I'm writing a program to validate credit card numbers and I have to use Luhn's Algorithm. Let me say beforehand, that I have just started to learn to program (we covered loops like last week), so there a lot of things I am unfamiliar with. I am having trouble with one of my functions that checks the arithmetic. Basically, it has to double every second digit from right to left and add everything together. But if you double a number, like 5, and you get 10, then you will have to add 1+0=1 to the total sum instead of 10. That's the part I'm stuck on. How can I put that in a program?
Sample code so far:
int
doubleEvenSum(string creditCardNumber) {
int evenSum;
int countPosition;
int doublePosition;
int length;
length = creditCardNumber.length ();
countPosition = creditCardNumber.at(length - 2);
evenSum = 0;
while(countPosition>0) {
if ((2 * countPosition) < 10) {
doublePosition = 2 * countPosition;
}
else if ((2 * countPosition) > 9) {
???
}
evenSum = evenSum + doublePosition;
}