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Python Program to Rotate a List

Beginner · Lists

Rotating a list shifts every element along by some number of places, and the elements that fall off one end come back at the other. Rotating [1, 2, 3, 4, 5, 6] left by 2 gives [3, 4, 5, 6, 1, 2], and rotating it right by 2 gives [5, 6, 1, 2, 3, 4].

Program

rotate_a_list.py
numbers = [1, 2, 3, 4, 5, 6]
k = 2 # or: k = int(input("Rotate by how many places? "))
print("Original list:", numbers)
# Rotating by 6 places brings a 6-element list back to the start
k = k % len(numbers)
# Left rotation: the first k elements move to the end
left = numbers[k:] + numbers[:k]
print(f"Rotated left by {k}:", left)
# Right rotation: the last k elements move to the front
right = numbers[-k:] + numbers[:-k]
print(f"Rotated right by {k}:", right)
Output
Original list: [1, 2, 3, 4, 5, 6]
Rotated left by 2: [3, 4, 5, 6, 1, 2]
Rotated right by 2: [5, 6, 1, 2, 3, 4]

How it works

  1. k % len(numbers) keeps k smaller than the length, so rotating by 8 places works the same as rotating by 2.
  2. numbers[k:] is everything from index k to the end, and numbers[:k] is the first k elements; joining them in that order rotates left.
  3. numbers[-k:] is the last k elements, and numbers[:-k] is everything before them; putting the last ones first rotates right.
  4. Slicing makes new lists, so numbers itself is not changed.

Other ways to write it

One step at a time

One step at a time
numbers = [1, 2, 3, 4, 5, 6]
for step in range(1, 3):
first = numbers.pop(0) # take the first element off
numbers.append(first) # and put it on the end
print(f"After step {step}:", numbers)
Output
After step 1: [2, 3, 4, 5, 6, 1]
After step 2: [3, 4, 5, 6, 1, 2]

Each step moves one element from the front to the back, which shows what a left rotation does. It changes the list itself.

Using collections.deque

Using collections.deque
from collections import deque
items = deque([1, 2, 3, 4, 5, 6])
items.rotate(2) # a positive number rotates right
print("Rotated right by 2:", list(items))
items.rotate(-3) # a negative number rotates left
print("Then left by 3:", list(items))
Output
Rotated right by 2: [5, 6, 1, 2, 3, 4]
Then left by 3: [2, 3, 4, 5, 6, 1]

A deque (double-ended queue) has a built-in rotate() that is fast even for long lists.

Try it yourself

Set k = 8 and check that the result is the same as for k = 2.

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