Implementation of lower_bound() and upper_bound() in List of Pairs in C++

In this article, we will discuss the implementation of the lower_bound() and upper_bound() in a list 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 List of Pairs lower_bound() for pair(x, y) will return an iterator pointing to the position of pair whose the first value is greater than or equals x and the second value is greater than equals to y.
  If the above-mentioned criteria are not met, then it returns an iterator to the index which out of the List of pairs.
• 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 List of Pairs upper_bound() for pair(x, y) will return an iterator pointing to the position of pair whose the first value is greater than x and the second value is greater than y.
  If the above-mentioned criteria are not met, then it returns an iterator to the index which out of the list of pairs.

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

Program:

C++

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

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

// Function to implement lower_bound()
void findLowerBound(
    list<pair<int, int> >& list,
    pair<int, int>& p)
{
    // Given iterator points to the
    // lower_bound() of given pair
    auto low = lower_bound(list.begin(),
                           list.end(), p);

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

// Function to implement upper_bound()
void findUpperBound(
    list<pair<int, int> >& list,
    pair<int, int>& p)
{
    // Given iterator points to the
    // upper_bound() of given pair
    auto up = upper_bound(list.begin(),
                          list.end(), p);

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

// Driver Code
int main()
{
    list<pair<int, int> > list;

    // Given sorted List of Pairs
    list.push_back(make_pair(1, 3));
    list.push_back(make_pair(1, 7));
    list.push_back(make_pair(2, 4));
    list.push_back(make_pair(2, 5));
    list.push_back(make_pair(3, 8));
    list.push_back(make_pair(8, 6));

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

    // Function Call to find lower_bound
    // of pair p in arr
    findLowerBound(list, p);

    // Function Call to find upper_bound
    // of pair p in arr
    findUpperBound(list, p);

    return 0;
}

Output:

lower_bound() for {2, 5} is at index: {2, 5 }
upper_bound() for {2, 5} is at index: {3, 8 }