Recursively iterate over decreasing list

Question

I want to recursively iterate over a list without considering the items i'm deleting

Code:

list_example = ['a', 'a', 'a', 'a', '(', 'a', 'a', ')']
for index, item in enumerate(list_example):
    if item == 'a':
        del list_example[index]

Now the removal works but the for loop doesn't know that the items are deleted, how do i make it so that the updated list is considered every time?

Use a while loop instead of a for loop and maintain your own index — Sayse, Oct 27 '20 at 16:00
What is "recursive iteration"? I don't see any recursion here. — user2390182, Oct 27 '20 at 16:04
If your problem is with the index, you could just shift the index by 1 every time you delete an element — Nik, Oct 27 '20 at 16:04

dannyadam · Answer 1 · 2020-10-28T03:22:12.187

1

One way to avoid the issue you've encountered is to iterate over the list in reverse. That way, the current index is independent of deletions for elements with greater indices.

list_example = ['a', 'a', 'a', 'a', '(', 'a', 'a', ')']
for index in reversed(range(len(list_example))):
    if list_example[index] == 'a':
        del list_example[index]

Another way to remove elements, if you don't need to modify the list in-place, is to use a list comprehension combined with a condition (if x != 'a').

list_example = ['a', 'a', 'a', 'a', '(', 'a', 'a', ')']
list_example = [x for x in list_example if x != 'a']

Additionally, Python has a filter function, which takes a function for specifying elements to drop.

list_example = ['a', 'a', 'a', 'a', '(', 'a', 'a', ')']
list_example = list(filter(lambda x: x != 'a', list_example))

edited Oct 28 '20 at 03:22

answered Oct 28 '20 at 02:53

dannyadam

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`reversed(range(len(list_example)))` might be a bit more readable than `range(len(list_example) - 1, -1, -1)` ? – fountainhead Oct 28 '20 at 03:19
@fountainhead, I agree and have updated the answer. Thanks! – dannyadam Oct 28 '20 at 03:22
1

Another way to remove elements, if you **do** need to modify the list in-place, is to use: `list_example[:] = [x for x in list_example if x!= 'a']`. – Mark Tolonen Oct 28 '20 at 15:53

score 0 · Answer 2 · answered Oct 28 '20 at 03:10

Try this:

list_example = ['a', 'a', 'a', 'a', '(', 'a', 'a', ')']
for item in reversed(list_example):
    if (item == 'a'):               # Found an 'a' while going right-to-left
        list_example.remove('a')    # Remove the first 'a' going from left-to-right
print (list_example)

Output:

['(', ')']

Recursively iterate over decreasing list

2 Answers2