Filling an array with random numbers from the range [-n, n] using Math.random

Question

I have a function in which I enter the size of the multidimensional array n. Next, I fill this array with random numbers in the range [-n, n], using Math.random ():

private int[][] enterMatrixSize() {
    System.out.print("enter matrix size (n): ");
    String input;
    while (!(input = in.next()).matches("\\p{Digit}+")) {
        System.out.print("Please enter a positive Integer: ");
    }
    int size = Integer.parseInt(input);
    int[][] array = new int[size][size];
    for (int i = 0; i < array.length; i++) {
        for (int j = 0; j < array[i].length; j++) {
            array[i][j] = (int) (Math.round(Math.random() * (size + 1)) - size / 2);
        }
    }
    for (int i = 0; i < array.length; i++, System.out.println()) {
        for (int j = 0; j < array[i].length; j++) {
            System.out.print(array[i][j]+" ");
        }
    }
    return array;
}

But it displays some incorrect values. For example, when I enter n equal to 1 - displays the numbers 0, 1 and 2. Which is strange. Since should output -1, 0, 1

Answer 1

I would change this line:

array[i][j] = (int) (Math.round(Math.random() * (size + 1)) - size / 2); 

to:

array[i][j] = ThreadLocalRandom.current().nextInt( -size, size + 1);

to generate a random int value in a specific range, here [-size, size]

Answer 2

I suggest using ThreadLocalRandom which provides a handy method: nextInt(int origin, int bound) . Which can then be used in your loop like this:

int[][] array = new int[size][size];
ThreadLocalRandom r = ThreadLocalRandom.current();
for (int i = 0; i < array.length; i++) {
    for (int j = 0; j < array[i].length; j++) {
        array[i][j] = r.nextInt(-size, size + 1);
    }
}

The first argument origin defines from where the numbers should start and the second argument bound limits the generated numbers to a given value exclusivly.

Answer 3

because round of 1.5 is 2, so in other words supposing random()=1 and size=1 then you have 1*(1+1))-1/2 so (2-1)/2

round(1.5)=2

instead use .floor(x) method or not use round, as you have casting to int, this should also work (as @Lino pointed out).

Summarizing the result of rounding for 1.5 is 2, use floor so you will have 1.