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.

Code :

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); } }