Java, input file to 2D array

Question

I'm really new to Java. I'm trying to take values from an input file, which I made in eclipse, and trying to save them to a 2D array. The input is:

31 22 23 79

20 -33 33 1

3 -1 46 -6

I can save it to a regular array fine, but not matter what I try I can't figure out how to get it to save to a 2d array in the form above. I tried for loops, but it saved all 12 numbers for each iteration of the loop. I tried using variables and just incrementing them like for the regular array and it just saved nothing. Any help on how to do this appreciated, code for regular array is below, prints the following to screen:

[31, 22, 23, 79, 20, -33, 33, 1, 3, -1, 46, -6]

import java.io.*;
import java.util.Arrays;
import java.util.StringTokenizer;

public class ArrayMatrix2d {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    if (args.length == 0){
        System.err.println("Usage: java Sum <filename>");
        System.exit(1);
    }

    try {
        //int[][] matrix = new int[3][4];
        int MyMat[] = new int[12];
        int num = 0;

        //int row = 0;
        //int column = 0;
        BufferedReader br = new BufferedReader(new FileReader(args[0]));
        String line;
        while((line = br.readLine()) != null) {
            StringTokenizer st = new StringTokenizer (line);
            while (st.hasMoreTokens()){
                int value1 = Integer.parseInt(st.nextToken());
                MyMat[num] = value1;
                num++;              
            }
        }
        System.out.println(Arrays.toString(MyMat));
        br.close();
    }       
    catch(Exception e) {}           
}

}

hamon · Accepted Answer · 2013-10-05 11:37:47Z

1

You could make your matrix like this

int[][] matrix=new int[3][]; //if the number of columns is variable

int[][] matrix=new int[3][4]; //if you know the number of columns

and in the loop you get

 int i=0;
 while((line = br.readLine()) != null) {
        StringTokenizer st = new StringTokenizer (line);
        int num=0;
        //the next line is when you need to calculate the number of columns
        //otherwise leave blank
        matrix[i]=new int[st.countTokens()];
        while (st.hasMoreTokens()){
            int value1 = Integer.parseInt(st.nextToken());
            matrix[i][num] = value1;
            num++;              
        }
        i++;
 }

edited Oct 5, 2013 at 11:37

answered Oct 5, 2013 at 11:32

hamon

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3 Comments

user2849168 Over a year ago

Thanks for your help. I'm getting the following output now: [[31, 22, 23, 79], null, null]. I've probably made a typo or something, will try track it down, but definitely going in the right direction.

hamon Over a year ago

Seems like your second and third row aren't constructed.

user2849168 Over a year ago

Aha, got it, thanks a million. Just had my print statement in the wrong place. Thank you!

Maxim Shoustin · Accepted Answer · 2013-10-05 11:37:48Z

1

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

String[][] root;

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

lines.removeAll(Arrays.asList("", null)); // <- remove empty lines

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

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

Now you have populated root[][]

Hope it will help you

edited Oct 5, 2013 at 11:37

answered Oct 5, 2013 at 11:31

Maxim Shoustin

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3 Comments

user2849168 Over a year ago

Hi, I really appreciate the help but my Java is so basic that I just don't understand what's going on here. Thanks though!

Maxim Shoustin Over a year ago

You can try to debug and observe how it works. With only 1st row you put file name and get List of lines. 2nd row removes all empty rows in file. So you cut off your code from 20 rows to 4

user2849168 Over a year ago

I'll try it out and see how it works, I'm definitely trying to learn anyway. Thanks!

Vik Ermolenko · Accepted Answer · 2017-09-21 10:20:53Z

0

With hope, that my code will be useful for you:

java.util.Scanner scan = new java.util.Scanner(System.in);
int [] ar= Arrays.stream(scan.nextLine().split(" ")).mapToInt(Integer::parseInt).toArray();
int m=ar[0];
int n=ar[1];
int myArray[][]=new int[m][n];
for (int i = 0; i < m; i++)
    myArray[i]= Arrays.stream(scan.nextLine().split(" ")).mapToInt(Integer::parseInt).toArray();

answered Sep 21, 2017 at 10:20

Vik Ermolenko

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