Why I am out of range by using x,y = y,x in python

Question

The request is to rotate a list. When K=3, you turn [1,2,3,4,5,6,7] into [5,6,7,1,2,3,4]; when K=1, you turn [1,2,3,4,5,6,7] into [7,1,2,3,4,5,6]

Why am I out of range? PyCharm informed me like this: IndexError: list index out of range

class Solution:
 def rotate(self, nums, k) -> None:
  k = k%len(nums)
  def rev(x, y, num):
      while y > x:
         num[x], num[y] = num[y], num[x]
         x += 1
         y -= 1
      rev(0, len(nums), nums)
      rev(0, k-1,nums)
      rev(k, len(nums), nums)
    
nums = [1,2,3,4,5,6,7]
s = Solution()
s.rotate(nums,3)

Answer 1

you're getting an index out of range error because you're trying to access an index in the list that doesn't exist.

Typically, your list nums has 7 elements which means you can do nums[i] if 0<=i<=6.

To fix your code you just need to replace rev(0, len(nums), nums) by rev(0, len(nums)-1, nums)

class Solution:
    def rotate(self, nums, k) -> None:
        k = k % len(nums)

        def rev(x, y, num):
            while y > x:
                num[x], num[y] = num[y], num[x]
                x += 1
                y -= 1
        rev(0, len(nums)-1, nums)
        rev(0, k - 1, nums)
        rev(k, len(nums)-1, nums)


nums = [1, 2, 3, 4, 5, 6, 7]
s = Solution()
s.rotate(nums, 3)

Alternatively, you can use this simpler implementation which returns another list:

class Solution:
    def rotate(self, nums, k) -> list:
        k = k % len(nums)

        return nums[-k:] + nums[:-k]


nums = [1, 2, 3, 4, 5, 6, 7]
s = Solution()
nums = s.rotate(nums, 3)

Answer 2

You have a off by one error as the indexing starts at 0 ends at len(list)-1. You call rev like this:

rev(0, len(nums), nums)

correct would be:

rev(0, len(nums)-1, nums)

Furthermore, due to the ability to add lists and index lists with negative indices in python you can also solve the problem this way:

nums = [1,2,3,4,5,6,7]

def rotate_list(list_to_rotate, k):
  return list_to_rotate[-k:] + list_to_rotate[:-k]

rotate_list(nums, 3)
# output: [5, 6, 7, 1, 2, 3, 4]