C validating a credit card number mark umbers dating
This also usually indicates a lack of experience in the proper handling of user submitted data.
Sending unvalidated user submitted data to an API, or a database, is a recipe for disaster if a malicious user comes along and decides they want to do bad things to a website.
This code sample demonstrates the algorithm for validating a credit card number.
Almost all credit card companies use the same method for generating card numbers.
Next, you do the same thing with the digits skipped in the first step, but this time you double the value of each digit and add the value of each digit in the result.As @Keith says, credit card numbers should be treated as strings, not numbers, and definitely not floating-point numbers.If you want to ensure that the input contains only digits (and maybe spaces), use To demonstrate its use, I've written a simple line counter (of only non-blank lines) using the POSIX standard one. [Another example of a number that isn't is a 'phone number'.] Using a string has the natural benefit that you can allow the I/O to have internal spaces as well - making it much more usable. User interface should allow the gaps to be captured.One of the major weaknesses of the Luhn check is that you can swap adjacent middle digits if they are both under 5 and the number will still pass..