Implementation of lower_bound() and upper_bound() on Map of Pairs in C++

Prerequisite: map lower_bound() function in C++ STL, map upper_bound() function in C++ STL

In this article, we will discuss the implementation of the lower_bound() and upper_bound() in the Map of pairs.

• lower_bound(): It returns an iterator pointing to the first element in the range [first, last) which has a value greater than or equals to the given value “val”. But in Map of Pairs lower_bound() for pair(x, y) will return an iterator pointing to the pair whose first value is greater than or equals x and second value is greater than equals to y.
  If the above-mentioned criteria are not met, then it returns an iterator which points to the pair {Map.size(), 0}.

  Syntax:

  mp.lower_bound({a, b})
  
  where,
  mp is the map of pairs
  and {a, b} whose lower_bound
  is to be found
  

• upper_bound(): It returns an iterator pointing to the first element in the range [first, last) which has a value greater than the given value “val”. But in Map of Pairs upper_bound() for pair(x, y) will return an iterator pointing to the pair whose first value is greater than x and second value is greater than y.
  If the above-mentioned criteria are not met, then it returns an iterator which points to the pair {Map.size(), 0}.

  Syntax:

  mp.upper_bound({a, b})
  
  where,
  mp is the map of pairs
  and {a, b} whose upper_bound
  is to be found
  

Below is the program to demonstrate lower_bound() and upper_bound() in Map of pairs:

Program 1:

// C++ program to demonstrate lower_bound()
// and upper_bound() in Map of Pairs

#include <bits/stdc++.h>
using namespace std;

// Function to implement lower_bound()
void findLowerBound(
    map<pair<int, int>, int>& mp,
    pair<int, int>& p)
{

    // This iterator points to the
    // lower_bound() of given pair
    auto low = mp.lower_bound(p);

    cout << "lower_bound() for {2, 5}"
         << " is: {"
         << (*low).first.first << ", "
         << (*low).first.second
         << "}" << endl;
}

// Function to implement upper_bound()
void findUpperBound(
    map<pair<int, int>, int>& mp,
    pair<int, int>& p)
{

    // This iterator points to the
    // upper_bound() of given pair
    auto up = mp.upper_bound(p);

    cout << "upper_bound() for {2, 5}"
         << " is: {"
         << (*up).first.first << ", "
         << (*up).first.second
         << "}" << endl;
}

// Driver Code
int main()
{
    // Declare map of Pairs
    map<pair<int, int>, int> mp;

    // Insert Pairs in Map
    mp.insert({ { 2, 3 }, 8 });
    mp.insert({ { 4, 1 }, 5 });
    mp.insert({ { 7, 1 }, 3 });
    mp.insert({ { 9, 3 }, 1 });
    mp.insert({ { 5, 0 }, 3 });

    // Given pair {2, 5}
    pair<int, int> p = { 2, 5 };

    // Function Call to find lower_bound
    // of pair p in map mp
    findLowerBound(mp, p);

    // Function Call to find upper_bound
    // of pair p in map mp
    findUpperBound(mp, p);

    return 0;
}

Output:

lower_bound() for {2, 5} is: {4, 1}
upper_bound() for {2, 5} is: {4, 1}