C50 Credit-Python

Question

Please note that i am a VERY confused BEGINNER but i'm enjoying the course and eager to learn. Below is my code and i'd like to understand a few things:

1. Why if i define main i get multiple errors such as: "name 'card' is not defined", whereas if i don't define main everything runs smoothly?

2. I copied the split_to_digits function and to my knowledge i assumed it returned a list, so i'm trying to loop through the list "everyotherdig" to sum the digits but if i try and print the sum it says "built in function sum"

3. Any other error i surely committed if you'd be so kind as to explain it to me i would be very grateful.

        # get user input 
     
        while True:

          try:
           
            card = int(input("Credit card number:"))''
           
            if card > 0:''
               break
            except ValueError:
             print("INVALID")
    
         #Convert card back into a string
      
       card = str(card)
    
       if len(card) not in (13,15,16):
          
         print("INVALID")
    
       #Store valid prefixes into a tuple
    
       prefixes = ["4","34","37","50","51","52","53","54","55"]
    
       #Check if user input is valid
    
       x = card.startswith(tuple(prefixes))
    
       if x == False:
       
          print("INVALID")
    
       def split_to_digits(card):
    
           return [int(i) for i in str(card)]


       #Store every other digit into a list
    
       everyotherdig = split_to_digits(card[-2::-2])
    
       allotherdig = split_to_digits(card[-1::-2])


       #Loop through string to apply Luhn's algorith
    
       for i in everyotherdig:
          
         if everyotherdig[i] > 5:
             
            sum = split_to_digits(everyotherdig[i]) * 2
          
         else:
             
            sum1 = everyotherdig[i] *2


    print(f"{sum}")

I managed to successfully get user input and get the card digits that i needed. Now i need to apply Luhn's algorithm to calculate whether the card number is valid. In order to do so i need to multiply every other digit by 2, starting with the number’s second-to-last digit, and then add those products’ digits together. Add the sum to the sum of the digits that weren’t multiplied by 2.

Answer 1

Why if i define main i get multiple errors such as: "name 'card' is not defined"

That sounds like a regular global-local variable mistake. In Python, unless you say that a variable is global, when you set a variable Python assumes it is local and not defining it in a function will cause 'not defined' problems. For example;

def func1():
  # These are expected to be local.
  x = 3
  y = 2

def func2():
  # Since you are not setting it, this is interpretted as a global variable
  print(a)

def func3():
  # Since this variable is set inside this function, does not matter before or after its usages, it is interpretted as a local variable. Because of that, this function causes a undefined error.
  print(a)
  a = 3

To declare a variable as global, you use

  global variableName

inside the function.

I copied the split_to_digits function and to my knowledge i assumed it returned a list, so i'm trying to loop through the list "everyotherdig" to sum the digits but if i try and print the sum it says "built in function sum"

You should not use 'sum' as a variable name, because that is a builtin function.