C++ multiset - equal_range() Function

The C++ multiset::equal_range function returns the bounds of a range which includes all elements in the multiset container that are equivalent to specified value. It returns a pair, with pair::first member as the lower_bound of the range, and pair::second member as the upper_bound of the range. This contains all elements in the range [pair::first, pair::second).

If no match is found, it will return the range with zero length and both iterators will point to the first element that is considered to go after the specified value.

pair<iterator,iterator> 
  equal_range (const value_type& val) const;

pair<const_iterator,const_iterator> 
  equal_range (const value_type& val) const;

pair<iterator,iterator> 
  equal_range (const value_type& val);

Parameters

val Specify value to compare.

Return Value

Returns a pair, with pair::first member as the lower_bound of the range, and pair::second member as the upper_bound of the range.

Time Complexity

Logarithmic i.e, Θ(log(n))

Example:

In the example below, the multiset::equal_range function returns the bounds of a range for specified value in MSet.

#include <iostream>
#include <set>
using namespace std;
 
int main (){
  multiset<int> MSet{10, 20, 20, 20, 30, 40, 50};
  multiset<int>::iterator it;

  cout<<"MSet contains:";
  for(it = MSet.begin(); it != MSet.end(); ++it)
    cout<<" "<<*it;

  //finding bound range of 20  
  pair<multiset<int>::iterator,
       multiset<int>::iterator> pit;
  pit = MSet.equal_range(20);

  cout<<"\nThe lower bound point is: "<<*pit.first<<"\n";
  cout<<"The upper bound point is: "<<*pit.second<<"\n";

  return 0;
}

The output of the above code will be:

MSet contains: 10 20 20 20 30 40 50
The lower bound point is: 20
The upper bound point is: 30

Example:

Lets consider another example where equal_range() function is used to specify bound range to delete elements from the multiset.

#include <iostream>
#include <set>
using namespace std;
 
int main (){
  multiset<int> MSet{10, 20, 20, 20, 30, 40, 50};
  multiset<int>::iterator it;

  cout<<"MSet contains:";
  for(it = MSet.begin(); it != MSet.end(); ++it)
    cout<<" "<<*it;

  //finding bound range of 20  
  pair<multiset<int>::iterator,
       multiset<int>::iterator> pit;
  pit = MSet.equal_range(20);

  //erasing the elements from the multiset
  MSet.erase(pit.first, pit.second);

  cout<<"\nMSet contains:";
  for(it = MSet.begin(); it != MSet.end(); ++it)
    cout<<" "<<*it;

  return 0;
}

The output of the above code will be:

MSet contains: 10 20 20 20 30 40 50
MSet contains: 10 30 40 50

❮ C++ <set> Library