std::rotate

Defined in header `<algorithm>`
	(1)
template< class ForwardIt > void rotate( ForwardIt first, ForwardIt n_first, ForwardIt last );			(until C++11)
template< class ForwardIt > ForwardIt rotate( ForwardIt first, ForwardIt n_first, ForwardIt last );			(since C++11) (until C++20)
template< class ForwardIt > constexpr ForwardIt rotate( ForwardIt first, ForwardIt n_first, ForwardIt last );			(since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt rotate( ExecutionPolicy&& policy, ForwardIt first, ForwardIt n_first, ForwardIt last );		(2)	(since C++17)

1) Performs a left rotation on a range of elements.

Specifically, std::rotate swaps the elements in the range [first, last) in such a way that the element n_first becomes the first element of the new range and n_first - 1 becomes the last element.

A precondition of this function is that [first, n_first) and [n_first, last) are valid ranges.

2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first	-	the beginning of the original range
n_first	-	the element that should appear at the beginning of the rotated range
last	-	the end of the original range
policy	-	the execution policy to use. See execution policy for details.
Type requirements
- `ForwardIt` must meet the requirements of ValueSwappable and ForwardIterator.
- The type of dereferenced `ForwardIt` must meet the requirements of MoveAssignable and MoveConstructible.

Return value

(none)	(until C++11)
The iterator equal to `first + (last - n_first)`	(since C++11)

Complexity

Linear in the distance between first and last

Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the three standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

template <class ForwardIt>
ForwardIt rotate(ForwardIt first, ForwardIt n_first, ForwardIt last)
{
    if(first == n_first) return last;
    if(n_first == last) return first;

    ForwardIt next = n_first;

    do {
        std::iter_swap(first++, next++);
        if (first == n_first) n_first = next;
    }
    while (next != last);

    ForwardIt ret = first;

    for(next = n_first; next != last; ) {
        std::iter_swap(first++, next++);
        if(first == n_first) n_first = next;
        else if(next == last) next = n_first;
    }

    return ret;
}

Example

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

Run this code

#include <vector>
#include <iostream>
#include <algorithm>

int main()
{
    std::vector<int> v{2, 4, 2, 0, 5, 10, 7, 3, 7, 1}; 

    std::cout << "before sort:      ";
    for (int n: v)
        std::cout << n << ' ';
    std::cout << '\n';

    // insertion sort
    for (auto i = v.begin(); i != v.end(); ++i) {
        std::rotate(std::upper_bound(v.begin(), i, *i), i, i+1);
    }

    std::cout << "after sort:       ";
    for (int n: v)
        std::cout << n << ' ';
    std::cout << '\n';

    // simple rotation to the left
    std::rotate(v.begin(), v.begin() + 1, v.end());

    std::cout << "simple rotate left : ";
    for (int n: v)
        std::cout << n << ' ';
    std::cout << '\n';

    // simple rotation to the right
    std::rotate(v.rbegin(), v.rbegin() + 1, v.rend());

    std::cout << "simple rotate right: ";
    for (int n: v)
        std::cout << n << ' ';
    std::cout << '\n';

}

Output:

before sort:      2 4 2 0 5 10 7 3 7 1 
after sort:       0 1 2 2 3 4 5 7 7 10 
simple rotate left: 1 2 2 3 4 5 7 7 10 0
simple rotate right: 0 1 2 2 3 4 5 7 7 10

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications