Python - Rotations of digits

Question

I'm trying to create code that will take a given integer, then output a set of integers, all of which are rotations of digits of the input integer.

Thus, if I inputted '197', the output should be '197', '971', and '791'.

However, when I try:

def rotation(n):
  rotations = set()
  for i in range( len( str(n) ) ):
    n = int( str(n)[i:] + str(n)[:i] )
    rotations.add(n)

with the input '197', it only returns '197', '971', yet NOT '791'.

Why is this happening?

Either reassign n (and work with element at index 0), or work with the index i (and assign the result to a temp variable. Don't do both. Write what you are trying to do on a piece of paper, you''ll understand. :) — UltraInstinct
– UltraInstinct, Commented Sep 22, 2018 at 22:30
@YunkaiXiao right. This is an error in the OP I didn't notice. — Jérôme
– Jérôme, Commented Sep 23, 2018 at 6:16

Jérôme · Accepted Answer · 2018-09-22 22:37:28Z

4

You're almost there. Except you're erasing the input n at each iteration. Use another variable name.

def rotation(n):
  rotations = set()
  for i in range( len( str(n) ) ):
    m = int( str(n)[i:] + str(n)[:i] )
    rotations.add(m)
  return rotations

print(rotation(197))

I would write it more like this, using a set comprehension:

def rotation(number):
    str_number = str(number)
    return {
        int( str_number[i:] + str_number[:i] )
        for i in range(len(str_number))
    }

Solution 2 by @Henry Woody is nice too. Rather than rotate input string by i at each iteration, rotate by 1 from last iteration.

edited Sep 22, 2018 at 22:37

answered Sep 22, 2018 at 22:34

Jérôme

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2 Comments

Sheldore Over a year ago

Haha, I was about to post this answer with the only difference that I used x instead of m :)

N Chauhan Over a year ago

(Or get rid of the variable altogether)

Henry · Accepted Answer · 2018-09-22 22:45:19Z

The way you have your code structured does duplicate rotation because you reassign n on each step of the loop and use the iteration variable i in your slices.

So the process in the loop from your example is:

i = 0
- You have n = 197 and the rotation logic does nothing with i = 0 so you add 197 to rotations
i = 1
- Again you have n = 197, and rotation logic makes n = 971 and you add that to rotations.
i = 2
- Now n = 971, and the rotation logic slices from index 2, but n has already been rotated so we have n = 197 again, which is added to rotations (and removed since rotations is a set). Basically n has already been rotated forward, and now it is being rotated forward 2 steps (back to the initial value and skipping over n = 719)

To fix this you can either:

1. Keep n at its initial value and on each step rotate n the full amount (i) and add that to rotation without modifying n. Like so:

def rotation(n):
  rotations = set()
  for i in range( len( str(n) ) ):
    rotations.add(int( str(n)[i:] + str(n)[:i] ))
  return rotations

2. Rotate n forward on each step, but only rotate it forward one position each time. Like so:

def rotation(n):
  rotations = set()
  for i in range( len( str(n) ) ):
    n = int( str(n)[1:] + str(n)[:1] )
    rotations.add(n)
  return rotations

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