map equal_range() in C++ STL

The map::equal_range() is a built-in function in C++ STL which returns a pair of iterators. The pair refers to the bounds of a range that includes all the elements in the container which have a key equivalent to k. Since the map container only contains unique key, hence the first iterator in the pair returned thus points to the element and the second iterator in the pair points to the next key which comes after key K. If there are no matches with key K and the key K is greater than largest key , the range returned is of length 1 with both iterators pointing to the an element which has a key denoting the size of map and elements as 0. Otherwise the lower bound and the upper bound points to the element just greater than the key K.
Syntax: 

iterator map_name.equal_range(key)

Parameters: This function accepts a single mandatory parameter key which specifies the element whose range in the container is to be returned. 
Return Value: The function returns a pair of iterators as explained above.
Below programs illustrate the above method: 
Program 1: 

CPP

// C++ program to illustrate the
// map::equal_range() function
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // initialize container
    map<int, int> mp;

    // insert elements in random order
    mp.insert({ 4, 30 });
    mp.insert({ 1, 40 });
    mp.insert({ 6, 60 });

    pair<map<int, int>::iterator,
         map<int, int>::iterator>
        it;

    // iterator of pairs
    it = mp.equal_range(1);
    cout << "The lower bound is "
         << it.first->first
         << ":" << it.first->second;

    cout << "\nThe upper bound is "
         << it.second->first
         << ":" << it.second->second;

    return 0;
}

Output:

The lower bound is 1:40
The upper bound is 4:30

Program 2: 

CPP

// C++ program to illustrate the
// map::equal_range() function
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // initialize container
    map<int, int> mp;

    // insert elements in random order
    mp.insert({ 4, 30 });
    mp.insert({ 1, 40 });
    mp.insert({ 6, 60 });

    pair<map<int, int>::iterator,
         map<int, int>::iterator>
        it;

    // iterator of pairs
    it = mp.equal_range(10);
    cout << "The lower bound is "
         << it.first->first << ":"
         << it.first->second;

    cout << "\nThe upper bound is "
         << it.second->first
         << ":" << it.second->second;

    return 0;
}

Output:

The lower bound is 3:0
The upper bound is 3:0