Choosing random numbers efficiently

Question

I have a method, which uses random samples to approximate a calculation. This method is called millions of times, so its very important that the process of choosing the random numbers is efficient.

I'm not sure how fast javas Random().nextInt really are, but my program does not seem to benefit as much as I would like it too.

When choosing the random numbers, I do the following (in semi pseudo-code):

// Repeat this 300000 times
Set set = new Set();
while(set.length != 5)
    set.add(randomNumber(MIN,MAX));

Now, this obviously has a bad worst-case running time, as the random-function in theory can add duplicated numbers for an eternity, thus staying in the while-loop forever. However, the numbers are chosen from {0..45}, so a duplicated value is for the most part unlikely.

When I use the above method, its only 40% faster than my other method, which does not approximate, but yields the correct result. This is ran ~ 1 million times, so I was expecting this new method to be at least 50% faster.

Do you have any suggestions for a faster method? Or maybe you know of a more efficient way of generation a set of random numbers.

To clarify, here is the two methods:

// Run through all combinations (1 million). This takes 5 seconds
 for(int c1 = 0; c1 < deck.length; c1++){
    for(int c2 = c1+1; c2 < deck.length; c2++){
     for(int c3 = c2+1; c3 < deck.length; c3++){
        for(int c4 = c3+1; c4 < deck.length; c4++){
         for(int c5 = c4+1; c5 < deck.length; c5++){
             enumeration(hands, cards, deck, c1, c2, c3, c4, c5);
         }
            } 
      }     
   }
   }

// Approximate (300000 combinations). This takes 3 seconds
Random rand = new Random();
HashSet<Integer> set = new HashSet<Integer>();
int[] numbers = new int[5];
while(enumerations < 300000){
set.clear();
while(set.size() != 5){
    set.add(rand.nextInt(deck.length));
}
Iterator<Integer> i = set.iterator();
int n = 0;
while(i.hasNext()){
    numbers[n] = i.next();
    n++;
}

After some testing and profiling, I found this method to be the most effective:

Random rand = new Random();
int[] numbers = new int[5];
ArrayList<Integer> list = new ArrayList<Integer>();
while(enumerations < 300000){
 while(list.size() != 5) {
     int i = rand.nextInt(deck.length);
        if(!list.contains(i)) list.add(i);
 }
 int index = 0;
 for(int i : list){ numbers[index] = i; index++; }
 enumeration(hands, cards, deck,numbers);
}

Answer 1

You can try using an existing Java implementation (or this one) for a Mersenne Twister.

Keep in mind most MT's are not cryptographically secure.