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

• Last Updated : 02 Sep, 2020

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 ``using` `namespace` `std;`` ` `// Function to implement lower_bound()``void` `findLowerBound(``    ``list >& 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 >& 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 > 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 }
```

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