2

I want my code to remove every single number in a list except for a specific number, which is 3. Instead it removes certain numbers but majority of them still remain in the list.

myList = [0,1,2,3,4,5]
i = 0
for i in myList:
    print(i)
    if i != 3:
        myList.remove(i)
    else:
        continue
    i += 1
    print(myList)
Nimantha
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rondon123
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  • why you are counting i ? you wanna have both indexes and values of your list ? – DRPK Dec 31 '20 at 00:22
  • You are changing the list as you iterate, skipping values. Is it okay to create a new list? Then its `myList = [v for v in myList if v == 3]`. – tdelaney Dec 31 '20 at 00:22
  • I think you are mixing i up with an index. In Python, when you iterate through a for loop, you are iterating over each element. Therefore, there's no reason to increment it, and you don't need to initialize it prior to the loop. – Matthew Kligerman Dec 31 '20 at 00:23
  • The reason this does not work is because in Python, you can't modify a list you are iterating over. That's why @tdelaney reassigned the value. – Matthew Kligerman Dec 31 '20 at 00:26
  • Does this answer your question? [How to remove items from a list while iterating?](https://stackoverflow.com/questions/1207406/how-to-remove-items-from-a-list-while-iterating) – Georgy Dec 31 '20 at 20:12
  • Also: [Strange result when removing item from a list while iterating over it](https://stackoverflow.com/q/6260089/7851470) – Georgy Dec 31 '20 at 20:12

3 Answers3

3

You've got a few issues. First, you're trying to modify the list in place, as you're trying to process it. You can fix that by changing:

for i in myList:

to:

for i in myList[:]:

That will allow your code to give the desired result. It makes a "copy" of the list over which to iterate, so you can modify the original list without messing up your iteration loop.

The other thing to note is that you assign a value to i in the for loop, but then you manually change it after your if-else block. That change gets discarded when you go back to the top of the loop.

Also, you're else: continue prevents incrementing i, but it doesn't matter, because that incremented value was just getting tossed anyway.

So... commenting out some of the unnecessary stuff gives:

myList = [0,1,2,3,4,5]
# i = 0
for i in myList[:]:
    print(i)
    if i != 3:
        myList.remove(i)
    # else:
        # continue
    # i += 1
    print(myList)
Nimantha
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GaryMBloom
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2

You have a couple of problems. First, the for loop iterates the list values, not its index. You can use enumerate to get both the index and the value. Second, if you delete values in a list, its remaining elements are shifted down by 1 but since the iterator also increments by 1, you miss a value. A trick is to iterate the list in reverse so that any deleted values have already been iterated.

>>> myList = [0,1,2,3,4,5]
>>> mlen = len(myList)
>>> for i, v in enumerate(reversed(myList), 1):
...     if v != 3:
...         del myList[mlen-i]
... 
>>> 
>>> myList

But this operation is slow. If you don't need to modify the original list, use a list comprehension

>>> myList = [0,1,2,3,4,5]
>>> myList = [v for v in myList if v==3]
>>> myList
[3]
tdelaney
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1

The issue is you are removing elements from the list you are iterating over. So your loop won't iterate over the entire list.

Andrew Biddle
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