std::ranges::rotate - cppreference.com (original) (raw)

1. Performs a left rotation on a range of elements. Specifically, ranges::rotate swaps the elements in the range [first, last) in such a way that the element *middle becomes the first element of the new range and *(middle - 1) becomes the last element.

The behavior is undefined if [first, last) is not a valid range or middle is not in [first, last).

1. Same as (1), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

{new_first, last}, where _newfirst_ compares equal to ranges::next(first, ranges::distance(middle, last)) and designates a new location of the element pointed by first.

[edit] Complexity

Linear at worst: ranges::distance(first, last) swaps.

[edit] Notes

ranges::rotate has better efficiency on common implementations if I models bidirectional_iterator or (better) random_access_iterator.

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

[edit] Possible implementation

See also the implementations in libstdc++ and MSVC STL.

struct rotate_fn { template<std::permutable I, std::sentinel_for S> constexpr ranges::subrange 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) { ranges::reverse(first, middle); auto last_it = ranges::next(first, last); ranges::reverse(middle, last_it);   if constexpr (std::random_access_iterator) { 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> constexpr ranges::borrowed_subrange_t operator()(R&& r, ranges::iterator_t middle) const { return (*this)(ranges::begin(r), std::move(middle), ranges::end(r)); } };   inline constexpr rotate_fn rotate {};

[edit] Example

ranges::rotate is a common building block in many algorithms. This example demonstrates insertion sort.

#include #include #include #include #include   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'; }

Output:

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!

[edit] See also