Challenge: Rotate a list by k elements

Write a function that rotates a list by k elements. For example, [1, 2, 3, 4, 5, 6] rotated by two becomes [3, 4, 5, 6, 1, 2].

Try solving this without creating a copy of the list. How many swap or move operations do you need?

Solution

Rotator in rotator.py rotates a list to the left, in place, without creating a copy of it.

• rotate(inputList) shifts every element one position to the left and moves the original first element to the end. The caller's list is mutated and nothing is returned.
• rotateRepeat(inputList, numRotations) calls rotate on the list numRotations times.

Move count

rotate saves the first element, shifts the remaining n - 1 elements one position to the left, and then writes the saved element into the last position — n moves for a list of n elements. rotateRepeat repeats that, so rotating by k costs n * k moves.

Running the demo

python3 main.py

main.py rotates [1, 2, 3, 4, 5, 6] once and prints the list before and after:

Num rotations: 1

Before:
[1, 2, 3, 4, 5, 6]

After:
[2, 3, 4, 5, 6, 1]

Running the tests

The tests are written for pytest and must be run from the repository root, since test_rotator.py and test_main.py import rotator and main as top-level modules and the repository has no packaging or conftest.py to place them on the import path.

python3 -m pytest