Random

Consideration about how to get unique random number fastest and unbiased probability.

Ways to get unique random number

The code below is written in Java.

1. Use HashSet and check duplicate

final int max = 1000; // exclude 0~999
final int count = 1000;

HashSet<Integer> set = new HashSet(max);
Random random = new Random();

for (int i = 0; i < count;)
{
    int rand = random.nextInt(max);
    
    if (set.contains(rand)) continue;
    
    set.add(rand);
    i++;
    System.out.println(rand);
}

This code can takes time infinity. also, It takes time longer when max and count are similar.

2. Use ArrayList

final int max = 1000;
final int count = 1000;

ArrayList<Integer> list = new ArrayList(max);
for (int i = 0; i < max; i++)
{
    list.add(i);
}

for (int i = 0; i < count; i++)
{
    int index = random.nextInt(list.length);
    int rand = list.remove(index);
    
    System.out.println(rand);
}

This code can takes time longer when max is big. (for create array list)

Compare Way 1(HashSet) and Way 2(ArrayList)

Think before the test

I think way2(ArrayList) is faster in most situations because it never occur duplicate.

Test Code

Test Result

After the test

but It's not. The creating array list and remove element is very slow.