Simple Error

chrislad

I'm probably just making a stupid math error, but I just can't see it. The code compiles and runs. It's basically a quiz question from the book I'm using. The point is to get the user to enter 9 digits for an ISBN code. The 10th digit is devised using digit 1 * 1 + digit 2 * 2 + digit 3 * 3 and so on and so forth with the total being modded by 11.

For entering 1 2 3 4 5 6 7 8 9, I am getting 1 2 3 4 5 6 7 8 9 208 (where 208 should be 3)

Bear in mind that I'm kinda jumping ahead of myself with arrays and that's based off stuff I did in college 5 years ago. The book hasn't hit do while or arrays yet, but I'm not to remember as best I can.

import java.util.Scanner;


public class ISBN {
	
	
	public static void main(String args[])
	{
		
		System.out.println("Enter ISBN with space between numbers (9 digits)");
		Scanner input = new Scanner(System.in);
		int[] ISBNinput;
		ISBNinput = new int[10];
		int counter = 0;
		
		do
		{
			ISBNinput[counter] = input.nextInt();
			counter++;
				
		}while (counter <= 8);
		
		ISBNinput[9] = (((ISBNinput[0] * 1) + (ISBNinput[1] * 2) + (ISBNinput[2] * 3) + (ISBNinput[3] * 4) 
				+ (ISBNinput[4] * 5) + (ISBNinput[5] * 6) + (ISBNinput[6] * 7) + (ISBNinput[7] * 8)
				+ (ISBNinput[8] * 9)) % 11);
		
		for (counter = 0; counter <= 9;)
		{
			System.out.print(ISBNinput[counter]); counter++;
		}
		
		}
	}

28064212

That code's correctly returning 12345678910 for me:
((1 * 1) + (2 * 2) + (3 * 3) + (4 * 4) + (5 * 5) + (6 * 6) + (7 * 7) + (8 * 8) + (9 * 9)) % 11 = 10

moycullen14

IIRC ISBN codes are supposed to sum to 0 mod 11 (it's a checksum/error correction thing)

(10 * digit1 + 9 * digit2 + 8 * digit3 +....... 1 * digit10) mod 11 = 0

Given the 9 digits, you need to work out the 10th (digit10)

So say S = (10 * digit1 + 9 * digit2 + 8 * digit3 +.......2*digit9) mod 11

then (S + digit10) mod 11 = 0

S      digit10       (11 - S) mod 11
0      0               0
1      10              10
2      9                9
...
9      2
10     1                1

So,

digit10 = (11 - S) mod 11

I think thats it?

chrislad

moycullen14 wrote: »

IIRC ISBN codes are supposed to sum to 0 mod 11 (it's a checksum/error correction thing)

(10 * digit1 + 9 * digit2 + 8 * digit3 +....... 1 * digit10) mod 11 = 0

Given the 9 digits, you need to work out the 10th (digit10)

So say S = (10 * digit1 + 9 * digit2 + 8 * digit3 +.......2*digit9) mod 11

then (S + digit10) mod 11 = 0

S      digit10       (11 - S) mod 11
0      0               0
1      10              10
2      9                9
...
9      2
10     1                1

So,

digit10 = (11 - S) mod 11

I think thats it?

Cheers, that's it. I misread the spec slightly. Wasn't overly concerned as it is just practice and the code compiled and ran fine. Thanks again!