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

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 originalNumbers = new ArrayList();
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();
}