C++ Using lower_bound() and upper_bound() methods in Map in STL

Hello Everyone!

In this tutorial, we will learn about the working of the lower_bound() and the upper_bound() methods in a Map in STL in the C++ programming language.

To understand the basic functionality of the Map Container in STL, we will recommend you to visit STL Map Container, where we have explained this concept in detail from scratch.

The lower_bound() method:

The lower_bound() method returns an iterator pointing to the first element which has a value not less than the given value.

The upper_bound() method:

The upper_bound() method an iterator pointing to the first element which has a value greater than the given value.

For a better understanding of its implementation, refer to the well-commented C++ code given below.

Code:

#include <iostream>
#include <bits/stdc++.h>

using namespace std;

int main()
{
    cout << "\n\nWelcome to Studytonight :-)\n\n\n";
    cout << " =====  Program to demonstrate the lower_bound() and upper_bound() in Map, in CPP  ===== \n\n\n";

    //Map declaration (Map with key and value both as integers)
    map<int, int> m;

    //Filling the elements by using the insert() method.
    cout << "Filling the Map with key-value pairs of integers in random order."; //Map automatically stores them in increasing order of keys

    //make_pair() is used to insert a key value pair into the map
    m.insert(make_pair(3, 30));
    m.insert(make_pair(2, 20));
    m.insert(make_pair(5, 50));
    m.insert(make_pair(9, 90));
    m.insert(make_pair(1, 10));

    cout << "\n\nThe number of elements in the Map are: " << m.size();

    cout << "\n\nThe elements of the Map m are: ";

    map<int, int>::iterator i;

    for (i = m.begin(); i != m.end(); i++)
    {
        cout << "( " << i->first << ", " << i->second << " ) ";
    }

    map<int, int>::iterator low, high;

    //lower_bound(x) returns the iterator to the first element that is greater than or equal to element with key x
    low = m.lower_bound(5);

    cout << "\n\nThe lower bound of 5 has key: " << low->first << " and value: " << low->second << ". ";

    low = m.lower_bound(6);

    cout << "\n\nThe lower bound of 6 has key: " << low->first << " and value: " << low->second << ". ";

    //upper_bound(x) returns the iterator to the first element that is greater than element with key x
    high = m.upper_bound(3);

    cout << "\n\nThe upper bound of 3 has key: " << high->first << " and value: " << high->second << ". ";

    high = m.upper_bound(4);

    cout << "\n\nThe upper bound of 4 has key: " << high->first << " and value: " << high->second << ". ";

    cout << "\n\n\n";

    return 0;
}

Output:

We hope that this post helped you develop a better understanding of the concept of the lower_bound() and the upper_bound() methods in the Map Container in STL and its implementation in CPP. For any query, feel free to reach out to us via the comments section down below.

Keep Learning : )