Pointers and Structures

Hello,
Im writing some code for a very simple card game and need to set up a deck.
To do this I have created a structure to hold the card information (shown below)

Code:

---

typedef struct
{
  int suit;
  int value;
}card;

---

To create the deck i call the following function from main.

Code:

---

card create_deck (void){
  int x = 0,i,j; 
  card *deck;
  deck = (card*) malloc (DECKSIZE * sizeof(card));

  if ( deck == NULL)  {
      fprintf( stderr, "Memory allocation failed\n");  }
  else  {
      for (i = 1; i <= 4; i++)      {
        for (j = 2; j < 15; j++)        {
            deck[x].suit = i;
            deck[x].value = j;
            x++;
        }
      }
  }
  return *deck;
}
}

---

I also need to shuffle this deck, and just to do this i am gng to switch two elements of deck using a random number generator and call the function a few times, however i havent got to that stage yet, so just to test if i am doing it correctly i have set up the following shuffle function

Code:

---

card shuffle_deck (card deck)
{
  int x = 6;
  int y = 9; 
  card temp;
//store values in deck[x] in temp   
  temp.suit = deck[x].suit;
  temp.value = deck[x].value; 
//copy values from deck[y] into deck[x]
  deck[x].suit = deck[y].suit;
  deck[x].value = deck[y].value;
//copy values from temp to deck[y]
  deck[y].suit = temp.suit;
  deck[y].value = temp.value;
}

---

Now the problem is i dont know how, from main, im meant to pass the deck or a pointer to that deck i have created to the shuffle function, below is what my main looks like.
int main (void)

Code:

---

{
  card p;
  p = create_deck();
  shuffle_deck(p);
  return 0;
}

---

Not sure if im going about this the right way or not,
Please help.

If you want to return a deck (rather than a single card) from CreateDeck, you should return a card *, and you need to pass this to the shuffle function.

You should also search for "shuffle" in this forum, as there's been some discussion on the subject, some shuffles are better than others.

--
Mats

Deck is a pointer to card, so create_deck() should have a return type of deck, not card, and return deck, not *deck. Then you have to remember to free() the deck when done. You might also consider passing create_deck() a flag to indicate whether you want the deck to be shuffled or not, and if so, do the shuffling inside create_deck() instead of having a separate function. If you do the shuffling inside create_deck, you can do it by doing a random shuffle of the integers indexing the deck. See

http://en.wikipedia.org/wiki/Knuth_shuffle

for an efficient algorithm.

Dot points rule, so here they are:

* Don't cast malloc see the FAQ
* Consider Enum'ing the suit, it makes for an easier read

Hi I modified the code to that it now looks like so:

Code:

---

CARD createDeck(int deckSize)
{
        int i, j;
        int calc = 0;
        CARD *deck;
       
        deck = (CARD *)malloc(deckSize * sizeof(CARD));
        if (deck == NULL) {
                fprintf (stderr, "Out of Memory!\n");
        }
       
        /* Create a deck of deckSize cards */
        for(i = 0; i <= (deckSize/13); i++) {
                for(j = 1; j < 14; j++) {
                        deck[(i*13) + j-1].value = j;
                        deck[(i*13) + j-1].suit = (i%4);
                }
        }
       
        calc = 63/13;
        printf("calc = %d\n", calc);
       
        for(i = 0; i < deckSize; i++) {
                printf ("v=%d s=%d\n", deck[i].value, deck[i].suit);
        }
       
        return *deck;
}

---

With main being:

Code:

---

int main()
{
        int deckSize = 104;
        CARD *d;
        d = createDeck(deckSize);
        return 0;
}

---

The error it still gives is
"error: incompatible types in assignment"
for the line
"d = createDeck(deckSize);"

Thanks for your responses so far but I still couldn't get this working.

Please help, Thanks :-)

Yes, you should return the type CARD* from the CreateDeck function, and your return statement shouldn't have a star in it - because that will just return the FIRST card in your deck.

--
Mats

You have design issues. deckSize is the product of the number of suits, and the number of cards per suit. You're letting the product be a variable, but in the code itself, you appear to have hardcoded each of the two factors (sort of - even there you don't really seem to have decided). If you want to create a generalized deck, you should pass two variables for the factors, say num_suits and num_cards_per_suit. If you don't want to do this, then just accept that the deck size is fixed at 52.

Isnt deck of type card?

Quote:

---

Originally Posted by DemolitionSquid

Isnt deck of type card?

---

No, deck is of type CARD * (in other words, pointer to CARD), according to your code.

Edit: And as mentioned above, you don't need to cast malloc(). Just make sure to #include <stdlib.h> in any source file that uses it.

Quote:

---

Originally Posted by DemolitionSquid

Isnt deck of type card?

---

Yes, but CreateDeck is declared as:

Code:

---

CARD createDeck(int deckSize)

---

It should be:

Code:

---

CARD *createDeck(int deckSize)

---

And consequently, you need to remove the * at the return, since you want to return the pointer to your deck, not the first card that your deck contains.

--
Mats

Thank you mats,
that solved the problems.

Quote:

---

Originally Posted by DemolitionSquid

Thank you mats,
that solved the problems.

---

I would say "That solved THAT problem" - I agree with Robatino - you should define your "number of suits" and "number of cards per suite" as constants of some sort, and use those constants within your code, rather than have "4", "13", etc hard-coded into the code.

--
Mats

Quote:

---

Originally Posted by matsp

I would say "That solved THAT problem" - I agree with Robatino - you should define your "number of suits" and "number of cards per suite" as constants of some sort, and use those constants within your code, rather than have "4", "13", etc hard-coded into the code.

---

Actually, I don't think that using the constants 4 and 13 is a problem, since a deck of cards normally has 4 suits and 13 cards per suit, and there's not a lot of demand for a generalized deck (unlike a dataset, say, where you want to let the dimensions vary). The problem is that the OP doesn't seem to have decided whether these numbers are constants or not (for example, "i&#37;4" suggests that the number of suits is fixed at 4, but "(deckSize/13)" suggests that it isn't (but that the number of cards per suit is fixed at 13). If both numbers are fixed, then the size of a deck is fixed, so it doesn't make sense to pass deckSize as if it made any sense for it to be a variable.

The deckSize will only be able to be multiples of 52. Its all a bit messy but it works if deckSize is a multiple of 52 where #suits = 4 and #cards per suit = 13 as in a standard deck.

(deckSize/13) is just my kludgey way of incrementing i so that deck[(i*13) + j-1] is in the correct position.

In that case, shouldn't the variable be called something like num_decks, so that you would pass num_decks = 2 instead of deckSize = 104? It just seems like a bad interface to require an argument that's a multiple of 52.