CODEWITHSUNDEEP
pythonpython-3.x
Anoni Moose
Anoni Moose

Reputation: 35

Python - Rotations of digits

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?

Upvotes: 2

Views: 4116

Answers (2)

Henry Woody
Henry Woody

Reputation: 15652

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

Upvotes: 1

Jérôme
Jérôme

Reputation: 14664

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.

Upvotes: 4

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