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

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 boundlower_bound(array_name, array_name + array_size, value, comparator_function);
// For upper boundupper_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:
array_name & array_size: The name and size of the array which represents the interval between [start, end).value: Value of the lower_bound()/upper_bound() to be searched in the range.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.
array_name & array_size: The name and size of the array which represents the interval between [start, end).
value: Value of the lower_bound()/upper_bound() to be searched in the range.
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 <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 Codeint 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

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