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Why does this code works like this?
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?
5 Antworten
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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.
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"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>.
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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
+ 1
Lothar '?' counts as multiple characters?
+ 1
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.