how to generate 6 unique java numbers

Question

Possible Duplicate:
Generating Unique Random Numbers in Java

Code will generate 6 random numbers from a range but need them to be unique been melting my brain any pointers would be greatly appreciated

import java.util.Random;
import java.util.Arrays;
public class numbers
{
private int[] lottoNumbers;
private int nextInt;
private Random r;

public numbers()
{
    lottoNumbers = new int [6];
    r = new Random();
    nextInt();
}
public void nextInt()
{
    System.out.print("***     ");
    for (int i = 0; i < lottoNumbers.length; i++)
    {
        lottoNumbers[i] = r.nextInt(49) +1;
        System.out.print(" " + lottoNumbers[i]);
    }
    System.out.println("     ***");
}
}

Answer 1

For each of the six numbers, you need to compare it against the already-generated numbers. If it's the same as any of them, you need to keep trying.

Answer 2

You can simply use the shuffle method and return the 6 first elements after shuffling:

// Create the list of possible numbers
List<Integer> numbers = new ArrayList<Integer>(50);
for(int i = 1 ; i <= 50 ; ++i) {
    numbers.add(i); 
}
// Shuffle the list
Collections.shuffle(numbers);

// Print the numbers
int count = 0;
for(int number : numbers) {
    System.out.println("Random number " + count + ": " + number);
    ++count; // break if count == 6?
}

The advantage of this method is that it runs in linear time (the time taken by the shuffle). You avoid picking x times (x can be very big) an element and checking if it has already been picked!

Answer 3

You need to add a loop that checks if a number exists among the items that you have already added. Define a boolean variable, set it to false, and go through the loop comparing the existing numbers against the newly generated one. If you see a duplicate, set the boolean to true. Once the loop is over, check your boolean to see if the new number has to be added or not.

int i = 0;
do {
    int next = r.nextInt(49) + 1;
    boolean seen = false;
    for (int j = 0 ; j != i ; j++) {
        if (lottoNumbers[j] == next) {
            seen = true;
            break;
        }
    }
    if (!seen) {
        lottoNumbers[i++] = next;
        System.out.print(" " + next);
    }
} while (i != lottoNumbers.length);

A better solution would involve using a Set<Integer>, but that is probably an advanced topic at this stage.

Answer 4

Just use a Set (thanks to @dty for the improved version!):

private static final Random RND = new Random();
private static final int COUNT = 6;

public int[] getRandomNumbers()
{
    final Set<Integer> set = new HashSet<Integer>(COUNT);

    /*
     * This works: .add() will not add the same element twice. As such, we just
     * need to check that the set has the expected size.
     */
    while (set.size() < COUNT)
        set.add(RND.nextInt());

    return set.toArray(new int[COUNT]);
}

Answer 5

public void nextInt()
{
    System.out.print("***     ");
    for (int i = 0; i < lottoNumbers.length; i++)
    {
        int temp;

        // check random int doesn't already exist (embedded assignment below)           
        // NOTE: while loop has empty body, note {} in line below
        while (Arrays.asList(lottoNumbers).contains(temp = r.nextInt(49)+1)){}

        lottoNumbers[i] = temp;
        System.out.print(" " + lottoNumbers[i]);
    }
    System.out.println("     ***");
}

Answer 6

I like the set approach but the example can be stremlined a bit

private static final Random RND = new Random();

public static Set<Integer> getRandomNumbers(int count) {
    final Set<Integer> set = new HashSet<>(count);
    while (set.size() < count) {
      set.add(RND.nextInt());
    }
    return set;
}

Answer 7

How about this way?

package rabdomnumbers;

    import java.util.ArrayList;
    import java.util.List;

    public class RabdomNumbers {
    public static int SIZE = 6;

      public static void main(String[] args) {
        Number[] numbers = new Number[SIZE];
        // set an array
    for (int x = 0; x < SIZE; x++) {
      numbers[x] = new Number();
    }
    // sort an array
    for (int x = 0; x < SIZE-1; x++) {
      for (int y = x + 1; y < SIZE; y++) {
        if (numbers[x].getNumber() > numbers[y].getNumber()) {
          Number buf = numbers[x];
          numbers[x] = numbers[y];
          numbers[y] = buf;
        }
      }
    }
    // print an array
    for (int x = 0; x < SIZE; x++) {
      System.out.println("" + numbers[x].getIndex());
    }
  }
}

Where Number is a simple class:

package rabdomnumbers;

import java.util.Random;

public class Number {
  private int index = 0;
  private int number = 0;
  private static int count = 0;
  private final static Random RND = new Random();

  public int getIndex() {
    return index;
  } 

  public int getNumber() {
    return number;
  }

  public Number() {
    index = count++;
    number = RND.nextInt();
  }
}