Trouble getting a card shuffle method to work

Question

Here is the problem description:

"The shuffle method shuffles the array of Card objects by looping through the array of Card objects one position at a time and exchanging (see slides 62-63 of the array lecture) the Card in that position with the Card in a random position (as determined by a Random number) between 0 and 51."

I've written the below code to do the above, but it's not shuffling my card objects.

public void shuffle()
{
    //reset nextCard to 0.
    nextCard = 0;

    Random randomGen = new Random();
    Card tempCard;

    int randomNum = randomGen.nextInt(DECK_SIZE);

    int i;
    for(i=0;i<DECK_SIZE;i++)
    {
        tempCard = deck[i];
        deck[i] = deck[randomNum];
        deck[randomNum] = tempCard;
    }//end for.
}//end shuffle().

What's the problem and how do I fix it?

P.S. I will post console output upon request.

Answer 1

You should generate randomNum for each iteration of the for loop

Rewriting your loop like this will give you expected result,

int randomNum;
int i;
for(i=0;i<DECK_SIZE;i++)
{
    randomNum = randomGen.nextInt(DECK_SIZE);
    tempCard = deck[i];
    deck[i] = deck[randomNum];
    deck[randomNum] = tempCard;
}//end for.

Answer 2

The method of @Karthik is incorrect because it uses randomGen.nextInt(DECK_SIZE) which returns a number [0..DECK_SIZE-1] each time. What you really want is to swap DECK[i] with one of the values [DECK[i+1]..DECK[N-1]] which is one of the remaining values not yet used. This is called the Knuth shuffle.

public static void shuffle(int[] a)
{
    int N = a.length;
    for (int i = 0; i < N; i++)
    { // Exchange a[i] with random element in a[i..N-1]
        int r = i + StdRandom.uniform(N-i);
        int temp = a[i];
        a[i] = a[r];
        a[r] = temp;
    }
}