STL_map——map::equal_range

Reference:
Returns a pair of iterators respectively to the first element in a map with a key that is greater than a specified key and to the first element in the map with a key that is equal to or greater than the key.

Function:
pair equal_range (
   const Key& _Key
) const;
pair equal_range (
   const Key& _Key
) const;

Parameter:
_Key:The argument key value to be compared with the sort key of an element from the map being searched.

Return Value:
A pair of iterators such that the first is the lower_bound of the key and the second is the upper_bound of the key.
To access the first iterator of a pair pr returned by the member function, use pr.first, and to dereference the lower bound iterator, use *(pr.first). To access the second iterator of a pair pr returned by the member function, use pr.second, and to dereference the upper bound iterator, use *(pr.second).

Example:
#include 
#include 

int main( )
{
   using namespace std; 
   typedef map  > IntMap;
   IntMap m1;
   map  :: const_iterator m1_RcIter;
   typedef pair  Int_Pair;

   m1.insert ( Int_Pair ( 1, 10 ) );
   m1.insert ( Int_Pair ( 2, 20 ) );
   m1.insert ( Int_Pair ( 3, 30 ) );

   pair  p1, p2;
   p1 = m1.equal_range( 2 );

   cout << "The lower bound of the element with "
        << "a key of 2 in the map m1 is: "
        << p1.first -> second << "." << endl;

   cout << "The upper bound of the element with "
        << "a key of 2 in the map m1 is: "
        << p1.second -> second << "." << endl;

   // Compare the upper_bound called directly 
   m1_RcIter = m1.upper_bound( 2 );

   cout << "A direct call of upper_bound( 2 ) gives "
        << m1_RcIter -> second << "," << endl
        << " matching the 2nd element of the pair"
        << " returned by equal_range( 2 )." << endl;

   p2 = m1.equal_range( 4 );

   // If no match is found for the key,
   // both elements of the pair return end( )
   if ( ( p2.first == m1.end( ) ) && ( p2.second == m1.end( ) ) )
      cout << "The map m1 doesn't have an element "
           << "with a key less than 40." << endl;
   else
      cout << "The element of map m1 with a key >= 40 is: "
           << p1.first -> first << "." << endl;
}

Output:
The lower bound of the element with a key of 2 in the map m1 is: 20.
The upper bound of the element with a key of 2 in the map m1 is: 30.
A direct call of upper_bound( 2 ) gives 30,
 matching the 2nd element of the pair returned by equal_range( 2 ).
The map m1 doesn't have an element with a key less than 40.

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