Why is my list getting sorted when I am sorting a list that's equivalent? Python

Question

I am trying to set a list = to another list and then sort the new list using .sort(). However, when I print them out to make sure they stored properly, both lists got sorted when I only set the new list to be. Why is this happening? The first print statement is printing the correct unsorted list, the third statement is correctly printing the new sorted list, but the fourth print statement is supposed to be printing the unsorted list but is printing a sorted list.

def twoSum(self, nums: List[int], target: int)->List[int]:

        print("original list", nums)
        sortedList = nums
        sortedList.sort()
        subset = []
        answer = []
        middle = math.floor((len(nums)/2))
        print("middle is ", middle)
        print("sorted list", sortedList)
        print("unsorted list", nums)

after you do `sortedList = nums` you have two names for the same list, so it doesn't matter which you sort. since there's still only one list, it's sorted. Make a copy instead, or use `sorted()` to make a copy and sort at the same time. — kindall, Feb 02 '22 at 19:14

score 1 · Answer 1 · answered Feb 02 '22 at 19:15

1

When you assign sortedList = nums you are merely copying a reference to nums. Thus if you sort nums, sortedList will refer to that same sorted list. If you don't want that to happen, take a copy - e.g., sortedList = nums.copy()

answered Feb 02 '22 at 19:15

DarkKnight

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Oh I never knew that, thank you a lot for the help – Orangutan Feb 02 '22 at 19:16
`nums.copy()`? that's a new one on me, and I've been using Python for years! – kindall Feb 02 '22 at 19:16
@kindall copy() has been available as a list builtin since Python 3. Of course you can still do *sortedList = nums[:]* if you need backward compatibility – DarkKnight Feb 02 '22 at 19:26
Yeah, it never occurred to me to go looking for a `copy()` method because the existing methods are satisfactory. :-) – kindall Feb 02 '22 at 22:22

score 1 · Answer 2 · answered Feb 02 '22 at 19:46

1

You can also use the builtin sorted() function which returns a new, sorted list.

list1 = [3, 2, 1]
list2 = sorted(list1)

answered Feb 02 '22 at 19:46

Cargo23

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I didn't even realize that I could do that, thank you for your feedback – Orangutan Feb 03 '22 at 04:43

score 0 · Accepted Answer · answered Feb 02 '22 at 19:15

0

That is because sortedList = nums means both lists point to the same memory location, this is called assignment by reference, if you want to copy the values only you can do as this thread suggests:

sortedList = nums[:]

answered Feb 02 '22 at 19:15

Hasan Aga

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Why is my list getting sorted when I am sorting a list that's equivalent? Python

3 Answers3