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Python Python Collections (2016, retired 2019) Lists Disemvowel

Posted by plejsyczejsy
plejsyczejsy
10,348 Points

Disemvowel, loop skipping double vowels

Can anyone help me?

disemvowel.py 
vowels = ["A", "E", "O", "I", "U"]

def disemvowel(word):
    word = list(word)
    word_copy = word
    for letter in word_copy:
        if letter.upper() in vowels:
            word.remove(letter)
    word = ''.join(word)
    return word

#looks fine, when I input for example "Warsaw" it outputs "Wrsw", but when I input "Warsaaw" it would give me "Wrsaw"
Ed H-P
Ed H-P
2,728 Points

Hi plejsyczejsy,

It looks like Nader has fixed your problem, but I thought I'd explain why it happens.

When the for loop is iterating through the word list, it's checking if each letter is a vowel. For your 'Warsaw' example above:

  • W (at index 0)
  • ^
  • is not a vowel
  • a (at index 1)
  • ^
  • is a vowel - this is removed

But before the for loop checks the list again, the list is moved 1 place, therefore the next letter checked is s not r. Therefore, in your original code, if you have two vowels side-by-side, this second vowel is skipped, because the for loop thinks it has already checked that index.

Does that make sense?

Nader Alharbi
Nader Alharbi
2,253 Points
Nader Alharbi
Nader Alharbi
2,253 Points

Isn't that happening because you are pointing to the same list with two variables (not really creating two lists)?

Ed H-P
Ed H-P
2,728 Points

Hey Nader,

Are you asking if the vowel skipping is caused by two variables looking at the same list? I don't think that's exactly true. As I understand it, the main issue is that the reference list the for loop iterates through checking for vowels, is the same list it's removing those vowels from. So when it removes a vowel, it thinks that index has already been checked, and so skips the following letter.

You're absolutely right that to avoid the above issue, you need to make a copy of the original list, and use that copy as your list to iterate through.

NB, if either of you didn't know, you can also copy a list to a new variable using [:] 

new_list = old_list[:]

>>> def disemvowel(word):
    word = list(word)
    word_copy = word[:]
    for letter in word_copy:
        if letter.upper() in vowels:
            word.remove(letter)
    return ''.join(word)

>>> disemvowel('Warsaw')
'Wrsw'
>>> disemvowel('Warsaaw')
'Wrsw'
>>>
Nader Alharbi
Nader Alharbi
2,253 Points
Nader Alharbi
Nader Alharbi
2,253 Points

You are absolutely right! Thank you for explaining that.

1 Answer

Nader Alharbi
Nader Alharbi
2,253 Points
Nader Alharbi
Nader Alharbi
2,253 Points

Hello there, Replace this code with the next one

word_copy = word

word_copy = word.copy()

Why is this happening? Honestly, i don't know :)

Nader Alharbi
Nader Alharbi
2,253 Points
Nader Alharbi
Nader Alharbi
2,253 Points

Just tested this,

print(vowels)
print(vowels_2)

vowels_2.append("test")

print(vowels)
print(vowels_2)

Edit: Reason is explained by Ed H-P post.

plejsyczejsy
plejsyczejsy
10,348 Points

Ohh, i didn't know i could do copy with this method. Thanks a lot!

Nader Alharbi
Nader Alharbi
2,253 Points
Nader Alharbi
Nader Alharbi
2,253 Points

It was noted by Craig more than once and i did not give it much attention my self. Looks like he was right after all haha