Python Array Rotation

Question

So I am implementing a block swap algorithm in python.

The algorithm I am following is this:

Initialize A = arr[0..d-1] and B = arr[d..n-1] 1) Do following u

Answer 1

A simple and shorthand syntax for array rotation in Python is

arr = arr[numOfRotations:]+arr[:numOfRotations]

Example:

arr = [1,2,3,4,5]
rotations = 4
then 

arr = arr[4:]+arr[:4]

gives us

[5,1,2,3,4]

Answer 2

I expect that when you pass a slice of a to your recursive call, you're not passing the same variable any more. Try passing a in its entirety and the upper / lower bounds of your slice as additional arguments to your function.

For instance consider this function:

def silly(z):
  z[0] = 2

I just tried the following:

>>> z = [9,9,9,9,9,9]
>>> silly(z)
>>> z
[2, 9, 9, 9, 9, 9]
>>> silly(z[3:])
>>> z
[2, 9, 9, 9, 9, 9]

Where you can see the modification to the slice was not retained by the full array

Out of curiosity, what outputs do you get & what outputs do you expect?

Answer 3

I found a problem that I needed Right and Left rotations for big values of k (where k is number of rotations), so, I implemented the following functions for any size of k.

Right Circular Rotation (left to the right: 1234 -> 4123):

def right_rotation(a, k):
   # if the size of k > len(a), rotate only necessary with
   # module of the division
   rotations = k % len(a)
   return a[-rotations:] + a[:-rotations]

Left Circular Rotation (right to the left: 1234 -> 2341):

def left_rotation(a, k):
   # if the size of k > len(a), rotate only necessary with
   # module of the division
   rotations = k % len(a)
   return a[rotations:] + a[:rotations]

Sources:

• https://stackoverflow.com/a/46846544/7468664
• https://stackoverflow.com/a/9457923/7468664

Answer 4

Do you actually need to implement the block swap or are you just looking to rotate the array? In python you can do CW and CWW rotations using

zip(*arr[::-1])

and

zip(*arr)[::-1]

Answer 5

def leftRotation():
    
    li = [int(x) for x in input().split()]

    d = int(input())
    ans = (li[d:]+li[0:d])

    for i in ans:
        print(i,end=' ')
    print()
    
leftRotation()

Answer 6

You can rotate a list in place in Python by using a deque:

>>> from collections import deque
>>> d=deque([1,2,3,4,5])
>>> d
deque([1, 2, 3, 4, 5])
>>> d.rotate(2)
>>> d
deque([4, 5, 1, 2, 3])
>>> d.rotate(-2)
>>> d
deque([1, 2, 3, 4, 5])

Or with list slices:

>>> li=[1,2,3,4,5]
>>> li[2:]+li[:2]
[3, 4, 5, 1, 2]
>>> li[-2:]+li[:-2]
[4, 5, 1, 2, 3]

Note that the sign convention is opposite with deque.rotate vs slices.

If you want a function that has the same sign convention:

def rotate(l, y=1):
   if len(l) == 0:
      return l
   y = -y % len(l)     # flip rotation direction
   return l[y:] + l[:y]

>>> rotate([1,2,3,4,5],2)
[4, 5, 1, 2, 3]
>>> rotate([1,2,3,4,5],-22)
[3, 4, 5, 1, 2]
>>> rotate('abcdefg',3)
'efgabcd'

---

For numpy, just use np.roll

>>> a
array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> np.roll(a, 1)
array([9, 0, 1, 2, 3, 4, 5, 6, 7, 8])
>>> np.roll(a, -1)
array([1, 2, 3, 4, 5, 6, 7, 8, 9, 0])

Or you can use a numpy version of the same rotate above (again noting the difference in sign vs np.roll):

def rotate(a,n=1):
    if len(a) == 0:
        return a
    n = -n % len(a)     # flip rotation direction
    return np.concatenate((a[n:],a[:n]))