Rotation of the numbers-python

Question

To find the rotations of a number I wrote a code like

def rotation(N):
    A=[]
    for i in range(len(N)):
        y=N.pop(0)
        N.append(y)
        A.append(N)
    return A
K=[1,9,7]
r=rotation(K)
print(r)

but it gives me an output like:

A=[[1, 9, 7], [1, 9, 7], [1, 9, 7]]

but it should be

A=[[1,9,7],[9,7,1],[7,1,9]]

and I didnt understand why this happens thanks

Answer 1

Use a simple list slicing:

def rotation(N):
    output = []
    for i in range(len(N)):
        output.append(N[i:] + N[:i])
    return output

K=[1,9,7]
r=rotation(K)
print(r)
# [[1, 9, 7], [9, 7, 1], [7, 1, 9]]

Answer 2

Use collections.deque

You should use collections.deque for this task and use the in-place method deque.rotate designed specifically for this purpose.

Using a list for this task would require expensive copying operations, while deque is optimised for fast addition and removal of elements from the beginning and end of a queue. See TimeComplexity for more details.

from collections import deque

A = deque([1, 9, 7])

for i in range(len(A)):
    print(A)
    A.rotate()

deque([1, 9, 7])
deque([7, 1, 9])
deque([9, 7, 1])

Why your code does not work

The reason your code does not work is because you are modifying the same object rather than a copy. The following will work:

def rotation(N):
    A = []
    for i in range(len(N)):
        N = N[:]
        N.append(N.pop(0))
        A.append(N)
    return A

K = [1,9,7]
r = rotation(K)

print(r)

[[9, 7, 1], [7, 1, 9], [1, 9, 7]]

Further explanation

If you modify the same object, A will consist of 3 lists with each list pointing to the same object and will therefore be guaranteed to be identical. Remember each list is just a bunch of pointers. If each pointer points to one object, changing it 3 times means the final assignment will be used for all the sublists.

Answer 3

def rotation(N):
    return [N[i:] + N[:i] for i in range(len(N))]
K = [1, 9, 7]
print(rotation(K))

Answer 4

I see what you want to do. First of all, here is an improvement: You can spare this variable "y", so like this:

N.append(N.pop(0))

And then, you must copy the list:

A.append(N[:])

so the entire code looks like:

def rotation(N):
A=[]
for i in range(len(N)):
    N.append(N.pop(0))
    A.append(N[:])
return A

K=[1,9,7] r=rotation(K) print(r)

That does work. This is because of how Python manages lists. Hope this helped.