c – partition_point和lower_bound有什么区别？

C 11包括算法std::partition_point().然而,对于我尝试过的所有情况,它给出与std::lower_bound()相同的答案.唯一的区别是方便的T&值参数.

我错过了什么,或者这两个功能或多或少都做了同样的事情？

解决方法:

它们基本相同.这将是lower_bound的有效实现：

template <typename ForwardIterator, typename T>
ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last,
    T const& value)
{
    return partition_point(first, last, [&](auto&& elem) {
        return elem < value;
    });
}

template <typename ForwardIterator, typename T, typename Compare>
ForwardIterator lower_bound(ForwardIterator first, ForwardIterator last,
    T const& value, Compare comp)
{
    return partition_point(first, last, [&](auto&& elem) {
        return comp(elem, value);
    });
}

这两种算法依赖于查找分区范围的分区点,它们只需要使用不同的参数进行搜索(对于partition_point的一元谓词,对于lower_bound的值或值和二进制谓词).

我们通常只考虑带有二元谓词的排序范围上下文中的lower_bound – 即使相对于这样的谓词的完全排序范围是not a requirement for that algorithm.

当我们在它的时候,upper_bound也可以用partition_point实现,只是操作数被翻转而谓词被否定：

template <typename ForwardIterator, typename T>
ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last,
    T const& value)
{
    return partition_point(first, last, [&](auto&& elem) {
        return !(value < elem);
    });
}

template <typename ForwardIterator, typename T, typename Compare>
ForwardIterator upper_bound(ForwardIterator first, ForwardIterator last,
    T const& value, Compare comp)
{
    return partition_point(first, last, [&](auto&& elem) {
        return !comp(value, elem);
    });
}

这两个措辞有多么不同,这有点奇怪.

lower_bound returns(上限措辞为similar)：

The furthermost iterator i in the range [first, last] such that for every iterator j in the range [first, i) the following corresponding conditions hold: *j < value or comp(*j, value) != false.

而partition_point returns

An iterator mid such that all_­of(first, mid, pred) and none_­of(mid, last, pred) are both true.

这些短语是等价的,因为要求是范围是分区的.但乍看之下肯定不是这样.

标签：c,c11,binary-search,std,partitioning
来源： https://codeday.me/bug/20190910/1801496.html