[Python] Credit card validator program in python , loop

I’m working on a credit card validator program in Python using the Luhn’s Algorithm, but I’m stuck on the for loop part.

Here’s the full code I have so far:

# Credit card validator by Ahum Maitra

credit_card_number = input("Enter your credit card number : \n>")
sum_odd_digits = 0
sum_even_digits = 0
total = 0

# Replace any '-' or spaces with nothing
temp_credit_card_number = credit_card_number
temp_credit_card_number = temp_credit_card_number.replace("-", "")
temp_credit_card_number = temp_credit_card_number.replace(" ", "")
temp_credit_card_number = temp_credit_card_number[::-1]

# Convert each digit into an integer
temp_credit_card_number = [int(digit) for digit in temp_credit_card_number]

# Loop through each digit with its index
for index, digit in enumerate(temp_credit_card_number):
if index % 2 == 0:
sum_odd_digits += digit
else:
doubled = digit * 2
if doubled > 9:
doubled -= 9
sum_even_digits += doubled

# Calculate the total
total = sum_even_digits + sum_odd_digits

# Check if valid
if total % 10 == 0:
print("Valid")
else:
print("Invalid")


The part I don’t understand is this section:

for index, digit in enumerate(temp_credit_card_number):
if index % 2 == 0:
sum_odd_digits += digit
else:
doubled = digit * 2
if doubled > 9:
doubled -= 9
sum_even_digits += doubled


I get that it’s looping through the reversed credit card number, but I’m confused about:

Why check if index % 2 == 0?

How exactly is the else part (doubling, subtracting 9) working in the context of Luhn’s algorithm?

For reference, here’s the Luhn’s algorithm problem:

https://cs50.harvard.edu/x/psets/1/credit/

Could someone please explain this loop in a beginner-friendly way?

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