std::lower_bound - cppreference.com

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Elemento definito nell'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.

Parametri

first, last	-	iterators defining the range to examine
value	-	valore per confrontare gli elementi Original: value to compare the elements to The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.
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`.

Valore di ritorno

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

Complessità

Logaritmica della distanza tra first e last

Original:

Logarithmic in the distance between first and last

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Possibile implementazione

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; }

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;
}

Esempio

#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:

Vedi anche


	torna gamma di elementi che corrispondono un tasto specifico Original: returns range of elements matching a specific key The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (funzione di modello) [modifica]

	ritorna un iteratore al primo elemento' maggiore di un certo valore Original: *returns an iterator to the first element greater* than a certain value The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (funzione di modello) [modifica]**