# std::rotate

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< cpp‎ | algorithm

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:

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

### See also

rotate_copy | copies and rotate a range of elements (function template)

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