Card number: the last digit

Python Advanced astronaut ages teen+ From CS Unplugged

The problem

Bank card numbers end in a check digit (the Luhn method). Write a function card_check_digit(first15) that gets the first 15 digits as text and gives back the last digit as a number. How: start from the RIGHT of first15. Double the 1st, 3rd, 5th ... digit from the right; if a doubled value is more than 9, subtract 9. Add everything up. The check digit is (10 - total % 10) % 10. (These are made-up test numbers, not real cards.)

You will practice: check digits, Luhn method, loops from the right, remainder (%)

Solve it myself Guide me step by step

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CC BY-SA 4.0

Adapted from a CS Unplugged programming challenge by the University of Canterbury Computer Science Education Research Group, licensed CC BY-SA 4.0.

Original challenge · License: CC BY-SA 4.0

Changes: Rewritten in our own words as a function with tests, hints and steps, in Python and JavaScript, by Open Cosmos. This version is also CC BY-SA 4.0.