Question

From Java Malik textbook- determine if an number is divisible by 11..

Code Solution provided:

import java.util.*;

public class Divby11
{
    static Scanner console = new Scanner(System.in);

    public static void main (String[] args)
    {
        int num, temp, sum;
        char sign;

        System.out.print("Enter a positive integer: ");
        num = console.nextInt();
        System.out.println();

        temp = num;

        sum = 0;
        sign = '+';

        do
        {
            switch (sign)
            {
            case '+' :
                sum = sum + num % 10;
                sign = '-';
                break;

            case '-' :
                sum = sum - num % 10;
                sign = '+';
            }

            num = num / 10;       //remove the last digit
        }
        while (num > 0);

        if (sum % 11 == 0)
            System.out.println(temp + " is divisible by 11");
        else
            System.out.println(temp + " is not divisible by 11");
    }

Why go through all the effort above and just say…

  if (sum % 11 == 0)
            System.out.println(temp + " is divisible by 11");
        else
            System.out.println(temp + " is not divisible by 11");

Can any of you experts see why the author would do it this way (long way)?

Answer 1

This code example isn’t actually dividing by eleven. If you see, it’s alternating between adding and subtracting each digit, then checks at the very end if the result is divisible by 11.

For example, look at the following number and how this algorithm works with it:

Start with sum=0, sign='+', num=517
First iteration: sum=7, sign='-', num=51
Second iteration: sum=6, sign='+', num=5
Final iteration: sum=11, sign='-', num=0

The final result is divisible by eleven.

EDIT: The algorithm does indeed look to be implementing the divisibility rule for 11 as dfa mentions in his answer.