std::upper_bound

Defined in header `<algorithm>`
template< class ForwardIt, class T > ForwardIt upper_bound( ForwardIt first, ForwardIt last, const T& value );	(1)
template< class ForwardIt, class T, class Compare > ForwardIt upper_bound( ForwardIt first, ForwardIt last, const T& value, Compare comp );	(2)

Returns an iterator pointing to the first element in the range [first, last) that is greater than value.

The range [first, last) must be at least partially ordered, i.e. partitioned with respect to the expression !(value < element) or !comp(value, element). A fully-sorted range meets this criterion, as does a range resulting from a call to std::partition.

The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Parameters

first, last	-	the range of elements to examine
value	-	value to compare the elements to
comp	-	comparison function object (i.e. an object that satisfies the requirements of `Compare`) which returns true if the first argument is less than the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function object must not modify the objects passed to it. The type Type1 must be such that an object of type T can be implicitly converted to Type1. The type Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type2.
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.

Return value

iterator pointing to the first element that is greater than value, or last if no such element is found.

Complexity

The number of comparisons performed is logarithmic in the distance between first and last (At most $log 2 (last - first) + O(1)$ comparisons). However, for non-RandomAccessIterators, the number of iterator increments is linear.

Possible implementation

First version

template<class ForwardIt, class T>
ForwardIt upper_bound(ForwardIt first, ForwardIt last, const T& value)
{
    ForwardIt it;
    typename std::iterator_traits<ForwardIt>::difference_type count, step;
    count = std::distance(first,last);
 
    while (count > 0) {
        it = first; 
        step = count / 2; 
        std::advance(it, step);
        if (!(value < *it)) {
            first = ++it;
            count -= step + 1;
        } else count = step;
    }
    return first;
}

Second version

template<class ForwardIt, class T, class Compare>
ForwardIt upper_bound(ForwardIt first, ForwardIt last, const T& value, Compare comp)
{
    ForwardIt it;
    typename std::iterator_traits<ForwardIt>::difference_type count, step;
    count = std::distance(first,last);
 
    while (count > 0) {
        it = first; 
        step = count / 2;
        std::advance(it, step);
        if (!comp(value, *it)) {
            first = ++it;
            count -= step + 1;
        } else count = step;
    }
    return first;
}

Example

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    std::vector<int> data = { 1, 1, 2, 3, 3, 3, 3, 4, 4, 4, 5, 5, 6 };
 
    auto lower = std::lower_bound(data.begin(), data.end(), 4);
    auto upper = std::upper_bound(data.begin(), data.end(), 4);
 
    std::copy(lower, upper, std::ostream_iterator<int>(std::cout, " "));
}

Output:

4 4 4

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