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.

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 types Type1 and Type2 must be such that an object of type T can be implicitly converted to both Type1 and Type2, and an object of type ForwardIt can be dereferenced and then implicitly converted to both Type1 and Type2.
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.

[edit] Return value

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

[edit] 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.

[edit] 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;
}

[edit] 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

[edit] See also

equal_range	returns range of elements matching a specific key (function template)
lower_bound	returns an iterator to the first element not less than the given value (function template)
partition	divides a range of elements into two groups (function template)

Language
Standard library headers
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Technical Specifications