For loop and .remove acting differently with same type / data

Question

I want to remove 'canBeGuessed' list items from the 'sortedList' items. The below code does that:

canBeGuessed = ['A', 'B', 'C']
SORTED_FREQUENCIES = 'ZQXJKVBPYGFWMUCLDRHSNIOATE'
sortedList = list(SORTED_FREQUENCIES)

for letter in SORTED_FREQUENCIES:
    if letter not in canBeGuessed:
        sortedList.remove(letter)

print(f"New sorted List: {sortedList}")

Output: New sorted List: ['B', 'C', 'A']

My question is, why doesn't this work when you change SORTED_FREQUENCIES to sortedList in the for loop? 'letter' in the for loop is the same type/output for both variables:

canBeGuessed = ['A', 'B', 'C']
SORTED_FREQUENCIES = 'ZQXJKVBPYGFWMUCLDRHSNIOATE'
sortedList = list(SORTED_FREQUENCIES)

for letter in sortedList:
    if letter not in canBeGuessed:
        sortedList.remove(letter)

print(f"New sorted List: {sortedList}")

Output: New sorted List: ['Q', 'J', 'V', 'B', 'Y', 'F', 'M', 'C', 'D', 'H', 'N', 'O', 'A', 'E']

I've checked the output and type of 'letter' and they are the same when looping over SORTED_FREQUENCIES and sortedList.

In your second code fragment you are modifying a list that you're iterating over. That will invariably not work. You should step through it with your favourite debugger to understand why — DarkKnight, Jul 09 '23 at 18:26

score 1 · Answer 1 · answered Jul 09 '23 at 18:26

here is what is happening

The loop starts with the first element, 'Z', at index 0.
'Z' is not in canBeGuessed, so it should be removed. However, the removal of 'Z' shifts all the remaining elements by one position to the left.
The loop moves to the next element, 'Q', but the current index (1) no longer corresponds to 'Q'. 'Q' was originally at index 1 but got shifted to index 0 due to the removal of 'Z'.
The loop continues with the shifted indices, leading to incorrect comparisons and removals. As a result, elements that should have been removed are skipped.

Thank you so much! I've read all the responses here but can't vote yet. — Noctiscreo, Jul 12 '23 at 12:35

Erich Kitzmueller · Answer 2 · 2023-07-09T19:05:31.020

1

Removing an entry from the list while iterating over it causes some entries to be skipped, since they take the position of the removed entry. Therefore, when you remove Z, Q takes the position of Z and in the next iteration, X instead of Q will be processed.

edited Jul 09 '23 at 19:05

answered Jul 09 '23 at 18:27

Erich Kitzmueller

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Thank you so much! I've read all the responses here but can't vote yet. – Noctiscreo Jul 12 '23 at 12:35

score 1 · Accepted Answer · answered Jul 09 '23 at 18:27

1

You're mutating the list while you're iterating over it. The first approach worked, because SORTED_FREQUENCIES remained unchanged during the loop, while in the second sortedList gets modified. After the first iteration, Z is removed, and the second iteration skips Q, now at index 0, to X, now at index 1.

A quick solution would be to use list slicing to create a copy of the list:

for letter in sortedList[:]:
    if letter not in canBeGuessed:
        sortedList.remove(letter)

answered Jul 09 '23 at 18:27

B Remmelzwaal

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Thank you so much! I've read all the responses here but can't vote yet. – Noctiscreo Jul 12 '23 at 12:35
You should mark the best solution to your problem as an answer :) – B Remmelzwaal Jul 12 '23 at 17:51

For loop and .remove acting differently with same type / data

3 Answers3