String index out of range despite iterating within a range

Question

I am trying to build a strong password checker using Python. The conditions of the password are as follows:

It has at least 6 characters and at most 20 characters.
It must contain at least one lowercase letter, at least one uppercase letter, and at least one digit.
It must NOT contain three repeating characters in a row ("...aaa..." is weak, but "...aa...a..." is strong, assuming other conditions are met).

Write a function strongPasswordChecker(s), that takes a string s as input, and return the MINIMUM change required to make s a strong password. If s is already strong, return 0.

Insertion, deletion or replace of any one character are all considered as one change.

The following is my attempt:

import re

class Solution:
    def strongPasswordChecker(self, s: str) -> int:

        # Holds the change
        change = 0

        # Checks if the password length is less than 6
        if len(s) < 6:
            change += 6 - len(s)

        # Checks if the password length is greater than 20
        elif len(s) > 20:
            change += len(s) - 20

        # Checks if the password has at least one digit
        elif re.search(r'\d', s):
            change += 1

        # Checks if the password has at least one upper case letter
        elif re.search(r'[A-Z]', s):
            change += 1

        # Checks if the password has at least one lower case letter
        elif re.search(r'[a-z]', password):
            change += 1

        # Checks for repeating characters
        for i in range(1, len(s)):
            if i >= 3 and i < len(s):
                if s[i] == s[i + 1] and s[i + 1] == s[i + 2]:
                    change += 1

        return change

Despite checking for the repeating characters with the if statement above, I'm still getting the following error:

IndexError: String Index out of range

On a separate issue: shouldn't all your regex expressions have not infront. For instance shouldn't elif re.search(r'\d', s): be elif not re.search(r'\d', s):? — DarrylG
– DarrylG, Commented Jan 7, 2020 at 16:20

Cory Kramer · Accepted Answer · 2020-01-08 12:24:20Z

4

The problem is this statement can go out of bounds potentially, for example when i == len(s) - 1 then s[i + 1] and s[i + 2] will both index out of bounds.

for i in range(1, len(s)):
    if i >= 3 and i < len(s):
        if s[i] == s[i + 1] and s[i + 1] == s[i + 2]:
            change += 1

If you want to make sure you don't have groups of 3 or longer, I'd use itertools.groupby

>>> any(len(list(g)) > 2 for k, g in groupby('aabbcc'))
False
>>> any(len(list(g)) > 2 for k, g in groupby('aabbbbbcc'))
True

To replace your for loop in your code, you'd use this like

elif any(len(list(g)) > 2 for k, g in groupby(s)):
    change += 1

edited Jan 8, 2020 at 12:24

answered Jan 7, 2020 at 16:12

Cory Kramer

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4 Comments

mchd Over a year ago

Doesn't your code just check only for aabbcc and aabbbbbcc?

Cory Kramer Over a year ago

Obviously that was just an example, you should pass the string s in your case

mchd Over a year ago

How do I use this in an if statement?

Cory Kramer Over a year ago

@OnurOzbek See the edit on the bottom of my answer, you can use this expression as another elif case

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