upper_bound in C++

upper_bound() is a standard library function in C++ defined in the header . It returns an iterator pointing to the first element in the range [first, last] that is greater than value, or last if no such element is found. The elements in the range shall already be sorted or at least partitioned with respect to val. 

Template :

Syntax 1: ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val);

Syntax 2: ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val, Compare comp);

first, last: The range used is [first, last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last. 
val: Value of the upper bound to search for in the range. 
comp: Binary function that accepts two arguments (the first of the type pointed by ForwardIterator, and the second, always val), and returns a value convertible to bool. The function shall not modify any of its arguments. This can either be a function pointer or a function object. 

Return type : An iterator to the upper bound of val in the range. If all the element in the range compare less than val, the function returns last.

Examples :

Input : 10 20 30 30 40 50
Output : upper_bound for element 30 is at index 4

Input : 10 20 30 40 50
Output : upper_bound for element 45 is at index 4

Input : 10 20 30 40 50
Output : upper_bound for element 60 is at index 5

Below are some C++ programs to illustrate the use of std::upper_bound : 

CPP

// CPP program to illustrate using
// std :: upper_bound with vectors
#include <bits/stdc++.h>

// Driver code
int main()
{
    std::vector<int> v{ 10, 20, 30, 40, 50 };

    // Print vector
    std::cout << "Vector contains :";
    for (int i = 0; i < v.size(); i++)
        std::cout << " " << v[i];
    std::cout << "\n";

    std::vector<int>::iterator upper1, upper2;

    // std :: upper_bound
    upper1 = std::upper_bound(v.begin(), v.end(), 35);
    upper2 = std::upper_bound(v.begin(), v.end(), 45);

    std::cout << "\nupper_bound for element 35 is at position : "
              << (upper1 - v.begin());
    std::cout << "\nupper_bound for element 45 is at position : "
              << (upper2 - v.begin());

    return 0;
}

Output:

Vector contains : 10 20 30 40 50
upper_bound for element 35 is at position : 3
upper_bound for element 45 is at position : 4

CPP

// CPP program to illustrate using
// std :: upper_bound with arrays
#include <bits/stdc++.h>
using namespace std;

// Main Function
int main()
{
    int arr[] = { 10, 20, 30, 40, 50 };

    // Print elements of array
    cout << "Array contains :";
    for (int i = 0; i < 5; i++)
        cout << " " << arr[i];
    cout << "\n";

    // using upper_bound
    int upper1 = upper_bound(arr, arr+5, 35) - arr;
    int upper2 = upper_bound(arr, arr+5, 45) - arr;

    cout << "\nupper_bound for element 35 is at position : "
              << (upper1);
    cout << "\nupper_bound for element 45 is at position : "
              << (upper2);

    return 0;
}

Output:

Array contains : 10 20 30 40 50
upper_bound for element 35 is at position : 3
upper_bound for element 45 is at position : 4

Time Complexity : The number of comparisons performed is logarithmic in the distance between first and last. i.e, (at most log2(last – first) + O(1) comparisons).

Important Points

• std::upper_bound() returns an iterator to the upper bound of the value passed to it.
• std::upper_bound() works only with sorted sequences. i.e. vector with sorted elements or array with sorted elements.