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# std::rotate

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
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)

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.

### [edit] 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
Type requirements
- `ForwardIt` must meet the requirements of `ValueSwappable` and `ForwardIterator`.
- The type of dereferenced `ForwardIt` must meet the requirements of `MoveAssignable` and `MoveConstructible`.

### [edit] Return value

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

### [edit] Complexity

Linear in the distance between `first` and `last`

### [edit] Possible implementation

    template <class ForwardIt>
    void rotate(ForwardIt first, ForwardIt n_first, ForwardIt last)
    {
        ForwardIt next = n_first;
        while (first != next) {
            std::iter_swap(first++, next++);
            if (next == last) {
                next = n_first;
            } else if (first == n_first) {
                n_first = next;
            }
        }
    }

| This section is incomplete Reason: implement C++11 signature

### [edit] 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

### [edit] See also

rotate_copy | copies and rotate a range of elements (function template) [edit]
std::experimental::parallel::rotate (parallelism TS) | parallelized version of `std::rotate` (function template) [edit]

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