Need to find a logic error in a card shuffling method

I'm trying to write a method that takes an array of integers (0-51, in that order), cuts it into two separate arrays (A and B in the below function by using the cut method, which I know for sure works) and then re-fuses the two arrays together by randomly selecting 0, 1 or 2 cards from the BOTTOM of either A or B and then adding them to the deck.

(ps- by "array" I mean linked list, I just said array because I thought it would be conceptually easier)

This is my code so far, it works, but there's a definite bias when it comes to where the cards land. Can anybody spot my logic error?

[code]
void Deck::shuffle(){
    IntList *A = new IntList();
    IntList *B = new IntList();
    cut(A, B);
    IntListNode *aMarker = new IntListNode;
    aMarker = A->getSentinel()->next;
    //cout<< A->getSentinel()->prev->prev->data <<'\n'<<'\n';
    IntListNode *bMarker = new IntListNode;
    bMarker = B->getSentinel()->next;
    //cout<< B->getSentinel()->prev->data;
    deckList.clear();

    srand(time(NULL));
    int randNum = 0, numCards = 0, totalNumCards = 0; 
    bool selector = true, aisDone = false, bisDone = false;

    while(totalNumCards < 52){
        randNum = rand() % 3;
        if(randNum == 0){
            selector = !selector;
            continue;
        }
        numCards = randNum;

        if(!aisDone && !bisDone){
            if(selector){
                for(int i = 0; i < numCards; i++){
                    deckList.push_back(aMarker->data);
                    aMarker = (aMarker->next);
                    if(aMarker == A->getSentinel()){
                        aisDone = true;
                        break;
                    }
                }
                selector = false;
            }else{
                for(int i = 0; i < numCards; i++){
                    deckList.push_back(bMarker->data);
                    bMarker = (bMarker->next);
                    if(bMarker == B->getSentinel()){
                        bisDone = true;
                        break;
                    }
                }
                selector = true;
            }
        }
        if(aisDone && !bisDone){
            for(int i = 0; i < (52 - totalNumCards); i++){
                deckList.push_back(bMarker->data);
                bMarker = (bMarker->next);
                if(bMarker == B->getSentinel()){
                    bisDone = true;
                    break;
                }
            }
            //return;
        }
        if(bisDon开发者_JAVA技巧e && !aisDone){
            for(int i = 0; i < (52 - totalNumCards); i++){
                deckList.push_back(aMarker->data);
                aMarker = (aMarker->next);
                if(aMarker == A->getSentinel()){
                    aisDone = true;
                    break;
                }
            }
            //return;
        }
        totalNumCards += numCards;
    }

    int tempSum = 0;
    IntListNode *tempNode = deckList.head();
    for(int j = 0; j < 52; j++){
        //cout<< (tempNode->data) << '\n';
        tempSum += (tempNode->data);
        tempNode = (tempNode ->next);
    }
    if(tempSum != 1326)
        system("PAUSE");

    return;
}
[/code]

---

What about just using std::random_shuffle? Yeah, it won't work for linked list, but you can change it to vector :)

---

If your instructor would have the moral to teach you programming the way it should be done then they'd encourage you to solve the problem like so, with four lines of code:

#include<algorithm>
#include<vector>

// ...
std::vector<int> cards;  // fill it in ...
std::random_shuffle(cards.begin(), cards.end());

Using the standard library is the right way of doing things. Writing code on your own when you can solve the problem with the standard library is the wrong way of doing things. Your instructor doesn't teach you right. If they want to get a point across (say, have you practice using pointers) then they should be more attentive in selecting the exercise they give you.

That speech given, here is a solution worse than the above but better than your instructor's:

1. 52 times do the following:

   1. Choose two random none-equal integers in the range [0,52).
   2. Swap the values in the array corresponding to these positions.

---

1. For most random number generators, the low bits are the least random ones. So your line

   randNum = rand() % 3;
   

   should be modified to get its value more from the high- to middle-order bits from rand.

2. Your expectations may be off. I notice that you swap the selector if your random value is 0. Coupled with the relative non-randomness of randNum, this may be your problem. Perhaps you need to make things less random to make them appear more random, such as swapping the selector every time through the loop, and always taking 1 or more cards from the selected deck.

---

Comments:

srand(time(NULL));

This should only be called once during an applications run. This it is usally best to call it in main() as you start.

int randNum = 0, numCards = 0, totalNumCards = 0; 
bool selector = true, aisDone = false, bisDone = false;

One identifier per line. Every coding standard written has this rule. It also prevents some subtle errors that can creep in when using pointers. Get used to it.

randNum = rand() % 3;

The bottom bits of rand are the lest random.

rand Num = rand() / (MAX_RAND / 3.0);

Question:

    if(!aisDone && !bisDone)
    {
         This can execute
         and set one of the above to isDone

         Example:
           Exit state aisDone == false bsiDone == false   // OK
           Exit state aisDone == true  bsiDone == false   // Will run below
           Exit state aisDone == false bsiDone == ture    // Will run below
    }
    if(aisDone && !bisDone)
    {
         Is this allowed to run if the first block above is run?
    }
    if(bisDone && !aisDone)
    {
         Is this allowed to run if the first block above is run?
    }

The rest is too complicated and I don't understand.

I can think of simpler techniques to get a good shuffle of a deck of cards:

for(loop = 0 .. 51)
{
    rand = rand(51 - loop);
    swap(loop, loop+rand);
}

The above simulates picking a card at random from the deck A and putting it on the top of deck B (deck B initially being empty). When the loop completes B is now A (as it was done in place).

Thus each card (from A) has the same probability of being placed at any position in B.