Bug in random numbers in Android

Question

TreeSet myNumbers = new TreeSet();
Random randGen = new Random();

for (int i = 1; i <= 16; i++) {
    // number generation here
    int randNum = randGen.nextInt(16          


        

Answer 1

To generate a random number in a range, it is like:

int min = ...
int max = ...
int randNumber = min + new Random().nextInt(max - min + 1);

So in your example where you want to generate a random number from [1, 16], it would look like:

int randNumber = 1 + new Random().nextInt(16 - 1 + 1);

Or if you choose to simplify:

int randNumber = 1 + new Random().nextInt(16);

Also, you should really be using a while loop instead of an infinite for loop:

    final TreeSet<Integer> myNumbers = new TreeSet<>();
    final Random rand = new Random();
    for(int i = 0; i < 16; i++){
        int n = 1 + rand.nextInt(16);
        while(!myNumbers.add(n))
            n = 1 + rand.nextInt(16);
    }

Answer 2

You're only generating one number in the range 1-15. You're then generating subsequent numbers with just nextInt:

if (myNumbers.add(randNum))
    break;
else
    randNum = randGen.nextInt();

That should be:

if (myNumbers.add(randNum))
    break;
else
    randNum = randGen.nextInt(16) + 1;

... and fix the initial call to nextInt to remove the "-1". (You don't need the 16 - 1, as explained in Josh's answer.)

Answer 3

It's not a big issue, if you are working in the range 1-16, but your code results in rejection of some randomly drawn numbers, if they have already been picked before. In your solution, the expected value of nextInt() calls is proportional to n log(n), where n is the number of total elements you want to shuffle (16 in your case) – and the actual values can be much higher for a single run. You might consider using a more efficient implementation.

A solution which always uses only n calls:

ArrayList<Integer> originalNumbers = new ArrayList<Integer>();
Random randGen = new Random();
int max = 16;
for (int i = 1; i <= max; i++) {
    // initializing ordered list so it becomes 1, 2, ..., max
    originalNumbers.add(i);
}
for (int i = max; i >= 1; i--) {
    // picking a random number from the ordered list, and swapping it
    // with the last unpicked element which is placed closer to the
    // end of list, where the already picked numbers are stored
    int randNum = randGen.nextInt(i);
    Collections.swap(originalNumbers, i - 1, randNum);
    Toast.makeText(getApplicationContext(), "" + originalNumbers[i - 1], 100).show();
}