How do I select an element randomly from a 2d array

Question

I'm trying to randomly select an element from a 2-d array. I specifically only one element e.g. x1

public static void main(String[] args) {
    Random rand = new Random();
    String[][] couples = {{"x1","y1"},{"x2","y2"},{"x3","y3"},{"x4","y4"},{"x5","y5"}};
    int rnd=rand.nextInt(couples.length);
    System.out.println(couples[rnd]);
}

This is what I've got so far but it prints out: [Ljava.lang.String;@4517d9a3

`couples[rnd][0]` or `couples[rnd][1]` – fantaghirocco Oct 28 '19 at 14:50 — fantaghirocco, Oct 28 '19 at 14:50
`Arrays.toString(couples[rnd])` could work too – baudsp Oct 28 '19 at 14:51 — baudsp, Oct 28 '19 at 14:51
random row or random col??? or both? what are you asking?? – ΦXocę 웃 Пepeúpa ツ Oct 28 '19 at 14:51 — ΦXocę 웃 Пepeúpa ツ, Oct 28 '19 at 14:51

score 1 · Answer 1 · answered Oct 28 '19 at 14:56

public static void main(String[] args) {
    Random rand = new Random();
    String[][] couples = {{"x1","y1"},{"x2","y2"},{"x3","y3"},{"x4","y4"},{"x5","y5"}};
    int rnd1 = rand.nextInt(couples.length);
    int rnd2 = rand.nextInt(couples[rnd1].length);
    System.out.println(couples[rnd1][rnd2]);
}

score 0 · Answer 2 · answered Oct 28 '19 at 14:56

Here you are only specifying random number for the element of first array. Since the element in first array is another array that contains two elements, you have to specify another random number to pick a one like below,

  public static void main(String[] args)
  {
    Random rand = new Random();
    String[][] couples = { { "x1", "y1" }, { "x2", "y2" }, { "x3", "y3" }, { "x4", "y4" }, { "x5", "y5" } };
    int rndOne = rand.nextInt(couples.length);
    int rndTwo = rand.nextInt(couples[rndOne].length);
    System.out.println(couples[rndOne][rndTwo]);
  }

score 0 · Answer 3 · answered Oct 28 '19 at 14:58

System.out.println(couples[rnd])

gives you

[Ljava.lang.String;@4517d9a3

This means your object is a String[]

Remember couples is a String[][]; a 2D array, so giving one index returns you another String[], not a single String.

You need to select another index from the String[]

Something like

int rnd=rand.nextInt(couples.length);
int rnd2 = rand.nextInt(couples[rnd]);
System.out.println(couples[rnd][rnd2]);

Additionally, have a look at https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html#getName to understand why you get [Ljava.lang.String;@4517d9a3.

How do I select an element randomly from a 2d array

3 Answers3