Codility Lesson 2: Arrays CyclicRotation (Solution in Python)

Question: Given a list of length N, print the elements of the list A after K right cyclic rotations

Description of Input Data: 

1. N,K are integers in the range of [0,100]

2. Elements in A lie in the range of [-1000,1000]

Example: 

Input Data: A=[3,8,9,7,6], N=5, K=3

Expected output: [9,7,6,3,8]

Logic:

1. Elements in the list A need to be reordered according to the value of K and length of list A

2. K elements from the end of the list are the first to appear

3. The remaining elements in the same order are placed after the K elements from the end of the list

Psedo Code:

1. Check the length of the input list( length = 0 or length > K or length < 0)

2. If length = 0 output is []

3. If length > K: then the output is concatination of A[-K:],A[:-K]

4. If length < K: then the output is concatination of A[-(K%length) :], A[: -(K%length)]

https://codility.com/programmers/lessons/2-arrays/cyclic_rotation/

Let us use some test cases to understand the functioning of the code:
Test Case 1:
When length of the input list is less than K
Input List: A = [3,8,9,7,6], K=3
Step 1: Compare length of the list to value of K
Length of A = 5 < K=3
Step 2: Building the output which is concatenation of A[-K : ],A[ : -K]

Using this picture here, which represents how indices in python are for accessing elements are defined, the solution can be easily identified.
A[-K : ] = A[-3 : ] = [9,7,6] and A[ : -K] = A[ : -3] = [3,8]
Concatenation of A[-K : ] and A[ : -K] = Concatenation of [9,7,6] and [3,8] = [9,7,6,3,8]

Now since we know how to compute the solution, let us see how this is the solution to cyclic rotation of the list.
When K=1 the cyclic rotation of the list [3,8,9,7,6] = [6,3,8,9,7] - last element of the list is the first element after the rotation
When K = 2 the cyclic rotation of the list [3,8,9,7,6] = [7,6,3,8,9] - the last element of K=1 cyclic rotation output is the first element on this list
Similarly when K=3 the cyclic rotation of the list [3,8,9,7,6] = [9,7,6,3,8]

Test Case 2: When the length of the list is greater than value of K
From the understanding of cyclic rotation it is evident that once the value of K = length of the list the list will remain unchanged. We will use this fact to solve this test case.
Input List: A = [3,8,9,7,6] and K=18
Step 1: Compare length of list A to value of K
Length of A = 5 < K=18
Step 2: Compute the solution, which is concatenation of A[-(K%len(A)):] and A[:-(K%len(A))]
K%len(A) = 18%5 = 3
This means A[-(K%len(A)):] and A[:-(K%len(A))] = A[-(3):] and A[:-(3)]
So the solution as discussed above will be [9,7,6,3,8]
Logically speaking with K=length of the A or multiples of length of A the cyclic rotation output becomes the input list A itself hence for all values of K > length A finding the cyclic rotation output of K%len(A) is equvalent to the finding the cyclic rotation value of K > len(A).

PS: You can also access this code @
https://github.com/samhithachalla/codilitysolutions/blob/codility-python-solutions-with-%3C100-score/CyclicRotation.py

How did you like my solution? Do you have a better and elegant way of completing this codility test? Is there a way I can improve my articles? Comment and let me know. Hope I helped a bit in your preparation. Looking forward to your next visit. :) Like always..Dont forget to like, share and subscribe.