Question

I want to generate random numbers within the range 1 to 4, 4 including.
Here is my code:

int num = r.nextInt(4) + 1;  //r is instance of Random.

However, I am running the above code in a loop and don’t want repeating random number.
What happens now is often I am getting:
1,1,1,2,3,1,4,2,2,1,4,2,4,4,2,1,4,3,3,1,4,2,4,1 as my output.

Here, though the numbers are random within the range(1-4), but often repeated like the number “1”in the first 3 iterations.

What I am looking for is a way to get non repeating random number within the loop.
One simple way I know is of keeping the last random number before current iteration and compare, but I am sure there must be better solution to this.
Thanks in advance.

Answer 1

Use random.nextInt(range-1) and then map that number to the output number with a function that excludes the previous number:

public class Test {
  private final Random random = new Random();
  private final int range;
  private int previous;

  Test(int range) { this.range = range; }

  int nextRnd() {
    if (previous == 0) return previous = random.nextInt(range) + 1;
    final int rnd = random.nextInt(range-1) + 1;
    return previous = (rnd < previous? rnd : rnd + 1);
  }


  public static void main(String[] args) {
    final Test t = new Test(4);
    for (int i = 0; i < 100; i++) System.out.println(t.nextRnd());
  }
}