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std::upper_bound

From cppreference.com
< cpp‎ | algorithm
 
 
 
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 criteria, 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.

Contents

[edit] 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.

[edit] Return value

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

[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>::distance_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>::distance_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

#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] Complexity

logarithmic in the distance between first and last

[edit] See also

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