std::rotate

 

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C library
qsort	bsearch

 

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)
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;elseif(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 sortfor(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

See also

rotate_copy

copies and rotate a range of elements (function template)