best way to pick a random subset from a collection?
- by Tom
I have a set of objects in a Vector from which I'd like to select a random subset (e.g. 100 items coming back; pick 5 randomly). In my first (very hasty) pass I did an extremely simple and perhaps overly clever solution:
Vector itemsVector = getItems();
Collections.shuffle(itemsVector);
itemsVector.setSize(5);
While this has the advantage of being nice and simple, I suspect it's not going to scale very well, i.e. Collections.shuffle() must be O(n) at least. My less clever alternative is
Vector itemsVector = getItems();
Random rand = new Random(System.currentTimeMillis()); // would make this static to the class
List subsetList = new ArrayList(5);
for (int i = 0; i < 5; i++) {
// be sure to use Vector.remove() or you may get the same item twice
subsetList.add(itemsVector.remove(rand.nextInt(itemsVector.size())));
}
Any suggestions on better ways to draw out a random subset from a Collection?
