Why does Check50 work for certain credit numbers and not others? I ran debug and found that for some credit numbers and not others it may randomly be always one away from a multiple of 10 preventing checksum and a valid card numbers checksum is not passed.
First, it's typically easier for people to help you if you copy/paste your code (or just the relevant parts of your code), so they can try running your code.
But go through what happens if i = 7, and multiplied_digits = 0 on line 27. i * 2 = 14, so you should end up with multiplied_digits = 5.
if((i * 2) > 9) // this is true, because (i * 2) is 14
{
multiplied_digits += ((i * 2) % 10); // ((i * 2) % 10) is 4, so multiplied_digits = 4
i = i- ((i * 2) % 10); // i = 7- ((7 * 2) % 10) = 7- (14 % 10) = 7- 4 = 3
i / 10; // i = 3 before this, so now i = 0 because the remainder is thrown away
multiplied_digits += i; // i = 0, so multiplied digits = 4 still
}
The fourth line of the snippet above is where you're going wrong, and why the Luhn sums you're getting are sometimes 1 off.
Also, it looks like you're not taking advantage of integer division. In C , 5 / 2 == 2, because the remainder gets thrown away. (If you want the remainder, you need to cast one as a float.) You don't need to subtract the last digit before dividing by 10, because that will be thrown away anyway.
in amex condition, number of zero's in the first condition is wrong. that is changing output of visa to amex..
