CODEWITHSUNDEEP
pythonlistloops
Sean
Sean

Reputation: 3385

How to modify list element within a for loop?

I'm trying to modify list elements and replace the original element with the newly modified one. However, I've noticed that the desired behavior differs depending on how I construct my for loop. For example:

samples = ['The cat sat on the mat.', 'The dog at my homework.']
punctuation = ['\'', '\"', '?', '!', ',', '.']

for sample in samples:
    sample = [character for character in sample if character not in punctuation]
    sample = ''.join(sample)

print(samples)

for i in range(len(samples)):
    samples[i] = [character for character in samples[i] if character not in punctuation]
    samples[i] = ''.join(samples[i])

print(samples)

This program outputs:

['The cat sat on the mat.', 'The dog at my homework.']
['The cat sat on the mat', 'The dog at my homework']

The second for loop is the desired output with the punctuation removed from the sentence, but I'm having trouble understanding why that happens. I've searched online and found this Quora answer to be helpful in explaining the technical details, but I'm wondering if it's impossible to modify list elements using the first method of for loops, and if I have to resort to using functions like range or enumerate to modify list elements within loops.

Thank you.

Upvotes: 1

Views: 154

Answers (3)

Achint Sharma
Achint Sharma

Reputation: 365

Try this out:

samples = ['The cat sat on the mat.', 'The dog at my homework.']
punctuation = ['\'', '\"', '?', '!', ',', '.']
new_sample = []

for sample in samples:
    sample = [character for character in sample if character not in punctuation]
    sample = ''.join(sample)
    new_sample.append(sample)
print(new_sample)

In this case, sample is an iterator, not the element of the list, so when you try to modify sample you are not updating the element.

Upvotes: 1

flakes
flakes

Reputation: 23624

You need to replace the item in the list, not update the local variable created by the for loop. One option would be to use a range and update by index.

for i in range(len(samples)):
    sample = [character for character in samples[i] if character not in punctuation]
    samples[i] = ''.join(sample)

That said, a more pythonic approach would be to use a comprehension. You can also use the regex library to do the substitution.

import re
clean_samples = [
    re.sub("['\"?!,.]", "", sample)
    for sample in samples
]

Upvotes: 1

U13-Forward
U13-Forward

Reputation: 71580

Modifying the iterator is not enough,

You need to modify the list as well:

Upvotes: 1

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