std::ranges::rotate
来自 cppreference.cn
| 定义于头文件 <algorithm> |
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| 调用签名 (Call signature) |
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| template< std::permutable I, std::sentinel_for<I> S > constexpr ranges::subrange<I> |
(1) | (C++20 起) |
| template< ranges::forward_range R > requires std::permutable<ranges::iterator_t<R>> |
(2) | (C++20 起) |
1) 对元素范围执行*左旋转*。具体来说，`ranges::rotate` 会交换范围 `[`first`， `last`)` 中的元素，使得元素 *middle 成为新范围的第一个元素，而 *(middle - 1) 成为最后一个元素。
如果 `[`first`， `last`)` 不是有效范围，或者 middle 不在 `[`first`， `last`)` 中，则行为未定义。
本页描述的类函数实体是 算法函数对象（非正式地称为 niebloids），即
目录 |
[编辑] 参数
| first, last | - | 定义要旋转的元素范围的迭代器-哨兵对 |
| r | - | 要旋转的元素范围 |
| middle | - | 指向旋转后范围开头应出现的元素的迭代器 |
[编辑] 返回值
{new_first, last}，其中 `new_first` 与 ranges::next(first, ranges::distance(middle, last)) 比较相等，并指向 `first` 所指向元素的新位置。
[编辑] 复杂度
最坏情况下为*线性*：ranges::distance(first, last) 次交换。
[编辑] 注意
如果 `I` 建模 `bidirectional_iterator` 或（更好）`random_access_iterator`，`ranges::rotate` 在常见实现中具有更好的效率。
当迭代器类型建模 `contiguous_iterator` 且交换其值类型不调用非平凡特殊成员函数或ADL查找的 `swap` 时，实现（例如 MSVC STL）可能会启用向量化。
[编辑] 可能的实现
另请参阅 libstdc++ 和 MSVC STL 中的实现。
struct rotate_fn { template<std::permutable I, std::sentinel_for<I> S> constexpr ranges::subrange<I> operator()(I first, I middle, S last) const { if (first == middle) { auto last_it = ranges::next(first, last); return {last_it, last_it}; } if (middle == last) return {std::move(first), std::move(middle)}; if constexpr (std::bidirectional_iterator<I>) { ranges::reverse(first, middle); auto last_it = ranges::next(first, last); ranges::reverse(middle, last_it); if constexpr (std::random_access_iterator<I>) { ranges::reverse(first, last_it); return {first + (last_it - middle), std::move(last_it)}; } else { auto mid_last = last_it; do { ranges::iter_swap(first, --mid_last); ++first; } while (first != middle && mid_last != middle); ranges::reverse(first, mid_last); if (first == middle) return {std::move(mid_last), std::move(last_it)}; else return {std::move(first), std::move(last_it)}; } } else { // I is merely a forward_iterator auto next_it = middle; do { // rotate the first cycle ranges::iter_swap(first, next_it); ++first; ++next_it; if (first == middle) middle = next_it; } while (next_it != last); auto new_first = first; while (middle != last) { // rotate subsequent cycles next_it = middle; do { ranges::iter_swap(first, next_it); ++first; ++next_it; if (first == middle) middle = next_it; } while (next_it != last); } return {std::move(new_first), std::move(middle)}; } } template<ranges::forward_range R> requires std::permutable<ranges::iterator_t<R>> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, ranges::iterator_t<R> middle) const { return (*this)(ranges::begin(r), std::move(middle), ranges::end(r)); } }; inline constexpr rotate_fn rotate {}; |
[编辑] 示例
`ranges::rotate` 是许多算法中常见的构建块。此示例演示了插入排序。
运行此代码
#include <algorithm> #include <iostream> #include <numeric> #include <string> #include <vector> int main() { std::string s(16, ' '); for (int k {}; k != 5; ++k) { std::iota(s.begin(), s.end(), 'A'); std::ranges::rotate(s, s.begin() + k); std::cout << "Rotate left (" << k << "): " << s << '\n'; } std::cout << '\n'; for (int k {}; k != 5; ++k) { std::iota(s.begin(), s.end(), 'A'); std::ranges::rotate(s, s.end() - k); std::cout << "Rotate right (" << k << "): " << s << '\n'; } std::cout << "\nInsertion sort using `rotate`, step-by-step:\n"; s = {'2', '4', '2', '0', '5', '9', '7', '3', '7', '1'}; for (auto i = s.begin(); i != s.end(); ++i) { std::cout << "i = " << std::ranges::distance(s.begin(), i) << ": "; std::ranges::rotate(std::ranges::upper_bound(s.begin(), i, *i), i, i + 1); std::cout << s << '\n'; } std::cout << (std::ranges::is_sorted(s) ? "Sorted!" : "Not sorted.") << '\n'; }
输出
Rotate left (0): ABCDEFGHIJKLMNOP Rotate left (1): BCDEFGHIJKLMNOPA Rotate left (2): CDEFGHIJKLMNOPAB Rotate left (3): DEFGHIJKLMNOPABC Rotate left (4): EFGHIJKLMNOPABCD Rotate right (0): ABCDEFGHIJKLMNOP Rotate right (1): PABCDEFGHIJKLMNO Rotate right (2): OPABCDEFGHIJKLMN Rotate right (3): NOPABCDEFGHIJKLM Rotate right (4): MNOPABCDEFGHIJKL Insertion sort using `rotate`, step-by-step: i = 0: 2420597371 i = 1: 2420597371 i = 2: 2240597371 i = 3: 0224597371 i = 4: 0224597371 i = 5: 0224597371 i = 6: 0224579371 i = 7: 0223457971 i = 8: 0223457791 i = 9: 0122345779 Sorted!
[编辑] 另请参阅
| (C++20) |
复制并旋转一个范围的元素 (算法函数对象) |
| (C++20) |
反转一个范围中元素的顺序 (算法函数对象) |
| 旋转一个范围中元素的顺序 (函数模板) |
