Recursion removing list elements Python

Question

need to write a recurive function to remove elements of list but keep the arrays in the result such as.

def remove_words(listx):
    for element in listx:
        if isinstance(element, list):
            remove_words(element)
        else:
            listx.remove(element)

    return listx

remove_words(["a", "b", ["c"]]) would return [[]]

My code returns ["b",[]] for some reason it's missing an element.

You remove an element of a list while iterating it. Do not do that. You remove the first element and when the index moves on, the new second element is the formerly third... find a way to avoid that! — user2390182
– user2390182, Commented Jan 9, 2018 at 9:46
You are removing element from list while iterating so just create a copy of list inside function and return that list — Vaseem Ahmed Khan
– Vaseem Ahmed Khan, Commented Jan 9, 2018 at 9:48
Do you want to modify your list in-place, or can you create a new list? — tobias_k
– tobias_k, Commented Jan 9, 2018 at 9:59
Can you provide a code example using this logic please? @tobias_k It doesn't matter, just needs to return proper nested lists. — King
– King, Commented Jan 9, 2018 at 10:03
Why did you specify a recursive solution? Mixing an iterative approach (for) with recursion is odd. If you want a recursive solution, this is not a very good start. — Tim Richardson
– Tim Richardson, Commented Jan 9, 2018 at 10:06

user2390182 · Accepted Answer · 2018-01-09 10:04:39Z

2

Do not remove elements from a collection while iterating it. Repeated calls to list.remove are also not ideal performance-wise since each removal (from a random index) is O(N). A simple way around both issues would be the following comprehension:

def remove_words(listx):
    return [remove_words(e) for e in listx if isinstance(e, list)]

The seemingly missing base case is a list with no nested lists where an empty list is returned.

If you want to modify the list in-place, you can use slice assignment:

def remove_words(listx):
    listx[:] = [remove_words(e) for e in listx if isinstance(e, list)]

edited Jan 9, 2018 at 10:04

answered Jan 9, 2018 at 9:52

user2390182

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englealuze · Accepted Answer · 2018-01-15 15:55:00Z

0

def remove_words(listx):
  if listx == []:
    return []
  elif not isinstance(listx[0], list):
    return remove_words(listx[1:])
  else:
    return [remove_words(listx[0])] + remove_words(listx[1:])

print(remove_words(["a", "b", ["c"]]))
print(remove_words(["a", ["b",["c","d"],"c"], [["f"],["g"]],"h"]))

answered Jan 15, 2018 at 15:55

englealuze

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