From the rightmost digit, which is the check digit, moving left,double the value of every second digit;
Question:
From the rightmost digit, which is the check digit, moving left,double the value of every second digit; if the product of thisdoubling operation is greater than 9 (e.g., 7 * 2 = 14), then sumthe digits of the products (e.g., 10: 1 + 0 = 1, 14: 1 + 4 =5).
Take the sum of all the digits.
If the total modulo 10 is equal to 0 (if the totalends in zero) then the number is valid according to the Luhnformula; else it is not valid.
Assume an example of an account number "7992739871" that willhave a check digit added, making it of the form 7992739871x:
| Account number | 7 | 9 | 9 | 2 | 7 | 3 | 9 | 8 | 7 | 1 | x |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Double every other | 7 | 18 | 9 | 4 | 7 | 6 | 9 | 16 | 7 | 2 | x |
| Sum of digits | 7 | 9 | 9 | 4 | 7 | 6 | 9 | 7 | 7 | 2 | =67 |
The check digit (x) is obtained by computing the sum of digitsthen computing 9 times that value modulo 10 (in equation form, (67* 9 mod 10)). In algorithm form:
Compute the sum of the digits (67).
Multiply by 9 (603).
The last digit, 3, is the check digit.
Credit Card error detection check -> Luhn Algorithm, (http://www.freeformatter.com/credit-card-number-generator-validator.html#cardFormats)
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