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

In this article we will discuss the implementation of the lower_bound() and upper_bound() in an array 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 Array 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 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 array 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 Array 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 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 array of pairs.

// For lower bound lower_bound(array_name, array_name + array_size, value, comparator_function); // For upper bound upper_bound(array_name, array_name + array_size, value, comparator_function);

1. array_name & array_size: The name and size of the array which represents the interval between [start, end).
2. value: Value of the lower_bound()/upper_bound() to be searched in the range.
3. comparator_function: Binary function that accepts two arguments as its input, namely an element of type pair from the array, and the second is the value for which lower_bound()/upper_bound() has to be found and returns a boolean value.

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

// C++ program to demonstrate lower_bound()
// and upper_bound() in Array of Pairs
#include <bits/stdc++.h>
using namespace std;

// Function to implement lower_bound()
void findLowerBound(pair<int, int> arr[],
                    pair<int, int>& p,
                    int n)
{
    // Given iterator points to the
    // lower_bound() of given pair
    auto low = lower_bound(arr, arr + n, p);

    cout << "lower_bound() for {2, 5}"
         << " is at index: "
         << low - arr << endl;
}

// Function to implement upper_bound()
void findUpperBound(pair<int, int> arr[],
                    pair<int, int>& p,
                    int n)
{
    // Given iterator points to the
    // lower_bound() of given pair
    auto up = upper_bound(arr, arr + n, p);

    cout << "upper_bound() for {2, 5}"
         << " is at index: "
         << up - arr << endl;
}

// Driver Code
int main()
{
    // Given sorted array of Pairs
    pair<int, int> arr[]
        = { { 1, 3 }, { 1, 7 }, { 2, 4 },
            { 2, 5 }, { 3, 8 }, { 8, 6 } };

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

    // Size of array
    int n = sizeof(arr) / sizeof(arr[0]);

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

    // Function Call to find upper_bound
    // of pair p in arr
    findUpperBound(arr, p, n);
    return 0;
}

lower_bound() for {2, 5} is at index: 3
upper_bound() for {2, 5} is at index: 4