Problem 45490. Luhn's Algorithm
Luhn's Algorithm is used as a checksum for credit card numbers or similar identifiers. It can detect single-digit changes and swapped digits. It consists of four steps:
- Double every second digit (starting with the last one).
- If a digit exceeds 9 due to the doubling, subtract 9.
- Add all the digits.
- Append the last digit of the sum to the original number.
More information can be found on https://en.wikipedia.org/wiki/Luhn_algorithm or in ISO/IEC 7812-1.
Your function takes the original numbers as a character vector, and it has to return a character vector that includes the checksum.
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