Why is [:] needed in python array when manipulating it? (rotation problem)-CodePudding

Why is the [:] needed in the array. I understand that slicing using [:] creates a copy but why is the copy needed here? Also I tried creating copy with a temp variable which didn't work. This is for array rotation problem.

class Solution:
    def rotate(self, nums, k):
      k=k%len(nums)
      g = len(nums) -k
      nums[:] = nums[g:]   nums[:g]

A=Solution()
MyList=[1,2,3,4]

A.rotate(MyList,2)
assert (MyList == [3, 4, 1, 2])

CodePudding user response：

After the assignment to MyList, the list [1,2,3,4] is created and MyList contains a reference to it.

When you call the function rotate() with MyList passed in as the nums argument, nums contains a reference to the same list [1,2,3,4] that MyList refers to.

If you want to modify that list within the rotate() function, you can do something like nums[0] = 99 or nums[1:2] = [33,66] or nums[:] = nums[g:] nums[:g] and these will act on the same list that was created in the original assignment to MyList.

However, if instead you were to write nums = nums[g:] nums[:g] inside the rotate() function, this would simply update nums to now be a reference to a brand new list (the result of the operation on nums[g:] and nums[:g]) which is completely different from the object that MyList refers to.

And this is the reason that the [:] syntax is needed on the left-hand side of the assignment to nums[:].

CodePudding user response：

The reason is because, inside of the function definition, 'nums' is a local variable that references whatever object was passed in as a function argument. Reassigning that variable does not change the object passed in, only the reference.

What the code:

templist = nums[g:]   nums[:g]
nums = templist

does is assign the local variable 'nums', inside of rotate, to point towards the templist object. The object passed in is unchanged.

In the case of the working code, [:] is NOT creating a copy of the list. Instead it is slice notation, telling the 'num' object to loop through its values and assign them to the values gives on the right of the = sign. It is mostly equivalent to:

templist = nums[g:]   nums[:g]
for i in range(len(templist)):
    nums[i] = templist[i]

CodePudding user response：

Well, the answer is: you don't! nums[:] = ... means that you are assigning everything in nums to something. Line five is the same as

    nums = nums[g:]   nums[:g]

don't forget to write

    return nums

at the end of the function to return nums.