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C: Credit Card Number Validator - Luhn Algorithm

This is a discussion on C: Credit Card Number Validator - Luhn Algorithm within the C Programming forums, part of the General Programming Boards category; Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start ...

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    C: Credit Card Number Validator - Luhn Algorithm

    Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start with:


    4 for Visa cards


    5 for Master cards


    37 for American Express cards


    6 for Discover cards


    In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card numbers. The algorithm is useful to determine if a card number is entered correctly or if a credit card is scanned correctly by a scanner. Almost all credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check, which can be described as follows (for illustration, consider the card number 4388576018402626):


    1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number.
    2 * 2 = 4
    2 * 2 = 4
    4 * 2 = 8
    1 * 2 = 2
    6 * 2 = 12 (1 + 2 = 3)
    5 * 2 = 10 (1 + 0 = 1)
    8 * 2 = 16 (1 + 6 = 7)
    4 * 2 = 8


    2. Now add all single-digit numbers from Step 1.
    4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 37


    3. Add all digits in the odd places from right to left in the card number.
    6 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 38


    4. Sum the results from Step 2 and Step 3.
    37 + 38 = 75


    5. If the result from Step 4 is divisible by 10, the card number is valid; otherwise, it is invalid. For example, the number 4388576018402626 is invalid, but the number 4388576018410707 is valid.




    Sample 1:
    Enter a credit card number as a long integer: 4246345689049834


    4246345689049834 is invalid


    Sample 2:
    Enter a credit card number as a long integer: 4388576018410707


    4388576018410707 is valid

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    That was a very educational report on credit card numbers! Nice job!

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