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Implementation of lower_bound() and upper_bound() in Vector of Pairs in C++
• Difficulty Level : Basic
• Last Updated : 29 Nov, 2020

In this article we will discuss the implementation of the lower_bound() and upper_bound() in vector 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 Vector 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 vectors 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 Vector 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 vectors of pairs.

Below is the program to demonstrate lower_bound() and upper_bound() in vector of pairs:
Program 1:

## CPP

 `// C++ program to demonstrate lower_bound()``// and upper_bound() in Vectors of Pairs` `#include ``using` `namespace` `std;` `// Function to implement lower_bound()``void` `findLowerBound(vector >& arr,``                    ``pair<``int``, ``int``>& p)``{``    ``// Given iterator points to the``    ``// lower_bound() of given pair``    ``auto` `low = lower_bound(arr.begin(), arr.end(), p);` `    ``cout << ``"lower_bound() for {2, 5}"``         ``<< ``" is at index: "` `<< low - arr.begin() << endl;``}` `// Function to implement upper_bound()``void` `findUpperBound(vector >& arr,``                    ``pair<``int``, ``int``>& p)``{``    ``// Given iterator points to the``    ``// upper_bound() of given pair``    ``auto` `up = upper_bound(arr.begin(), arr.end(), p);` `    ``cout << ``"upper_bound() for {2, 5}"``         ``<< ``" is at index: "` `<< up - arr.begin() << endl;``}` `// Driver Code``int` `main()``{``    ``// Given sorted vector of Pairs``    ``vector > arr;``    ``arr = { { 1, 3 }, { 1, 7 }, { 2, 4 },``            ``{ 2, 5 }, { 3, 8 }, { 8, 6 } };` `    ``// Given pair {2, 5}``    ``pair<``int``, ``int``> p = { 2, 5 };` `    ``// Function Call to find lower_bound``    ``// of pair p in arr``    ``findLowerBound(arr, p);` `    ``// Function Call to find upper_bound``    ``// of pair p in arr``    ``findUpperBound(arr, p);` `    ``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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