# 25 Algorithms library [algorithms]

## 25.7 Mutating sequence operations [alg.modifying.operations]

### 25.7.14 Shift [alg.shift]

```template<class ForwardIterator> constexpr ForwardIterator shift_left(ForwardIterator first, ForwardIterator last, typename iterator_traits<ForwardIterator>::difference_type n); template<class ExecutionPolicy, class ForwardIterator> ForwardIterator shift_left(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, typename iterator_traits<ForwardIterator>::difference_type n); ```
Preconditions: n >= 0 is true.
The type of *first meets the Cpp17MoveAssignable requirements.
Effects: If n == 0 or n >= last - first, does nothing.
Otherwise, moves the element from position first + n + i into position first + i for each non-negative integer i < (last - first) - n.
In the first overload case, does so in order starting from i = 0 and proceeding to i = (last - first) - n - 1.
Returns: first + (last - first - n) if n < last - first, otherwise first.
Complexity: At most (last - first) - n assignments.
```template<class ForwardIterator> constexpr ForwardIterator shift_right(ForwardIterator first, ForwardIterator last, typename iterator_traits<ForwardIterator>::difference_type n); template<class ExecutionPolicy, class ForwardIterator> ForwardIterator shift_right(ExecutionPolicy&& exec, ForwardIterator first, ForwardIterator last, typename iterator_traits<ForwardIterator>::difference_type n); ```
Preconditions: n >= 0 is true.
The type of *first meets the Cpp17MoveAssignable requirements.
ForwardIterator meets the Cpp17BidirectionalIterator requirements ([bidirectional.iterators]) or the Cpp17ValueSwappable requirements.
Effects: If n == 0 or n >= last - first, does nothing.
Otherwise, moves the element from position first + i into position first + n + i for each non-negative integer i < (last - first) - n.
In the first overload case, if ForwardIterator meets the Cpp17BidirectionalIterator requirements, does so in order starting from i = (last - first) - n - 1 and proceeding to i = 0.
Returns: first + n if n < last - first, otherwise last.
Complexity: At most (last - first) - n assignments or swaps.