NAME¶

std::ranges::rotate - std::ranges::rotate

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::permutable I, std::sentinel_for<I> S >

constexpr ranges::subrange<I> (1) (since C++20)

rotate( I first, I middle, S last );
template< ranges::forward_range R >

requires std::permutable<ranges::iterator_t<R>> (2) (since C++20)
constexpr ranges::borrowed_subrange_t<R>

rotate( R&& r, ranges::iterator_t<R> middle );

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).
2) 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 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.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

first, last - the range of elements to rotate
r - the range of elements to rotate
middle - the iterator to the element that should appear at the beginning of the
rotated range

Return value¶

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

Complexity¶

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

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.

Possible implementation¶

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

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 {};

Example¶

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

// Run this code

#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';
}

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!

See also¶

ranges::rotate_copy copies and rotate a range of elements
(C++20) (niebloid)
ranges::reverse reverses the order of elements in a range
(C++20) (niebloid)
rotate rotates the order of elements in a range
(function template)