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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 LegacyForwardIterator.
-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 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

See also the implementations in libstdc++ and libc++.

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 read      = n_first;
   ForwardIt write     = first;
   ForwardIt next_read = first; // read position for when "read" hits "last"
 
   while(read != last) {
      if(write == next_read) next_read = read; // track where "first" went
      std::iter_swap(write++, read++);
   }
 
   // rotate the remaining sequence into place
   (rotate)(write, next_read, last);
   return write;
}

Example

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

#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