Filling empty cells of a matrix with random values in java

Question

I am trying to make a matrix in Java that is going to be filled with random values only in that positions that are empty. I am trying these code but it doesn't work. I made two nested for loops, one for the rows and one for the columns. And for every position, I ask if it's 0 (I'm not sure how to make a control on that) and if it does not generate a random coordinate and print it in the format (i,j). But it is always printing 0.

public int[][] getRandomPosition(int matrixlength) {
       int[][] grid = new int[matrixlength][matrixlength];

       for (int i = 0; i < grid.length; i++) {
          for (int j = 0; j < grid[i].length; j++) {
             if (grid[i][j] == 0) {
                grid[i][j] = (int) Math.random();
                System.out.print(grid[i][j]);
                return grid;
             }
          }
       }
       return null;
}

Any idea of how I can resolve the problem at hand?

score 0 · Answer 1 · answered May 27 '17 at 19:26

Math.radnom() returns a double that's greater or equal than 0.0 and less than 1.0. When you cast it to an int, it will be truncated to 0. Instead, you need to generate a random int. Additionally, you shouldn't return the grid before filling it up completely. E.g.:

Random rand = new Random();
for (int i = 0; i < grid.length; i++) {
    for (int j = 0; j < grid[i].length; j++) {
        if (grid[i][j] == 0) {
            grid[i][j] = rand.nextInt();
        }
     }
}

Ousmane D. · Accepted Answer · 2017-05-27T19:46:14.920

The reason as to why it's always printing zero is because of Math.random() returns a number from 0.0 to 0.9 inclusive therefore once you cast the result returned from Math.random() to an int, it will always result in 0.

Solution to your problem; you could either use the Random class and utilize its nextInt method or use the formula below:

grid[i][j] = (int)(Math.random() * (max - min) + min); // where min is the minimum value you want to generate and max is the max you want to generate (exclusive).

Though I'd recommend the former, and you'd first need to create a Random object prior to the outer loop i.e

Random randomGen = new Random();

then within your, if block, you can do:

grid[i][j] = randomGen.nextInt();

There's also another Random#nextInt overload which enables you to specify the bounds of the random numbers being generated.

Filling empty cells of a matrix with random values in java

2 Answers2