Thread: Java error correcting, testing for a square issue i think

Hi all, i am having an issue with my code.

First of all this is a program that reads in 10 digits, and there are 4 possible cases.

No error occured
single error occured - correct the error and print out correct 10 digits
double error occured - correct the errors and print out correct 10 digits
More than two errors occured - print out a message saying more than 2 errors

The progra actually works well to a certain extent

Now i think the issue with this program is with the following condition: (quoted from task spec)

4. If (Q2-4PR) is not a non-zero square (i.e. √(Q2-4PR) hasn’t got a root in GF(11)) or if position value i or j is zero there are at least three errors have occurred.

The reason i think there is a problem here is because if i type in 5545644889 as the 10 digits for example. then i get the following output:

Please enter a 10 digits number to decode:
5545644889
QQ - 4PR = 1
(QQ - 4PR) is not a square. More than 2 errors
BUILD SUCCESSFUL (total time: 2 seconds)

Now 1 is obviously a square number (1x1) so im not sure if im calculating this condition correctly.

Any ideas?

I will try and find another 10 digits example because once 4 was the value QQ - 4PR, and it still had this form of output, when obviously 4 is a square.


Here is the program anyway.

    public static void decode(String code) {
 
        // If input is less than 10 digits then warn the user of an invalid input
        if (code.length() != 10) {
            System.out.println("Invalid input! Input must be 10 digits");
            return;
        }
 
        // Make an Array out of the String code  
        int d[] = new int[]{0,
            code.charAt(0) - '0',
            code.charAt(1) - '0',
            code.charAt(2) - '0',
            code.charAt(3) - '0',
            code.charAt(4) - '0',
            code.charAt(5) - '0',
            code.charAt(6) - '0',
            code.charAt(7) - '0',
            code.charAt(8) - '0',
            code.charAt(9) - '0'
        };
 
        // Generating the Syndrome Vector
        int s1 = (d[1] + d[2] + d[3] + d[4] + d[5] + d[6] + d[7] + d[8] + d[9] + d[10]) % 11;
        int s2 = (d[1] + 2 * d[2] + 3 * d[3] + 4 * d[4] + 5 * d[5] + 6 * d[6] + 7 * d[7] + 8 * d[8] + 9 * d[9] + 10 * d[10]) % 11;
        int s3 = (d[1] + 4 * d[2] + 9 * d[3] + 5 * d[4] + 3 * d[5] + 3 * d[6] + 5 * d[7] + 9 * d[8] + 4 * d[9] + d[10]) % 11;
        int s4 = (d[1] + 8 * d[2] + 5 * d[3] + 9 * d[4] + 4 * d[5] + 7 * d[6] + 2 * d[7] + 6 * d[8] + 3 * d[9] + 10 * d[10]) % 11;
 
        // Test to see if s1, s2, s3, s4 = 0, 0, 0, 0
        if (s1 == s2 && s2 == s3 && s3 == s4 && s4 == 0) {
            System.out.println("No error!");
            return;
        }
 
        //Calculate P & if P is negative, add 11 to obtain positive modulus result
        int P = (s2 * s2 - s1 * s3) % 11;
        if (P < 0) {
            P += 11;
        }
 
        //Calculate Q & if Q is negative, add 11 to obtain positive modulus result
        int Q = (s1 * s4 - s2 * s3) % 11;
        if (Q < 0) {
            Q += 11;
        }
 
        //Calculate R & if R is negative, add 11 to obtain positive modulus result
        int R = (s3 * s3 - s2 * s4) % 11;
        if (R < 0) {
            R += 11;
        }
 
        // If P, Q & R are all equal to 0 then there is a single error
        if (P == Q && Q == R && R == 0) {
            d[s2 / s1] -= s1;
            if (d[s2 / s1] < 0) {
                d[s2 / s1] += 11;
            }
            System.out.println("Single error!");
            System.out.println("At position " + s2 / s1 + " by magnitude " + s1);
            String correct = "";
            for (int i = 1; i <= 10; i++) {
                correct += "" + d[i];
            }
            // Print out the correct codeword
            System.out.println("The correct code: " + correct);
            return;
        }
 
        // Calculate (QQ - 4PR)%11
        int delta = (Q * Q - 4 * P * R) % 11;
        if (delta < 0) {
            delta += 11;
        }
 
        if (delta > 0) {
            int de = 2 * P % 11;
            int mapping[] = new int[]{0, 1, 6, 4, 3, 9, 2, 8, 7, 5, 10};
            int deinv = mapping[de];
 
            // Calculate i (position of error)
            int i = ((int) (-Q + Math.sqrt(delta)) * deinv) % 11;
            if (i < 0) {
                i += 11;
            }
 
            //Calculate j (position of error)
            int j = ((int) (-Q - Math.sqrt(delta)) * deinv) % 11;
            if (j < 0) {
                j += 11;
            }
 
            // Test to see if (QQ - 4PR)%11 is a non-zero square
            int x = (Q * Q - 4 * P * R) % 11;
            if (x < 0) {
                x += 11;
            }
            if (x != 1 || x != 4 || x != 9) {
                //(QQ - 4PR)%11 is not a square
                System.out.println("(QQ - 4PR)%11 = " + x);
                System.out.println("(QQ - 4PR)%11 is not a square. More than 2 errors");
                return;
            }
 
            // Test to see if i or j are equal to 0
            if (i == 0 || j == 0) {
                System.out.println("i or j is equal to 0. More than two errors!");
                return;
            }
 
            // Calculate b (Magnitude of error)
            int b = ((i * s1 - s2) / (i - j)) % 11;
            if (b < 0) {
                b += 11;
 
                // Calculate a (Magnitude of error)
            }
            int a = s1 - b;
            System.out.println("a " + a + " b " + b);
            d[j] -= b;
 
            if (d[j] < 0) {
                d[j] += 11;
            }
            d[i] -= a;
            if (d[i] < 0) {
                d[i] += 11;
            }
            System.out.println("Two errors!");
            String correct = "";
            for (int k = 1; k <= 10; k++) {
                correct += "" + d[k];
            }
 
            // Print out the corrected codeword
            System.out.println("The correct code: " + correct);
            return;
        }
        System.out.println("More than two errors!");
    }
 
    // Main method
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Please enter a 10 digits number to decode: ");
        String number = scanner.nextLine();
        decode(number);
 
    }
}

In an OR statement only one condition has to be true.

When x == 1, x != 4 is true
When x == 4, x != 1 is true
When x == 9, x != 1 is true

Your if statement will always be true no matter what value x has.

What happened when you tried it? You did try it instead of wasting time asking us, didn't you?