Stuck at CS50 Python Week2 pset question "Vanity Plates" - Not sure what went wrong in my code

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04-10-2024

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Stuck on CS50 Python Week 2 "Vanity Plates"? Here's How to Debug Your Code

The CS50 Python Week 2 "Vanity Plates" problem can be tricky. You're tasked with writing a function that checks if a given string is a valid vanity plate. But, if you're stuck, you're not alone! Let's break down common issues and how to fix them.

The Problem:

You need to write a function is_valid(s) that takes a string s as input and returns True if s is a valid vanity plate according to the following rules:

• The plate must start with at least two letters.
• The plate can contain a maximum of 6 characters.
• The plate must contain only letters and numbers.
• Numbers cannot appear before the letters.
• The plate cannot contain any spaces or punctuation.

Here's a common approach:

def is_valid(s):
  if len(s) < 2 or len(s) > 6:
    return False

  for char in s:
    if not char.isalnum():
      return False
    if char.isdigit() and not s.startswith(char):
      return False
    if char.isdigit() and not s[0].isalpha():
      return False

  return True

Let's analyze:

1. Length Check: The first condition checks if the plate is within the valid length range.
2. Character Validation: The loop iterates through each character in the plate.
   • isalnum(): This checks if each character is alphanumeric (letter or number).
   • isdigit(): This checks if the character is a number.
   • startswith(): This verifies that the plate starts with a letter if a number is present.
   • isalpha(): This ensures that the first character is a letter if a number is present.

Debugging Tips:

• Test with Example Plates: Create a list of valid and invalid plates and test your code against them. This helps identify specific cases that might be causing issues.
• Print Statements: Use print statements inside your loop to see the value of char and the current state of the plate. This can reveal unexpected behavior.
• Break Down Logic: If you're struggling with the logic, break the function into smaller, more manageable parts. Focus on validating the length, character type, and number placement individually.

Common Errors:

• Incorrect Length Check: Make sure your length check considers the minimum and maximum lengths correctly.
• Missing isalnum() Check: Ensure you are validating all characters to ensure they are only letters and numbers.
• Incorrect Number Placement Check: Verify that the number placement logic is correctly implemented, making sure numbers don't appear before letters.

Example:

>>> is_valid("CS50")
True
>>> is_valid("CS50P")
False 
>>> is_valid("CS 50")
False
>>> is_valid("CS!50")
False

Remember: Practice and experimentation are key. Understand the problem and its rules, break it down into smaller steps, test your code thoroughly, and don't be afraid to seek help if needed. Good luck!