std::rotate (3) - Linux Manuals

std::rotate: std::rotate

NAME

std::rotate - std::rotate

Synopsis

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

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:
// 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