std::lower_bound – cppreference.com

std::lower_bound

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C++
 
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Partitionierungsoperationen
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Partitioning operations
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Sorting operations (on sorted ranges)
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definiert in Header <algorithm>
template< class ForwardIt, class T > ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value );
 (1)
template< class ForwardIt, class T, class Compare > ForwardIt lower_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 not less than value. The first version uses operator< to compare the elements, the second version uses the given comparison function comp.

Parameter

first, last - iterators defining the range to examine
value -
Wert auf die Elemente zu vergleichen
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value to compare the elements to
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comp - comparison function 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 must not modify the objects passed to it.
The type Type1 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type1. The type Type2 must be such that an object of type T can be implicitly converted to Type2. ​

Type requirements
-
ForwardIt must meet the requirements of ForwardIterator.

Rückgabewert

Iterator pointing to the first element that is not less than value, or last if no such element is found.

Komplexität

Logarithmisch im Abstand zwischen first und last
Original:
Logarithmic in the distance between first and last
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Mögliche Implementierung

First version 
template<class ForwardIt, class T>
ForwardIt lower_bound(ForwardIt first, ForwardIt last, const T& value)
{
    ForwardIt it;
    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 (*it < value) {
            first = ++it; 
            count -= step + 1; 
        } else count = step;
    }
    return first;
}
Second version 
template<class ForwardIt, class T, class Compare>
ForwardIt lower_bound(ForwardIt first, ForwardIt last, const T& value, Compare comp)
{
    ForwardIt it;
    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(*it, value)),
            first = ++it;
            count -= step + 1;
        } else count = step;
    }
    return first;
}

Beispiel

#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

Siehe auch