset container with the emplace function in C++. |
| multiset equal_range() | It yields the pair iterator. The pair denotes a range that contains all of the objects in the container that have the key y. |
| multiset::emplace_hint() | In the multiset, adds a new element. |
| multiset::rbegin() | Responsible for reversing iterator pointing to the multiset container's final element. |
| multiset::rend() | Provides a reverse iterator directing to the theoretical element just before the multiset container's initial element. |
| multiset::cbegin() | Provides a fixed iterator pointing to the container's initial element. |
| multiset::get_allocator() | The allocator object corresponding with the multiset is copied. |
| b.erase() | Remove all elements from the multiset that have the same value. |
| b.erase((b.find()) | Just one instance of an element with the same value should be removed from a multiset. |
Example Explaining Multiset Functions
1) Removing Elements from Multiset
Let's learn how to remove an element from a multiset in C++ with some value:
Output:
2) Swapping Multisets Using swap()
Output:
Let's try swapping two string multisets.
Output:
3) Finding the Size of a Multiset in C++ using size()
Output:
4) Removing all the Elements from the Multiset Container Using the clear() Function.
Output:
Template Parameters
There are three template parameters the key, Allocator, and the comparison class.
- Key: The kind of elements included in the container is determined by the key. A multiset's key is each of its elements.
- Compare: A boolean value is returned by a class that takes 2 arguments of a similar type as the container elements. If a is to be placed before b in a strict weak ordering action, the expression comp(a,b), where a comp is an object of this class, and a and b are components of the container, should yield true. This can be a class that implements a function call operator or a function pointer. The default value is less, which is equivalent to using the less-than operator (all times).
- Allocator: The storage allocation paradigm is defined by the kind of allocator object. The allocator class template for type Key is used by default, specifying the simplest memory allocation paradigm that is value agnostic.
Conclusion
- The C++ STL ( STL stands for Standard Template Library) includes multisets.
- Multisets are associative containers similar to Sets that store sorted values (the value is the key of type T).
- Unlike sets, multisets can have various redundant keys.
- Multisets compare the keys by default with the operator.
- The value of a multiset's items can be added or removed but not changed (The items are always const).
- The content of a multiset is a key, which is of type T, but unlike Sets, they can contain redundant keys.