Python problem 13 of 30

Rotate a list

Python
Easy

Write rotate(xs, k) that returns a new list rotated k steps to the right. k may be larger than the list, and the list may be empty.

rotate([1, 2, 3, 4, 5], 2)   # [4, 5, 1, 2, 3]
rotate([1, 2, 3], 7)         # [3, 1, 2]
rotate([], 4)                # []

Show hint

Rotating by the list length changes nothing, so reduce k with %. Watch out for dividing by zero.

Show solution

def rotate(xs, k):
    if not xs:
        return []
    k %= len(xs)
    return xs[-k:] + xs[:-k]

The last k items move to the front. The empty check avoids k % 0. When k is 0, xs[-0:] is the whole list and xs[:-0] is empty, so the result is still correct.

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