how to add two integers in a number

Question

I am writing a credit card validation program. I am trying to see if a product is more that 1 digit long (ie: 10), if it is I need to add the two integers together. For example 10 would be 1 + 0 , which equals 1, how do I do this?

This is what I have so far:

public class CreditCard{
public static void main(String args[]){

    //Take in a 16 digit credit card number
    Scanner in=new Scanner (System.in);
    int num[]=new int[16];
    int i=0;


    for (i=0;i<num.length;i++) {
        System.out.println("Please enter a 16 digit credit card number");
        num[i]=in.nextInt();

    }
    if (num.length < 16 || num.length > 16)
        {
            System.out.println("Invalid");
        }
     else
    {

        //multiply every other number by 2, starting with place 16, and find the sum(sum1)
         int num1 = (num[16] * 2);
         int num2 = (num[14] * 2);
         int num3 = (num[12] * 2);
         int num4 = (num[10] * 2);
         int num5 = (num[8] * 2);
         int num6 = (num[6] * 2);
         int num7 = (num[4] * 2);
         int num8 = (num[2] * 2);

         if( num1 > 9)

         int sum1 = (num[16] * 2) + (num[14] * 2) + (num[12] * 2) + (num[10] * 2) + (num[8] * 2) + (num[6] * 2) + (num[4] *2) + (num[2] *2);
         int sum2 = (num[15] + num[13] + num[11] + num[9] + num[7] + num[5] + num[3] + num[1]);

        int totalSum = sum1 + sum2;
        if (totalSum % 10 != 0)
        {
            System.out.println("Invalid card number!!");
        }
        else

        {  
            System.out.println(" Valid Credit Card Number!!");


        }

      }      
}
}

Answer 1

Rather than re-inventing the wheel you could have a look at the Luhn algorithm which is a widely used checksum algorithim used to validate credit card numbers. Here is a Java version .

Answer 2

It looks like you're trying to implement the Luhn checkdigit algorithm .

Try the apache commons LuhnCheckDigit() utility method of the Apache commons-validator library .

Answer 3

您可能希望首先将整数拆分为数字： 将int值拆分为单独的数字

Answer 4

If Blender's comment is correct, you could do it like this:

int num=123; // or whatever
String str=Integer.toString(num);
int total=0;
if(str.length()>1){
    for(int i=0; i<str.length(); i++){
        total+=Integer.parseInt(str.charAt(i));
    }
}
System.out.println(total);

(I haven't run this, so there are probably some errors in there)

Answer 5

I think a simple way would be

if (num1 > 9) num1 = num1-9

This works if you look at how the original numbers look after multiplying by 2.

num  num*2  result
0    0      0
1    2      2
2    4      4
3    6      6
4    8      8
5    10     1
6    12     3
7    14     5
8    16     7
9    18     9

Answer 6

I think your code to calculate the total should be

int totalSum = 0;
for (i=0; i<num.length; i+=2) {
    totalSum+=num[i+1];
    if (num[i+2]>4) {
        totalSum+=1+((num[i+2]*2) % 10); 
    } else {
        totalSum+=num[i+2]*2;
    }
}

This avoids any slow string conversions.