std::upper_bound

 

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Technical Specifications

 

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C library
qsort	bsearch

 

Defined in header <algorithm>
	(1)
template< class ForwardIt, class T > ForwardIt upper_bound( ForwardIt first, ForwardIt last, const T& value );			(until C++20)
template< class ForwardIt, class T > constexpr ForwardIt upper_bound( ForwardIt first, ForwardIt last, const T& value );			(since C++20)
		(2)
template< class ForwardIt, class T, class Compare > ForwardIt upper_bound( ForwardIt first, ForwardIt last, const T& value, Compare comp );				(until C++20)
template< class ForwardIt, class T, class Compare > constexpr ForwardIt upper_bound( ForwardIt first, ForwardIt last, const T& value, Compare comp );				(since C++20)

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 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.

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	-	binary predicate which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the predicate function should be equivalent to the following:  bool pred(const Type1 &a, const Type2 &b); The signature does not need to have const &, but the function 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.
- Compare must meet the requirements of BinaryPredicate. it is not required to satisfy Compare

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 270	C++98	Compare was required to be a strict weak ordering	only a partitioning is needed; heterogeneous comparisons permitted

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)