Picking out a random element from an organised ArrayList

Question

So the idea here is that I have an ArrayList with a set of words, I want to sort the list so that it only includes entries with even numbered indecies and then picks out an entry at random. I gave this a bash and I managed to get it to show only odd entries like so:

 int i = 0;

    for (Iterator<Phrase> it = phrases.iterator(); it.hasNext(); i++)
    {
        Phrase current = it.next(); 

        if (i % 2 == 0)
        {
            System.out.println(current);    
        }            
    }

And this prints off every odd numbered element on the ArrayList which is fine, but I don't know how to select one at random from the odd numbered ones. This is what I tried putting into the if statement but it doesn't do what I want it to, it does print off the elements at random but it also includes the odd numbered elements when I only want the even ones

Random r = new Random();
int x = r.nextInt(phrases.size());
System.out.println(phrases.get(x));

Any help would be very appreciated here, thanks.

Answer 1

but I don't know how to select one at random from the odd numbered ones.

what about ensuring x to be half of the size and multiply it with 2 to have the even index. Try the following:

 Random r = new Random();
 int x = r.nextInt(phrases.size()/2) + (list.size() & 1) - 1; 
   // size is divided by 2 
  // so that x is randomly 0 to (size/2 -1) inclusive
  System.out.println(phrases.get(x * 2)); // ensuring the accessing index are even

Answer 2

You could loop until you get one, but technically, that may never happen. So instead, just make sure x is a multiple of 2.

            Random r = new Random();
            int x = r.nextInt(phrases.size()); // Might be even or odd
            x = x % 2 != 0 ? x + 1 : x; // if x is not divisible by 2, x + 1, else x
            // x  is is now a multiple of two
            if(x >= phrases.size()){ // make sure x is still within the 
                                                  // index boundaries.
                  x = x-2;
                  if(x < 0){
                      x = 0;
                  }   
            }
            System.out.println(phrases.get(x));