Credit card test confusion

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I am supposed to find if cc numbers are valid based on these rules:

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.

If the total’s last digit is 0 (or, put more formally, if the total modulo 10 is congruent to 0), the number is valid!

When I work-it-out by hand, I dont get valid for these numbers... but they are (from the solutions site). Can I see how its done correctly?

371449635398431

5105105105105100

4111111111111111

4012888888881881

Thanks

What I have tried:

I worked it out on paper. I tried my best to follow the rules exactly. I did get it right on other numbers, but not these.

Posted 26-Dec-16 14:42pm

Member 12919791

Updated 26-Dec-16 20:41pm

Maciej Los

v2

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[no name] 26-Dec-16 22:11pm

We aren't here to teach you programming or write code for you. Do what you have already been told and learn how to use the debugger and find out why your code is not behaving as you expect it to instead of reposting your not a question over and over.

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CPallini · Answer 1 · 2016-12-26T20:41:00

Let's perform the operation on the first input code:

371449635398431

every other digit (starting from second-to-last)
3 8 3 3 9 4 7
doubled and summed up
6 + 16 + 6 + 6 + 18 + 8 + 14 = 74
remaining digits (starting from the left), summed up
3 + 1 + 4 + 6 + 5 + 9 + 4 + 1 = 33

Total = 107

So, at least I can confirm that, according to the rules depicted, the first input code should be rejected.

Either we're right (and the 'solution site' is wrong) or we're wrong (bad computation) or the reported rules are miswritten (or misunderstood).

[update]
According to this site Credit Card Number Generator & Validator - FreeFormatter.com[^] such input is a valid credit card number. However the rules followed are a bit different, namely they follow the Luhn algorithm. Since the algorithm is well described in the same site, I suggest you to have a look at it.
[/update]