Why does this code works like this?

Question

https://www.sololearn.com/en/compiler-playground/c7v2xI4BjGBI

Why did i need to create a new list to perform comprehension on it, without later calling it, in order to get the desired output?

The task: Given a list of words, use a list comprehension to create a new list that includes only the words that are longer than 4 letters.

Why does printing the newly created list (word4) gives [None, None] as output?

Answer 1

words = ["tree", "button", "cat", "window", "defenestrate"]

# Use a list comprehension to filter out words longer than four letters
[words.remove(w) for w in words if len(w) < 5]
word4 = words

# Display the filtered list
print(words)
print(word4)

In the line, "[words.remove(w) for w in words if len(w) < 5]", it actually edits the list interatively. If you wanted to have a copy of that list in words4, do the assignment after and it works. Honestly you don't need to make a copy of the list. The variable words4 is unnecessary.

Answer 2

"Why does printing the newly created list (word4) gives [None, None] as output?"

The remove() method you used inside the comprehension construction returns nothing (None). That is why you get `None` inside <word4>, for when a method returns nothing, a `None` will be there as you have seen.

The two `None` in <word4> represents "tree" and "cat", two words whose length was less than 5.

Add more short words in <words> and you will see, more `None` will appear in <word4>.

P.S. You don't need to use remove() method, look here...

word5 = [w for w in words if len(w) < 5]

It generates matching data without altering <words>.

Answer 3

the use of:

[words_1.remove(w) for w in words_1 if len(w) < 5]

is not a real comprehension, but it uses the internal for loop of it. comprehensions are designed to create a new list output from an input iterable.

so the task can also be done in a simple `for loop` (3 lines code + output):

words_1 = ["tree", "button", "cat", "window", "defenestrate"]
for word in words_1:
if len(word) < 5:
words_1.remove(word)
print(words_1) # => result will be: ['button', 'window', 'defenestrate']

> an issue that has *NOT* been adressed yet in this code is index shifting when elements are removed in a for loop when iterating from the left side. so in some cases not all elements that match the condition will be removed.

see samples in the attached file:

https://sololearn.com/compiler-playground/ctJlID88z6gh/?ref=app

Answer 4

Lothar '?' counts as multiple characters?

Answer 5

Igor Matić ,

in a string, `?` is a regular character like a letter or a digit. you can try any other character instead.

the issue happens if 2 words < length 5 follow each other.