Java random values and duplicates

Question

I have an array (cards) of 52 cards (13x4), and another array (cardsOut) of 25 cards (5x5). I want to copy elements from the 52 cards into the 25 card array by random.

Also, I dont want any duplicates in the 5x5 array. So here's what I have:

        double row=Math.random() *13;
        double column=Math.random() *4;

        boolean[][] duplicates=new boolean[13][4];

        pokerGame[][] cardsOut = new pokerGame[5][5];
        for (int i=0;i<5;i++)
            for (int j=0;j<5;j++){
                if(duplicates[(int)row][(int)column]==false){
                    cardsOut[i][j]=cards[(int)row][(int)column];
                    duplicates[(int)row][(int)column]=true;
                }
            }

2 problems in this code. First, the random values for row and column are only generated once, so the same value is copied into the 5x5 array every time. Since the same values are being copied every time, I'm not sure if my duplicate checker is very effective, or if it works at all.

How do I fix this?

Answer 1

I'd recommend shuffling the deck in place first, then selecting the first 25 cards to place in your 5x5 array. Trying to mix these two steps is unnecessary complication.

If you need to optimize for performance (you probably don't) then it is worth noting that you can use the Fisher-Yates shuffle and stop the algorithm after the first 25 random cards have been selected without shuffling the remaining cards.

Answer 2

I think your approach makes it more difficult than it should be. Try using a class named card, and two collections - one named CardsRemaining and another named CardsUsed.

Initially, CardsRemaining contains all 52 card objects, and CardsUsed is empty. As cards are added to CardsUsed, they are removed from CardsRemaining, naturally preventing duplicates from occuring.

Your program might end up looking something like this (sorry it's in c#):

  class Program
    {
        static void Main(string[] args)
        {
            List<Card> CardsRemaining=new List<Card>();
            for (int i = 0; i < 12; i++)
            {
                for (int j = 0; j < 4; j++)
                {
                    Card c = new Card(i, j);
                    CardsRemaining.Add(c);
                }
            }
            List<Card> CardsUsed = new List<Card>();
            for(int i=0;i<25;i++)
            {
                int cardIndex = getRandomNumber(CardsRemaining.Count);
                Card c = CardsRemaining[cardIndex];
                CardsRemaining.Remove(c);
                CardsUsed.Add(c);
            }
        }
    }

    class Card
    {
        public int Number;
        public int Color;
        public Card(int number, int color)
        {
            this.Number = number;
            this.Color = color;
        }
    }