How to generate a list of random numbers between a certain range, then get numbers not included in that list

Question

I have generated a list of 20 unique random numbers between 1 and 300, and used that for what I need to use it for. However, I also need the numbers that have not made it to the list to be added into another list for use in another function.

Here is the code I have used to generate the random list of 20 numbers:

JToggleButton[][] p = new JToggleButton[5][4];
ArrayList<Integer> list = new ArrayList<Integer>();

    Random rand = new Random();
    for (int i = 0; i < p.length; i++) {
        for (int j = 0; j < p[i].length; j++) {

            int randomNum = rand.nextInt((300 - 1) + 1) + 1;

            while (list.contains(randomNum)) {
                randomNum = rand.nextInt((300 - 1) + 1) + 1;
            }

            list.add(randomNum);

// rest of code that I need the random number list for

I need to get the other 280 numbers not included in this 20 item list into another list, but I am unsure how to actually get these 'unused' numbers

Answer 1

If you truly need two lists exactly as you say, then here's a method of doing that:

final int numberOfInts = 300;
List<Integer> excluded = new ArrayList<>();
for (int i = 1; i <= numberOfInts ; i++) {
    excluded.add(i);
}

Random rand = new Random();
List<Integer> included = new ArrayList();
for (int i = 0; i < p.length; i++) {
    included.add(excluded.remove(rand.nextInt(excluded.size())));
}

Answer 2

Took me forever to understand what your question was asking.

    ArrayList<Integer> otherNumbers = new ArrayList<Integer>();
    for(int x = 0; x < list.size(); x++) {
        if(!list.contains(x+1)) {
            otherNumbers.add(x+1);
        }
    }

You can just populate a new list by skipping over the values the 20 number list contains.

Answer 3

I think the best way to do this would be to generate a separate list and populate it with every number between 1 and 300. After that, just iterate through your list of generated random numbers and remove those elements from your 1 through 300 list. Something like this..

ArrayList<Integer> list300 = new ArrayList<Integer>();
For (int i = 1; i <= 300; i++) {
    list300.add(i);
}

For (int i = 0; i < list.size(); i++) {
    if (list300.contains(list[i])) {
        list300.remove(Integer.vaueOf(list[i]));
    }
}

Answer 4

You can create a List with all 300 numbers to begin with, and then when you add a random number to the other list, just remove the same number from the List with all of the values:

Create the list:

    ArrayList<Integer> excludedNumbers = new ArrayList<>(300);
    for (int i = 1; i <= 300; i++){
        excludedNumbers.add(i);
    }

Add a single line to your current code:

ArrayList<Integer> randomList = new ArrayList<>();
Random rand = new Random();

for (int i = 0; i < p.length; i++) {
    for (int i = 0; i < p[i].length; i++) {

            int randomNum = rand.nextInt((300 - 1) + 1) + 1;

            while (randomList.contains(randomNum)) {
                randomNum = rand.nextInt((300 - 1) + 1) + 1;
            }

            excludedNumbers.remove((Integer) randomNum); // the new line
            randomList.add(randomNum);
    }
} 

The cast of Integer ensures the List uses the Object remove function rather than the int remove option.

Answer 5

I hope I got your question right as it was kind of hard to understand for me. I would do it like this:

ArrayList<Integer> notIncludedNumbers = new ArrayList<Integer>();
for (int i = 0; i <= 300; i++){
   if (!list.contains(i))
      notIncludedNumbers.add(i);
}

You iterate over all the numbers from 1 to 300 and add them to the notIncludedNumbers list if they are not contained in your list with the 20 random numbers. I hope I could help.