Question

I’m having a little trouble with some code I’m writing. Basically, I’m trying to “shuffle” a stack collection like a deck of cards, but for some reason, one of the temporary stacks I’m using won’t empty completely, and this causes an empty collection exception on the next run around. I traced the code and the output by hand, and the elements are being left in temporary stack 1 (the code is below). I’m really not sure why this is happening! If you have any insight on this, it’d be really helpful.

Here is a link to the problem method: http://pastebin.com/cxJCmemZ

public void shuffleCards(LinkedStack<UnoCard> deck) {
        int tempIndex;
        LinkedStack<UnoCard> tempCardStack1 = new LinkedStack<UnoCard>();
        LinkedStack<UnoCard> tempCardStack2 = new LinkedStack<UnoCard>();

        //Fisher-Yates shuffle
        for (int i = (deck.size() - 1); i >= 0; i--) {
            tempIndex = ((int)(i * Math.random()));

            System.out.println("i is: " + i);
            System.out.println("tempIndex is: " + tempIndex);

            //swap if cards are different
            if (tempIndex != i) {
                //pop face down cards up to first card onto temporary stack
                System.out.println("Popping up to first card");
                for(int j = 0; j <= tempIndex; j++) {
                    UnoCard tempCard = faceDownCards.pop();
                    System.out.println(tempCard.toString());
                    tempCardStack1.push(tempCard);
                }

                //pop face down cards up to second card onto temporary stack
                System.out.println("Popping up to second card");
                for(int j = (tempIndex + 1); j <= i; j++) {
                    UnoCard tempCard = faceDownCards.pop();
                    System.out.println(tempCard.toString());
                    tempCardStack2.push(tempCard);
                }

                //replace first card in second card position
                System.out.println("Replacing first card");
                UnoCard tempCard = tempCardStack1.pop();
                System.out.println(tempCard.toString());
                faceDownCards.push(tempCard);

                //place second card in temporary stack
                System.out.println("Transferring second card");
                tempCard = tempCardStack2.pop();
                System.out.println(tempCard.toString());
                tempCardStack1.push(tempCard);

                //replace temporary stack
                System.out.println("Replacing second stack");
                for(int j = 0; j < tempCardStack2.size(); j++) {
                    tempCard = tempCardStack2.pop();
                    System.out.println(tempCard.toString());
                    faceDownCards.push(tempCard);
                }

                //replace second card in first card position
                System.out.println("Replacing second card");
                tempCard = tempCardStack1.pop();
                System.out.println(tempCard.toString());
                faceDownCards.push(tempCard);

                //replace temporary stack
                System.out.println("Replacing first stack");
                for(int j = 0; j < tempCardStack1.size(); j++) {
                    tempCard = tempCardStack1.pop();
                    System.out.println(tempCard.toString());
                    faceDownCards.push(tempCard);
                }
            }
        }
    }

Answer 1

if you pop a stack the size shrinks so the for loop will only run for size/2 times

so the ending loops should really be

while(!tempCardStack1.isEmpty()){
    tempCard = tempCardStack1.pop();
    System.out.println(tempCard.toString());
    faceDownCards.push(tempCard);
}