Why my set approach works in some cases and failed in others?

July 2, 2022

I tried to solve with set in python .
for most case my code works.

class Solution:
def findWords(self, words: List[str]) -> List[str]:
    set_top = {'q','w','e','r','t','y','u','i','o','p'}
    set_mid = {'a','s','d','f','g','h','j','k','l'}
    set_low = {'z','x','c','v','b','n','m'}
    
    for i in words:
        set_i = set(i.lower())
        
        if set_i-set_top and set_i-set_mid and set_i-set_low :
            words.pop(words.index(i))
    
    return words

I dont know where i went wrong.
refer image for test case failed

Test case 14 failed image

>Solution :

You’re removing items from the same list you’re iterating over.

Better would be to only collect the ones that do match, and return those:

def answer():
    ...
    valid_words = []
    for word in words:
        if ...:
           valid_words.append(word)
    return valid_words

adding a couple of debugging prints shows this more clearly:


In [81]: findWords(['abdfs', 'cccd', 'a', 'qwwewm'])
0: words = ['abdfs', 'cccd', 'a', 'qwwewm'], word=abdfs
removing abdfs: 0
1: words = ['cccd', 'a', 'qwwewm'], word=a
2: words = ['cccd', 'a', 'qwwewm'], word=qwwewm
removing qwwewm: 2
Out[81]: ['cccd', 'a']

It never actually checks cccd because you’ve fiddled with the list context and the iterator skips over it.