C++ multiset rbegin()

C++ multiset rbegin() function is used to return a reverse iterator referring to the last element of the multiset container.

A reverse iterator of multiset moves in reverse direction and incrementing it until it reaches to the beginning (First element) of the multiset container.

Syntax

      reverse_iterator rbegin();                            //until C++ 11

const_reverse_iterator rbegin() const;                //until C++ 11

      reverse_iterator rbegin() noexcept;              //since C++ 11

const_reverse_iterator rbegin() const noexcept;  //since C++ 11

Parameter

None

Return value

It returns an iterator in reverse (reverse iterator) which points to the last element of the multiset.

Complexity

Constant.

Iterator validity

No changes.

Data Races

The multiset is accessed neither the non-const nor the const versions modify the multiset container. Concurrently accessing the elements of a multiset is safe.

Exception Safety

This function never throws exception.

Example 1

Let's see the simple example for rbegin() function:

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  multiset<int> mymultiset= {20,10,20,40,10,30};
  
  // show content:
  cout<<"Elements are: "<<endl;
  multiset<int>::reverse_iterator rit;
  for (rit=mymultiset.rbegin(); rit!=mymultiset.rend(); ++rit)
    cout << *rit<< '\n';

  return 0;
}

Output:

Elements are: 
40
30
20
20
10
10

In the above example, rbegin() function is used to return a reverse iterator pointing to the last element in the mymultiset multiset.

Because multiset stores the elements in sorted order of keys therefore, iterating over a multiset will result in above order i.e. sorted order of keys.

Example 2

Let's see a simple example to iterate over the multiset in reverse order using while loop:

#include <iostream>
#include <set>
#include <string>
#include <iterator>

using namespace std;
 
int main() {
 
	// Creating & Initializing a multiset of String 
	multiset<string> multisetEx = {"aaa", "ccc", "ddd", "bbb", "aaa", "bbb"};
 
	// Create a multiset iterator and point to the end of multiset
	multiset<string, int>::reverse_iterator it = multisetEx.rbegin();
 
	// Iterate over the multiset using Iterator till beginning.
	while (it != multisetEx.rend()) {
		// Accessing KEY from element pointed by it.
		string word = *it;
  
		cout << word << endl;
 
		// Increment the Iterator to point to next entry
		it++;
	}
	return 0;
}

Output:

ddd
ccc
bbb
bbb
aaa
aaa

In the above example, we are using while loop to iterate over the multiset in reverse order and rbegin() function initializing the last element of the multiset.

Because multiset stores the elements in sorted order of keys therefore, iterating over a multiset will result in above order i.e. sorted order of keys.

Example 3

Let's see a simple example to get the first element of the reversed multiset:

#include <set>  
#include <iostream>  
  
int main( )  
{  
   using namespace std;   
   
   multiset <int> s1;  
   multiset <int>::iterator s1_Iter;  
   multiset <int>::reverse_iterator s1_rIter;  
  
   s1.insert( 10 );  
   s1.insert( 20 );  
   s1.insert( 30 ); 
   s1.insert( 20 );
  
   s1_rIter = s1.rbegin( );  
   cout << "The first element in the reversed multiset is "  
        << *s1_rIter << "." << endl;  
  
   // begin can be used to start an iteration   
   // throught a multiset in a forward order  
   cout << "The multiset is:";  
   for ( s1_Iter = s1.begin( ) ; s1_Iter != s1.end( ); s1_Iter++ )  
      cout << " " << *s1_Iter;  
   cout << endl;  
  
   // rbegin can be used to start an iteration   
   // throught a multiset in a reverse order  
   cout << "The reversed multiset is:";  
   for ( s1_rIter = s1.rbegin( ) ; s1_rIter != s1.rend( ); s1_rIter++ )  
      cout << " " << *s1_rIter;  
   cout << endl;  
  
   // A multiset element can be erased by dereferencing to its key   
   s1_rIter = s1.rbegin( );  
   s1.erase ( *s1_rIter );  
  
   s1_rIter = s1.rbegin( );  
   cout << "After the erasure, the first element "  
        << "in the reversed multiset is "<< *s1_rIter << "." << endl;  
        
return 0;        
}    

Output:

The first element in the reversed multiset is 30.
The multiset is: 10 20 20 30
The reversed multiset is: 30 20 20 10
After the erasure, the first element in the reversed multiset is 20.

Example 4

Let's see a simple example to sort and calculate the highest marks:

#include <iostream>
#include <string>
#include <set> 

using namespace std;

int main ()
{
  multiset<int> marks = {410, 450, 465, 290, 410, 450};

   cout << "Marks" << '\n';
   cout<<"______________________\n";
   
  multiset<int>::reverse_iterator rit;
  for (rit=marks.rbegin(); rit!=marks.rend(); ++rit)
    cout << *rit<< '\n';

    auto ite = marks.rbegin();
 
    cout << "\nHighest Marks is: "<< *ite <<" \n";

  return 0;
  }

Output:

Marks
______________________
465
450
450
410
410
290

Highest Marks is: 465

In the above example, a multiset marks is implemented where the marks are the keys. This enables us to take advantage of the auto sorting in multisets and lets us to identify the highest marks. ,

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