string index out of range

[kenny koh]

Hi to all helpful ppl, i am writing this code and getting an error. java.lang.StringIndexOutOfBoundsException: String index out of range: -2.
at java.lang.String.charAt(String.java:695)

import java.util.*;
public class CreditCardVerifier {
	public static void main(String[]args) {
		Scanner sc = new Scanner(System.in);
		System.out.print("Please enter the 16 digit credit card no for verification: ");
		String credit = sc.nextLine();
		
		int leng = credit.length();
		int total = 0;
		
		for(int x=0;x<leng;x++) {
			if(x%2==0) {
				int don = (int)credit.charAt(x)-48;
					if(x*2>10) {
						x-=9;
					}
					else {
						x=x*2;
					}
				total+=x;	
			}
		}
	
	if(total%10==0) {
		System.out.println("Your credit card no, " + credit + " is valid!");
	}
	else
		System.out.println("Your credit card no, " + credit + " is not valid!");
	}
}

[Rob Spoor]

Well, look at line 16. If x * 2 > 10 you decrease x by 9. That may cause x to become negative; after all, x * 2 > 10 && x < 9 is true for 6, 7 and 8 as values of x. Recognizing a Luhn-check, you want to change lines 12 to 21 to use don instead of x:

int don = credit.charAt(x) - '0';   
if (x % 2 == 0) {   
    don = don * 2;
    if(don > 9) {   
        don -= 9;   
    }   
}
total += don;

Note that I changed the 48 to '0'. They have the same value, but '0' is easier to read. The cast to int is not needed. Also, you need to add don always - you just need to modify it when x is even.

[kenny koh]

Omg, hey thanks alot man. your help is much appreciated.

Cheers

[Campbell Ritchie]

Welcome to the Ranch

Quite a lot to comment about in that code. Please put spaces around your binary operators, which makes it easier to read. don’t use number literals like 48. You should avoid number literals altogether, if possible. It is not clear what 48 means. If you wish to subtract the digit 0 from it, you can do arithmetic on chars, with - '0'. That is much easier to read.
There is something odd about subtracting 9. If you start with the value 7, then twice 7 will be more than 10, so you subtract 9 and get -2 The JVM doesn’t like looking for the minus 2nd letter in a String.

[Rob Spoor]

Ken, please note that I fixed another bug while you were replying.

[Harsha Smith]

what rule are you applying here to validate the given string? I don't understand the code

[Rob Spoor]

http://en.wikipedia.org/wiki/Luhn_algorithm

[Harsha Smith]

May be you can add some code to check if the input string is of length 16 and also it contains only numbers by pattern matching .put that code in a while loop until the condition satisfies. use variable names like doubleTheNumber(instead of don) proceed with the rest of the code.

Had you used an IDE you would have got a warning like the local variable don never read

[Rob Spoor]

Credit card numbers don't necessarily have to be 16 characters.