std::ranges::rotate
來自 cppreference.com
| 定義於標頭檔案 <algorithm> |
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| 呼叫簽名 (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) 對元素範圍執行*左旋轉*。具體來說，`ranges::rotate` 會交換範圍 `[`first`， `last`)` 中的元素，使得元素 *middle 成為新範圍的第一個元素，而 *(middle - 1) 成為最後一個元素。
如果 `[`first`， `last`)` 不是有效範圍，或者 middle 不在 `[`first`， `last`)` 中，則行為未定義。
本頁描述的類函式實體是 演算法函式物件（非正式地稱為 niebloids），即
目錄 |
[編輯] 引數
| first, last | - | 定義要旋轉的元素範圍的迭代器-哨兵對 |
| r | - | 要旋轉的元素範圍 |
| middle | - | 指向旋轉後範圍開頭應出現的元素的迭代器 |
[編輯] 返回值
{new_first, last}，其中 `new_first` 與 ranges::next(first, ranges::distance(middle, last)) 比較相等，並指向 `first` 所指向元素的新位置。
[編輯] 複雜度
最壞情況下為*線性*：ranges::distance(first, last) 次交換。
[編輯] 注意
如果 `I` 建模 `bidirectional_iterator` 或（更好）`random_access_iterator`，`ranges::rotate` 在常見實現中具有更好的效率。
當迭代器型別建模 `contiguous_iterator` 且交換其值型別不呼叫非平凡特殊成員函式或ADL查詢的 `swap` 時，實現（例如 MSVC STL）可能會啟用向量化。
[編輯] 可能的實現
另請參閱 libstdc++ 和 MSVC STL 中的實現。
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 {}; |
[編輯] 示例
`ranges::rotate` 是許多演算法中常見的構建塊。此示例演示了插入排序。
執行此程式碼
#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'; }
輸出
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!
[編輯] 另請參閱
| (C++20) |
複製並旋轉一個範圍的元素 (演算法函式物件) |
| (C++20) |
反轉一個範圍中元素的順序 (演算法函式物件) |
| 旋轉一個範圍中元素的順序 (函式模板) |
