python 3 How to append items into a list from another list

Question

I'm trying to get each number that is not 6 into a separate list named valid. With my current knowledge with loops, i try to loop over each element in nums and append into 'valid', but it is not returning ALL NUMBERS that are != 6 (Not 6) even though i'm appending first then deleting the value. My current python version is 3.6

def sum6(nums):
    valid = []

    for item in nums:
        if item != 6:
            valid.append(item)
            nums.remove(item)
        else:
            pass
    print(valid)
    print(nums)

sum6([1,2,3,6,4,5])

Prints valid: [1,3,4] Instead of [1,2,3,4,5]
Prints nums:[2,6,5] Instead of [6]

Is `nums.remove(item)` necessary? Without it the code would run fine if you do not need to print a list of 6s at the end of the func — Iain Shelvington, Dec 29 '19 at 11:07
I was trying to just modify the original list which would contain all 6's instead of creating another list. — Lil boat, Dec 29 '19 at 11:18
The problem is that you are removing items from the original list while iterating. See the duplicate for some solutions. — Thierry Lathuille, Dec 29 '19 at 11:21

score 2 · Answer 1 · answered Dec 29 '19 at 11:08

2

Here's one way to do this:

def sum6(nums):
    valid = [x for x in nums if x != 6]
    nums = [x for x in nums if x == 6]
    print(valid)
    print(nums)

>>> sum6([1,2,3,4,5,6])
    [1, 2, 3, 4, 5]
    [6]

answered Dec 29 '19 at 11:08

YOLO

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score 2 · Accepted Answer · answered Dec 29 '19 at 11:10

You are changing the nature of the thing that is being iterated on while it's being iterated on. This produces unexpected results, as you've found out. Make a copy of the item for your for loop instead, and keep the specific object you're iterating on intact for the duration of the loop.

So, e.g., maintaining the logical thinking in your code, you would change it to something like this:

def sum6(nums):
    valid = []
    invalid = []
    for item in nums:
        if item != 6:
            valid.append(item)
        else:
            invalid.append(item)
    print(valid)
    print(invalid)

sum6([1,2,3,6,4,5])

Ohh... I see now. Thanks really appreciate it. – Lil boat Dec 29 '19 at 11:15 — Lil boat, Dec 29 '19 at 11:15

python 3 How to append items into a list from another list

2 Answers2