Question

I’m writing a program to inverse matrix
so I stored the result in

double matrix[][] = new double[n][2*n];

and when I print the result in consol it’ll be correct
but now I try to improve the program and I want to print array in

JOptionPane.showMessageDialog

so I write

StringBuilder builder = new StringBuilder(n*n);
for (i = 0; i < n; i++){
    for(j = 0; j < n; j++){
        builder.append(matrix[i][j]);
        builder.append(",");
    }
    builder.append("\n");
}
JOptionPane.showMessageDialog(null, builder.toString(), "The inverse matrix is:", JOptionPane.INFORMATION_MESSAGE);

now the problim output should be double for example like this

-0.14285714285714285    0.2857142857142857
0.4285714285714286  -0.35714285714285715    

But every time with any input matrix I get same result

1.0,0.0,
-0.0,1.0,

Thank you.. and sorry my English is not good

---

Ok, It solved
I Keep the size of matrix as

double matrix[][] = new double[n][2*n];

and correct the loop as Reimeus said with some modifying

for (int j = n; j < n*2; j++) {

Thank you all..
It is the first question in this great site and I got answer quickly.. Thank you very much

Answer 1

I believe your matrix is n*n. You need to correct this line:

  double matrix[][] = new double[n][2*n];

To

  double matrix[][] = new double[n][n];

If size is correct, please correct the iteration.

For formatting, please use formatting class DecimalFormat as below:

 String fPattern = "0.00000000000000000"; //please supply the right format pattern
 DecimalFormat dFormat = new DecimalFormat(fPattern);
 StringBuilder builder = new StringBuilder(n*n);
 for (i = 0; i < n; i++){
    for(j = 0; j < n; j++){
      builder.append(dFormat.format(matrix[i][j]));
      builder.append(",");
    }
    builder.append("\n");
 }

Please use 0.00000000000000000 as pattern if you always wish to have fixed length of decimals. If you want varying length, please use #.#################.