Unordered_multiset in C++

Functions in unordered_multiset in C++

Following are the functions present to use in STL unordered_miltiset.

Example

This program depicts how unordered_multiset in C++ is declared and how its functions are used. Functions such as empty(), find(val), size() and count() has been used in the example shown below.

Code:

// Program to demonstrate different functions of unordered_multiset in //C++
#include <bits/stdc++.h>
using namespace std;

// typedef for declaring iterator
typedef unordered_multiset<int>::iterator umit;

// Function for printing unordered_multiset
void DisplayUms(unordered_multiset<int> umts)
{
	// begin() returns iterator to first element of unordered_multiset
	umit it;
	for (it = umts.begin(); it != umts.end(); it++)
		cout << *it << " ";
	cout << "\n";
}

// Program to check all functions
int main()
{
	// Empty initialization
	unordered_multiset<int> umset1;

    // Initialization by assignment
	umset1 = {2, 4, 5, 2, 0, 3, 4, 5};

	// Initialization by intializer list
	unordered_multiset<int> umset2 ({2, 3, 2, 7, 2, 3, 4, 2, 6});

	// empty() returns true if unordered_multiset is empty
	if (umset1.empty())
		cout << "1st unordered multiset is empty\n";
	else
		cout << "1st unordered multiset is not empty\n";
        // Otherwise false
         
	// size() returns number of elements present in data structure
	cout << "The size of second unordered multiset  is : "<< umset2.size() << endl;
    int val = 3;
    cout<<"Printing 1st unordered mutiset"<<endl;
	DisplayUms(umset1);

	umset1.insert(7);
    cout<<"Printing 1st unordered mutiset after inserting the element 7"<<endl;
	DisplayUms(umset1);

	/* Find is returning the iterator to first position of val, if exists otherwise it returns iterator
	to end*/
	if (umset1.find(val) != umset1.end())
		cout << "1st unordered multiset contains "<< val << endl;
	else
		cout << "1st unordered multiset does not contains "<< val << endl;

	// Count function is returning total number of occurrences in set
	val = 5;
	int cnt = umset1.count(val);
	cout << val << " occurs " << cnt << " times in unordered multiset 1 \n";

	val = 9;

	// If count return greater than 0 value then element exists otherwise not
	if (umset1.count(val))
		cout << "1st unordered multiset contains "<< val << endl;
	else
		cout << "1st unordered multiset does not contains "<< val << endl;

	val = 1;

	/* Equal_range returns a pair,where first is iterator to first position of val and second is iterator to last position to val*/
	pair<umit, umit> enrange_it = umset2.equal_range(val);
	if (enrange_it.first != enrange_it.second)
		cout << val << " occured atleast once in unoredered_multiset \n";

    cout<<"Printing 2nd unordered mutiset"<<endl;
	DisplayUms(umset2);

	// Erase function deletes all instances of val
	umset2.erase(val);
    cout<<"Printing 2nd unordered mutiset after calling erase function for the element 1"<<endl;
	DisplayUms(umset2);

	// Clear function deletes all entries from set
	umset1.clear();
	umset2.clear();

	if (umset1.empty())
		cout << "1st unordered multiset is empty\n";
	else
		cout << "1st unordered multiset is not empty\n";
}

You can also try this code with Online C++ Compiler

Run Code

Output:

1st unordered multiset is not empty
The size of second unordered multiset  is : 9
Printing 1st unordered mutiset
3 0 5 5 4 4 2 2 
Printing 1st unordered mutiset after inserting the element 7
7 3 0 5 5 4 4 2 2 
1st unordered multiset contains 3
5 occurs 2 times in unordered multiset 1 
1st unordered multiset does not contains 9
Printing 2nd unordered mutiset
6 4 7 3 3 2 2 2 2 
Printing 2nd unordered mutiset after calling erase function for the element 1
6 4 7 3 3 2 2 2 2 
1st unordered multiset is empty

Here, we can see that most of the functions used in this program are similar to unordered_set. But there are some different functions as well in unordered_multiset. Let's look at the list of functions of unordered_multiset in the next section.

Check out this problem - First And Last Occurrences Of X

Must Read Dynamic Binding in C++

FAQs

1. What is hash_function() in unordered_multiset?
   It is a unary function that accepts a single parameter and returns a unique value having type size_t.
    
2. What is rehash() in unordered_multiset?
   It is used for setting the number of buckets to N or more in the container.
    
3. What is the difference between multiset and vector?
   A multiset is slower to insert any object than a vector, but they are still sorted. But multiset takes more space because of the pointer used in the inner implementation.
    
4. What is reserve() in unordered_multiset?
   It is used for setting the number of buckets in the container or bucket_count such that it at least contains n elements.
    
5. What is operator ‘=’ in unordered_multiset?
   '=' is an operator used for C++ STL to copy or move one unordered_multiset to another unordered_multiset, and the operator function for this purpose is unordered_multiset::operator=.

Key Takeaways

In this article, we have extensively discussed the concepts of Unordered_multiset its functions, along with an example and frequently asked questions. 

We hope that this blog has helped you enhance your knowledge regarding Unordered_multiset and if you would like to learn more, check out our articles on STL containers in c++. Do upvote our blog to help other ninjas grow. Happy Coding!