Finding duplicate random numbers in an ArrayList

Question

public class LotteryNumbers {
private ArrayList <Integer> numbers;

public LotteryNumbers() {
    this.numbers = new ArrayList <Integer> ();
    this.drawNumbers();
}

public ArrayList <Integer> numbers() {
    return this.numbers;
}

public void drawNumbers() {
    Random random = new Random ();
    int counter = 0;

    while (counter < 7) {
        this.numbers.add(random.nextInt(39) + 1);
        counter++;
    }

}

This is a class used for printing 7 numbers from 1..39.

It does that job but the problem is I want the 7 random numbers to be different.

How do I check if an arrayList contains the same number since it is random?

Thanks for reading.

have you tried contains() ?? .. Or have you tried using a set? — TheLostMind, Jan 22 '14 at 15:26
http://stackoverflow.com/questions/4228975/how-to-randomize-arraylist — gefei, Jan 22 '14 at 15:26

Christian Tapia · Accepted Answer · 2014-01-22T15:33:46.367

You could try using the contains() method from the ArrayList numbers:

public void drawNumbers()
{
    Random random = new Random();
    int counter = 0;
    int choice;
    while (counter < 7) {
        choice = random.nextInt(39) + 1;
        if (numbers.contains(choice)) {
            continue;
        }
        numbers.add(choice);
        counter++;
    }
}

From Java Docs:

public boolean contains(Object o): Returns true if this list contains the specified element.

So, if the ArrayList already contains the choice (randomly generated), it will continue to the next iteration (counter won't be increased) and choose another random number. If it doesn't contains the choice, it will add it to the array and increase counter.

This can also be done by this way (without using continue)

if (!numbers.contains(choice)) {
    numbers.add(choice);
    counter++;
}

Thanks Christian! This has worked. I tried something like this and I now know why it did not work. My mistake was that I put int choice = random.nextInt(39) + 1; outside of the loop, which kept giving me the same random number, but after reading your code I can now understand that I had to do it in the loop so I get a new random number every time. Thanks again. — user3116280, Jan 22 '14 at 15:36

Bludzee · Answer 2 · 2014-01-22T15:38:54.967

2

How do I check if an ArrayList contains the same number since it is random?

Like this (example):

public void drawNumbers() {
    Random random = new Random ();
    int counter = 0;

    while (counter < 7) {
        int newNumber = random.nextInt(39) + 1;
        if (! numbers.contains(newNumber)) {
            this.numbers.add(newNumber);
            counter++;
        }
    }
}

edited Jan 22 '14 at 15:38

answered Jan 22 '14 at 15:31

Bludzee

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score 0 · Answer 3 · answered Jan 22 '14 at 15:53

You could use contains as as the earlier responses suggest, however contains on an array list in inefficient with O(n) complexity. One of the comments by @TheLostMind suggest using a Set, the best Set implementation to use in this instance is BitSet, note it does not confirm to the java.util.Set interface specification.

public class LotteryNumbers {
private final int[] numbers = new int[7]

public LotteryNumbers() {
    this.drawNumbers();
}

public int[] numbers() {
    return this.numbers;
}

public void drawNumbers() {
    BitSet selected = new BitSet(40);
    Random random = new Random ();
    int counter = 0;

    while (counter < 7) {
        int num = random.nextInt(39) + 1;
        if(!selected.get(num)) {
          selected.flip(num);
          numbers[counter++] = num;
        }
    }

}

This implementation, tho unlikely, does not guarantee that you will always get a result.

score 0 · Answer 4 · answered Jan 22 '14 at 16:06

You could also put your numbers in a list and use COllections.shuffle and get the first 7 occurences.

You do not need to check if duplicate...

ArrayList list = new ArrayList(); list.add(1); list.add(2); ....

Collections.shuffle(list);

loop and get your numbers...

int num = Integer.intValue(list.get(i));

Finding duplicate random numbers in an ArrayList

4 Answers4