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Implementation of lower_bound() and upper_bound() in Array of Pairs in C++
• Last Updated : 01 Aug, 2020

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.

Syntax:

// 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);

Parameters: The function lower_bound() and upper_bound() in array of pairs accepts the following parameters:

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.

Return Type: It returns an iterator pointing to the first element of the array whose first parameter is greater than or equal to the value.

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

## C++

 `// C++ program to demonstrate lower_bound()``// and upper_bound() in Array of Pairs``#include ``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);`` ` `    ``// 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;``}`
Output:
```lower_bound() for {2, 5} is at index: 3
upper_bound() for {2, 5} is at index: 4
```
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