如何从文件（Java）将双精度数添加到2d数组中？

Question

All the examples that i have seen involve specifying the number of rows and columns at the start of the file but the method I'm working on reads a file with the following:

1.0 2.0
3.0 4.0

and using this data creates a 2d array and stores it without specifying the number of rows and columns.

Here's the code I have written:

 public static double[][] readMatrixFrom(String file) throws FileNotFoundException {
     Scanner input = new Scanner(new FileReader(file));
     int rows =0;
     int columns =0;

     while(input.hasNextLine()){
         String line = input.nextLine();
         rows++;
         columns = line.length();     
     }
     double[][] d = new double[rows][columns]
     return d;      
}

I'm unsure of how to add these values now that I have created the 2d array. i tried this but got an InputMismatchException .

Scanner s1 = new Scanner(file);
double[][] d = new double[rows][columns]

for (int i= 0;i<rows;i++) {
    for (int j= 0;i<rows;j++) {
         d[i][j] = s1.nextDouble();
     }
}

Answer 1

if you just want to use the basic arrays you can achieve it with something like

     Scanner input = new Scanner(new FileReader(file));

     int row=0;
     int col =0;
     String s="";

     //count number of rows
     while(input.hasNextLine()) {
         row++;
         s=input.nextLine();
     }

     //count number of columns
     for(char c: s.toCharArray()) {
         if(c==' ')
             col++;
     }

     col++; // since columns is one greater than the number of spaces

     //close the file
     input.close();

     // and open it again to start reading it from the begining
     input = new Scanner(new FileReader(file));
     //declare a new array
     double[][] d = new double[row][col];   

     int rowNum=0;
     while(input.hasNextLine()) {
         for(int i=0; i< col; i++) {
             d[rowNum][i]= input.nextDouble();
         }
         rowNum++;
     }

However if you prefer to use java collection you can avoid reading the file again. Just store the strings in a list and iterate over the list to extract elements from it.

Answer 2

Based on your input, Your columns = line.length(); is returning 7 rather than 2 as it returns the String length.

Hence try calculating the no of columns in the row columns = line.split(" ").length;

Also while trying to read your input you were using index i for the 2nd for-loop . It should be like below,

for (int i= 0;i<rows;i++) {
    for (int j= 0;j<columns;j++) {
         d[i][j] = s1.nextDouble();
     }
}

Answer 3

In order to work with arrays of unknown size you should read the data into a Collection (such as a List ). However, Collection (s) only work with the wrapper-types; so you will need to copy the elements back into an array of double (s) if that is what you really need. Something like,

public static double[][] readMatrixFrom(String file) throws FileNotFoundException {
    Scanner input = new Scanner(new FileReader(file));
    List<List<Double>> al = new ArrayList<>();
    while (input.hasNextLine()) {
        String line = input.nextLine();
        List<Double> ll = new ArrayList<>();
        Scanner sc = new Scanner(line);
        while (sc.hasNextDouble()) {
            ll.add(sc.nextDouble());
        }
        al.add(ll);
    }
    double[][] d = new double[al.size()][];
    for (int i = 0; i < al.size(); i++) {
        List<Double> list = al.get(i);
        d[i] = new double[list.size()];
        for (int j = 0; j < d[i].length; j++) {
            d[i][j] = list.get(j);
        }
    }
    return d;
}

Which I tested by creating a file in my home folder with your contents and running it like so

public static void main(String[] args) {
    String file = System.getProperty("user.home") + File.separator + "temp.txt";
    try {
        System.out.println(Arrays.deepToString(readMatrixFrom(file)));
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    }
}

And I get (as I assume you wanted)

[[1.0, 2.0], [3.0, 4.0]]