The following checksum formula is widely used by banks and credit card companies

Question

The following checksum formula is widely used by banks and credit card companies to validate legal account numbers:
d0 + f(d1) + d2 + f(d3) + d4 + f(d5) + d6 + ... = 0 (mod 10)
The di are the decimal digits of the account number and f(d) is the sum of the decimal digits of 2d (for example, f(7) = 5 because 2 * 7 = 14 and 1 + 4 = 5). For example, 17327 is valid because 1 + 5 + 3 + 4 + 7 = 20, which is a multiple of 10. Implement the function f and write a program to take a 10-digit integer as a command-line argument and print a valid 11-digit number with the given integer as its first 10 digits and the checksum as the last digit.

Explanation / Answer

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Program CreditCardCheckSum.java

====================================================

class CreditCardCheckSum
{
    public static void main(String args[])
    {
        String n=args[0];
        if(n.length()!=10)
        {
            System.out.println("Wrong number of digits entered");
            System.exit(1);
        }
        int i,s=0;
        for(i=n.length()-1;i>=0;i--)
        {
            if(i%2!=0)
            {
                s=s+f((2*Integer.parseInt(n.charAt(i)+""))+"");
            }
            if(i%2==0)s=s+Integer.parseInt(n.charAt(i)+"");
        }
        int c=0;
        while(true)
        {
            if(s%10!=0)
            {
                c++;
                s=s+c;
            }
            else break;
        }
        String fv=""+n+c;
        System.out.println("The final number is: "+fv);
    }
    public static int f(String a)
    {
        if(a.length()==1)return Integer.parseInt(a);
        if(a.length()==2)return (Integer.parseInt(a.charAt(0)+"")
                            +Integer.parseInt(a.charAt(1)+""));
        return 0;
    }
}

===========================================================

sample output:

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