C++ set equal_range() Function

C++ set equal_range() Function with tutorial for beginners and professionals with examples on constructor, destructor, operator=(), begin(), cbegin(), cend(), end(), crbegin(), empty(), max_size(), clear(), emplace_hint(), key_comp(), swap() etc.

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C++ set equal_range()

C++ set equal_range() function is used to return the boundary of the range containing all elements in the container that are equal to val. Since there is no duplication of values in the set container, this range includes at most one element.

If val does not match any value in the container, the return value range will be length 0 and both iterators will point to the nearest value greater than val. Otherwise, if val is greater than all elements in the container, it points to end.

Syntax

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

The range is defined by two iterators, one points to the first element that is not less than value val and another points to the first element greater than value val.

Parameter

val: value to be searched in the set container.

Return value

This function returns pair. Where pair:: first is at the lower boundary of the range with the same value that lower_bound (val) would return, pair :: second is the same value as the upper_bound (val) would return, upper bound of the range it corresponds to.

Complexity

Logarithmic in size.

Iterator validity

No changes.

Data Races

The container is accessed (neither the const not non-const versions modify the container).

Concurrently accessing the elements of a set is safe.

Exception Safety

If an exception is thrown, there are no changes in the container.

Example 1

Let's see the simple example:

#include <iostream>
#include <set>

using namespace std;

int main(void) {
   set<char> m = {'a','b','c','d'};

   auto ret = m.equal_range('b');

   cout << "Lower bound of b is: " << *ret.first<< endl;

   cout << "Upper bound of b is: " << *ret.second<< endl;

   return 0;
}

Output:

Lower bound of b is: b
Upper bound of b is: c

In the above example, lower bound of b is b and upper bound of b is c.

Example 2

Let's see a simple example:

#include <iostream>
#include <set>

using namespace std;
 
int main()
{
     // initialize container
    set<int> mp;
 
    // insert elements in random order
    mp.insert( 4 );
    mp.insert( 1 );
    mp.insert( 6 );
 
    pair<set<int>::const_iterator,set<int>::const_iterator> ret;
 
    ret = mp.equal_range(10);
    cout << "The lower bound is: " << *ret.first;
    cout << "\nThe upper bound is: " << *ret.second;
 
    return 0;
}

Output:

The lower bound is 3
The upper bound is 3

In the above example, equal_range() function returns to the end() i.e. 3 because it tries to find 10 which is not present in the set mp.

Example 3

Let's see a simple example:

#include <set>  
#include <iostream>  
  
int main( )  
{  
   using namespace std;  
   typedef set<int, less< int > > IntSet;  
   IntSet s1;  
   set <int, less< int > > :: const_iterator s1_RcIter;  
  
   s1.insert( 10 );  
   s1.insert( 20 );  
   s1.insert( 30 );  
  
   pair <IntSet::const_iterator, IntSet::const_iterator> p1, p2;  
   p1 = s1.equal_range( 20 );  
  
   cout << "The upper bound of the element with "  
        << "a key of 20 in the set s1 is: "  
        << *(p1.second) << "." << endl;  
  
   cout << "The lower bound of the element with "  
        << "a key of 20 in the set s1 is: "  
        << *(p1.first) << "." << endl;  
  
   // Compare the upper_bound called directly   
   s1_RcIter = s1.upper_bound( 20 );  
   cout << "A direct call of upper_bound( 20 ) gives "  
        << *s1_RcIter << "," << endl  
        << "matching the 2nd element of the pair"  
        << " returned by equal_range( 20 )." << endl;  
  
   p2 = s1.equal_range( 40 );  
  
   // If no match is found for the key,  
   // both elements of the pair return end( )  
   if ( ( p2.first == s1.end( ) ) && ( p2.second == s1.end( ) ) )  
      cout << "The set s1 doesn't have an element "  
           << "with a key less than 40." << endl;  
   else  
      cout << "The element of set s1 with a key >= 40 is: "  
           << *(p1.first) << "." << endl; 
           
           return 0;
}

Output:

The upper bound of the element with a key of 20 in the set s1 is: 30.
The lower bound of the element with a key of 20 in the set s1 is: 20.
A direct call of upper_bound( 20 ) gives 30,
matching the 2nd element of the pair returned by equal_range( 20 ).
The set s1 doesn't have an element with a key less than 40.

Example 4

Let's see a simple example:

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  std::set<int> myset;

  for (int i=1; i<=5; i++) myset.insert(i*10);   // myset: 10 20 30 40 50

  pair<std::set<int>::const_iterator,set<int>::const_iterator> ret;
  ret = myset.equal_range(30);

  cout << "The lower bound points to: " << *ret.first << '\n';
  cout << "The upper bound points to: " << *ret.second << '\n';

  return 0;
}

Output:

The lower bound points to: 30
The upper bound points to: 40

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C++ set equal_range() Function

C++ set equal_range()

Syntax

Parameter

Return value

Complexity

Iterator validity

Data Races

Exception Safety

Example 1

Example 2

Example 3

Example 4

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