Java - Trouble creating a 2D array from a text file

Question

I have a text file that is formatted as a square array. I am trying to convert this to a 2 dimensional array in Java. "st" is the text file described below.

This is my code:

public createArray(String st) throws NumberFormatException, IOException {

InputStream is;

is = new FileInputStream(st);

BufferedReader br = new BufferedReader(new InputStreamReader(is, Charset.forName("UTF-8")));
    int row = 0;

    while ((line = br.readLine()) != null){
        String[] nums = line.split(" ");

        for (int col = 0; col < nums.length; col++){

            int n = Integer.parseInt(nums[col]);

            myArray[row][col] = n;
        }
        row++;

    }

I am getting a null pointer exception on the line:

myArray[row][col] = n;

If I print out the values of row, col, and n at each iteration, they appear to coincide correctly. However, I am still getting a NullPointerException.

The text file is formatted as:

1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50
51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70
71 72 73 74 75 76 77 78 79 80
81 82 83 84 85 86 87 88 89 90
91 92 93 94 95 96 97 98 99 0

myArray is an instance variable and is declared as int[][] myArray; — user2121620
– user2121620, Commented Oct 15, 2013 at 4:52
How do determine the number of rows and columns stored in the file? — MadProgrammer
– MadProgrammer, Commented Oct 15, 2013 at 4:54
My problem was that I did not initialize myArray with any size declaration before populating it. — user2121620
– user2121620, Commented Oct 15, 2013 at 4:59

sanbhat · Accepted Answer · 2013-10-15 04:53:58Z

5

I think you have not initialized int[][] myArray;

You should do something like this

int[][] myArray = new int[100][100];

OR

find the total rows in the file and initialize the row. And in each loop calculate the column and create the desired array

int[][] myArray;
{
    int rows = getTotalRows();
    myArray = new int[rows][];
}

then

while ((line = br.readLine()) != null){
        String[] nums = line.split(" ");
        myArray[row] = new int[nums.length]; //initialize dynamically here
        for (int col = 0; col < nums.length; col++){

            int n = Integer.parseInt(nums[col]);

            myArray[row][col] = n;
        }
        row++;
}

answered Oct 15, 2013 at 4:53

sanbhat

17.6k6 gold badges52 silver badges65 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Maxim Shoustin · Accepted Answer · 2013-10-15 05:02:04Z

3

Regards to your comment you didn't initiate array.

int[][] myArray = new int[100][]; // enough to define rows

Other way:

If you use Java 7 you can load text file to List. As I know this is a shortest way to create String[][]

String[][] myArray;

List<String> lines = Files.readAllLines(Paths.get("<your filename>"), StandardCharsets.UTF_8);

myArray = new String[lines.size()][]; 

for(int i =0; i<lines.size(); i++){
  myArray[i] = lines.get(i).split("[ ]+"); // you can use just split(" ") but who knows how many empty spaces
}

Now you have populated myArray[][]

edited Oct 15, 2013 at 5:02

answered Oct 15, 2013 at 4:54

Maxim Shoustin

78k29 gold badges212 silver badges229 bronze badges

Collectives™ on Stack Overflow

Java - Trouble creating a 2D array from a text file

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related