For loop/if statement skipping elements in list?

Question

I'm just doing a simple python exercise that looks like this:

list1 = [47, 48, 49, 50, 51, 52, 53, 54]
list2 = []

for num in list1:
  if num >= 50:
    list1.remove(num)
    list2.append(num)

print(list1)
print(list2)

List 1 should only have numbers less than 50 and list 2 should have numbers greater than or equal to 50. However, the output says otherwise:

List1: [47, 48, 49, 51, 53]
List2: [50, 52, 54]

How can I fix this?

Do not modify list you are iterating over *(unless you are iterating backwards using indexes)*. — Olvin Roght, Jun 29 '22 at 16:26
You could try to run your program `as it is` in here - https://pythontutor.com/ To see what's happening then. — Daniel Hao, Jun 29 '22 at 16:33

score 0 · Answer 1 · answered Jun 29 '22 at 16:28

0

This is not a good way doing it. you are removing items from list during iteration.

when you remove items from a list during a loop. it will effect the indexing of the list. that is why you see your code jumps through some of items.

it is easier to use list comprehensions:

list1 = [47, 48, 49, 50, 51, 52, 53, 54]


list2 = [i for i in list1 if i>=50]
list1 = [i for i in list1 if i<50]

answered Jun 29 '22 at 16:28

Mehrdad Pedramfar

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3

Actually, you can do it. `list2 = [list1.pop(i) for i in range(len(list1) - 1, -1, -1) if list1[i] >= 50]` – Olvin Roght Jun 29 '22 at 16:31

score 0 · Answer 2 · answered Jun 29 '22 at 16:39

You can also filter,

list1 = [47, 48, 49, 50, 51, 52, 53, 54]

list2 = filter(lambda x: x<50,  list1)   # print(list(list2))
list3 = filter(lambda x: x>=50, list1)   # print(list(list3))

Or use indexing:

import numpy as np 

l1 = np.array(list1)
list2 = l1[l1 <  50]
list3 = l1[l1 >= 50]

For loop/if statement skipping elements in list?

2 Answers2