Remove and add items to iterated lists

Question

I am iterating through two lists like so:

list1 = [z]
list2 = [z-t,z-s,s-a,a-n, g-z]
for e in list1:
 for t in list2:
  # if item from list2 contains item from list1 (before "-")
  # remove the item from list2 and append part after "-" to list1
  # iterate again until no change is made or list2 is empty

The problem I can't solve is that when I remove the item from list2 it goes to next item and I am not able to prevent it. For example

list2 = [z-t,z-s,s-a....]
          ^ removing this while iterating
next iteration jumps over one item
list2 = [z-s, s-a,...]
               ^ this is where I am second iteration
list2 = [z-s, s-a,...]
          ^ this is where I want to be

Is there any way to do this?

**Don't** change the length of the list while iterating over it? Alternatively, manage the current index yourself, and only increment it as required. — jonrsharpe, Apr 15 '15 at 19:27
@Kasra expected output is in tihs case list1 = [z, t, s, a ,n] list2 = [g] — Croolman, Apr 15 '15 at 19:32

score 1 · Accepted Answer · answered Apr 15 '15 at 19:29

1

There are a few ways to achieve this, one is iterating backwards over the list, ie

for e in list1:
    for t in list2[::-1]:  # This iterates over the list backwards
        # Do your check
        # Remove the element

If you need to process it forwards, you can iterate over a copy so you can mutate the original list while iterating over the initial content, ie

for e in list1:
    for t in list2[:]:  # The [:] syntax feeds you your data via a new list.
        # Do your check
        # Remove the element, and not worry about mutating the original list length

answered Apr 15 '15 at 19:29

Christian Witts

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That works! Didn't know about this fancy thing – Croolman Apr 15 '15 at 19:36
More importantly than iterating over the list backwards, the first snippet iterates over a backwards **copy** of the list... – jonrsharpe Apr 15 '15 at 19:38

Remove and add items to iterated lists

1 Answers1