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// Stuart Reges handout #12
// 10/22/04
//
// This program prompts the user for all but the last digit of a credit card
// and tells you what the last digit should be for the card to be legal.
// This is an implementation of the Luhn algorithm.
public class CreditCard {
public static void main(String[] args) {
giveIntro();
Scanner console = new Scanner(System.in);
System.out.print("partial card # (all but last digit)? ");
String number = console.nextLine();
System.out.println("The last digit should be " + lastDigitFor(number));
}
// Explains the program to the user.
public static void giveIntro() {
System.out.println("This program will tell you the last digit of a");
System.out.println("credit card (assuming it is legal). Please");
System.out.println("enter all but the last digit.");
System.out.println();
}
// This method takes a partial credit card number (all but the last
// digit) and computes what the last digit should be, returning it
// as an int. The method ignores non-digit characters like dashes and
// spaces.
public static int lastDigitFor(String partialCard) {
int multiplier = 2;
int sum = 0;
for (int i = partialCard.length() - 1; i >= 0; i--)
if (Character.isDigit(partialCard.charAt(i))) {
int next = digitFor(partialCard.charAt(i)) * multiplier;
sum += next/10 + next % 10;
if (multiplier == 2)
multiplier = 1;
else
multiplier = 2;
}
return 10 - sum % 10;
}
// This method expects a digit character like '0', '1', ..., '9'. It
// returns the int equivalent of the digit.
public static int digitFor(char ch) {
return ch - '0';
}
}
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