Java 2d Array If statement

Question

I made a 2d array to display a map, i'm having trouble making it so that if the random number "1" can be placed anywhere in the array except in [0][0], I've tried a lot of things with strange results.

public static void main (String[] args)
{
    int rnd = (int) (Math.random()*4);
    int[][]map;
    int rows = 4;
    int columns = 4;
    map = new int[rows][columns];
    map[rnd][rnd] = 1;

    for(int i=0; i<map.length; i++){
        for(int j=0; j<map[0].length; j++){    
                System.out.print(map[i][j] + " ");
        }
        System.out.println("");         
    }
    }
}

anyone got a way to do it?

Answer 1

One way is to choose two random numbers and loop until they're not both zero.

Random random = new Random();
int i, j;
do {
    i = random.nextInt(4);
    j = random.nextInt(4);
} while (i == 0 && j == 0);
int[][] map = new int[4][4];
map[i][j] = 1;

An alternative is to choose a random number from 1 to 15 and then use division and remainder.

Random random = new Random();
int i = random.nextInt(15) + 1;
int[][] map = new int[4][4];
map[i/4][i%4] = 1;

It is not obvious what the second version does, so it probably requires a comment to explain it.

Answer 2

Right now, you are using the same random number for both dimensions, so either (0, 0), (1, 1), (2, 2), or (3, 3) could be set to 1 , but no other spots.

Choose 2 different random numbers, one for each dimension. It's best to create a java.util.Random object, so you can call nextInt(4) to get the range you need. In a while loop, if both numbers are 0 , choose 2 different random numbers again.