upper_bound in C++

upper_bound is a Standard Template Library (STL) function used to find the first element that is strictly greater than a given value in a sorted range. It is commonly used for efficient searching and range queries in sorted containers.

Example: Basic Usage with Vector

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> v = {10, 20, 30, 40, 50};

    // Finding upper bound for value 30 in vector v
    cout << *upper_bound(v.begin(), v.end(), 30);

    return 0;
}

40

Explanation:

• The vector is already sorted
• upper_bound(30) skips 10, 20, 30
• Returns iterator pointing to 40
• Dereferencing the iterator prints 40

Syntax

iterator upper_bound(iterator first, iterator last, const T& value);

Parameters

• first: Iterator to the first element of the range
• last: Iterator to one past the last element of the range
• value: The value to compare against
• comp (optional): Custom comparison function

Return Value

• Returns an iterator to the first element greater than value
• Returns last if no such element exists

Note: The behavior of std::upper_bound() is undefined if the range is not sorted

The below examples demonstrate some of its common uses.

Example 1: Find Upper Bound in an Array

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    int arr[5] = {10, 20, 30, 40, 50};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Finding upper bound for value 30 in array arr
    cout << *upper_bound(arr, arr + n, 30);

    return 0;
}

40

Explanation: This code uses upper_bound() to find and print the first element in the sorted array that is strictly greater than 30, which is 40.

Example 2: Use upper_bound() with Custom Comparator

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

bool comp(const string &a, const string &b)
{
    return lexicographical_compare(a.begin(), a.end(), b.begin(), b.end(),
                                   [](char c1, char c2) {