Thread: Question. Lots of Maths!

Hi.
First of all i am in the process of building an error correcting code program for my Cryptography and Coding Systems Module.

The program is not finished but it runs to a certain extent.

The problem i am having is i think to do with the modulus or % operator.

I quote below from my specification that if the value 8899880747 is entered as the 10 digit codeword. Then variables P, Q and R should equal 10, 7 & 10 respectively.

i.e. (from spec.) :

"P = (7*7-2*3) mod 11 = 43 mod 11 = 10
Q = (2*3-7*3) mod 11 = -15 mod 11 = -4 mod 11 = 7
R = (3*3-7*3) mod 11 = - 12 mod 11 = -1 mod 11 = 10"


now if you run my program, i have just ran a small test to see if my variables are bieng calculated properly. By just printing out the values of P, Q and R.

I am actually getting 10, -4, and -1 respectively for P, Q and R . This is kind of strange as if you look above at the quoted spec Q and R are actually -4 and -1, before they get to 7 and 10.

Why is this?

Here is my program so far. It is far from finished but i cant really move on until all my variables are bieng calculated properly.

public static void main(String[] args) {
 
        int d1, d2, d3, d4, d5, d6, d7, d8, d9, d10; // the 10 digits from user input
        int s1, s2, s3, s4; // if all = 0 then there are no errors
        int s1Squared, s2Squared, s3Squared, s4Squared;
        int P, Q, R; // if all = 0, then there is a single error
        int qSquared; // the Product of Q * Q
        int a, b; // the magnitude of the errors
        int i, j; // the positions of the errors
 
        String input; // the 10 digits input by the user
 
        Scanner scan = new Scanner(System.in);
 
        System.out.println("Enter a potential 10 digit codeword: ");
        input = scan.nextLine();
 
        d1 = Integer.parseInt(String.valueOf(input.charAt(0)));
        d2 = Integer.parseInt(String.valueOf(input.charAt(1)));
        d3 = Integer.parseInt(String.valueOf(input.charAt(2)));
        d4 = Integer.parseInt(String.valueOf(input.charAt(3)));
        d5 = Integer.parseInt(String.valueOf(input.charAt(4)));
        d6 = Integer.parseInt(String.valueOf(input.charAt(5)));
        d7 = Integer.parseInt(String.valueOf(input.charAt(6)));
        d8 = Integer.parseInt(String.valueOf(input.charAt(7)));
        d9 = Integer.parseInt(String.valueOf(input.charAt(8)));
        d10 = Integer.parseInt(String.valueOf(input.charAt(9)));
 
        s1 = (d1 + d2 + d3 + d4 + d5 + d6 + d7 + d8 + d9 + d10) % 11;
        s2 = (d1 + 2 * d2 + 3 * d3 + 4 * d4 + 5 * d5 + 6 * d6 + 7 * d7 + 8 * d8 + 9 * d9 + 10 * d10) % 11;
        s3 = (d1 + 4 * d2 + 9 * d3 + 5 * d4 + 3 * d5 + 3 * d6 + 5 * d7 + 9 * d8 + 4 * d9 + d10) % 11;
        s4 = (d1 + 8 * d2 + 5 * d3 + 9 * d4 + 4 * d5 + 7 * d6 + 2 * d7 + 6 * d8 + 3 * d9 + 10 * d10) % 11;
 
        s1Squared = (int) Math.pow(s1, 2);
        s2Squared = (int) Math.pow(s2, 2);
        s3Squared = (int) Math.pow(s3, 2);
        s4Squared = (int) Math.pow(s4, 2);
 
        P = (s2Squared - (s1 * s3)) % 11;
        Q = ((s1 * s4) - (s2 * s3)) % 11;
        R = ((s3Squared) - (s2 * s4)) % 11;
 
        qSquared = (int) Math.pow(Q, 2);
 
        i = (int) ((Math.sqrt(qSquared - 4 * P * R) - Q) / (2 * P)) % 11;
        j = (int) (-Q - (Math.sqrt(qSquared - 4 * P * R)) / (2 * P)) % 11;
 
        b = ((i * s1 - s2) / (i - j)) % 11;
        a = s1 - b;
 
        System.out.println(P);
        System.out.println(Q);
        System.out.println(R);
 
        if (s1 == 0 && s2 == 0 && s3 == 0 && s4 == 0) {
            System.out.println("There are no errors!");
        }
        if (P == 0 && Q == 0 && R == 0 && s1 != 0 && s2 != 0 && s3 != 0 && s4 != 0) {
            System.out.println("There is 1 error");
        }
 
 
    }
}

Do you have understanding of Operators precedence?

As a hint,
* has higher precedence than +. So, it will be calculated before +.
Now look at your program.

There are no + operations in the calculations of P, Q & R

for the record, s1, s2, s3, & s4 are bieng calculated correctly. I realise the multiplication is done before addition.

Try placing System.out.println after every statement and see where exactly the problem is or if you are good to debug this, try debugging it.

What do you mean by debugging it mate

Thanks

I have tried that, i have printed out the values of s1, s2, s3, s4 and they are all bieng calculated correctly, therefore ive nailed it down to the problem bieng in the calculation of P, Q, & R

Oh right, I wasnt aware of that rule.