Random Numbers with Objective-C

Question

Hey Guys,
I'm creating a blackjack program and I need to randomize 52 numbers, ensuring not to repeat any number.
I know I can use the following to generate a number between 1-52, but I need the to eliminate the possibility of the same card being drawn twice:
cardNumber = arc4random() % 52 + 1;
Any idea on how I would do this? Code example would be helpful!
Thanks,
Justin

Answer 1

Hi Justin -
Justin Carman wrote:
I'm creating a blackjack program and I need to randomize 52 numbers, ensuring not to repeat any number.
I would initialize an array (the deck) with the card values, then use your random number as the index into the array. Each time a card is chosen just remove its value from the deck and decrement the modulus of the random numbers. NSMutableArray makes a nice deck for this purpose since its count is automatically decremented each time a value is removed. E.g.:
// AppController.m
#import "AppController.h"
#define N_DECK 52
@implementation AppController
@synthesize cardArray; // @property (nonatomic, retain) NSMutableArray *cardArray;
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
self.cardArray = [NSMutableArray arrayWithCapacity:N_DECK];
- (IBAction)newDeck:(id)sender {
[cardArray removeAllObjects];
for (int i = 0; i < N_DECK; i++) {
[cardArray addObject:[NSNumber numberWithInt:i+1]];
// NSLog(@"newDeck: cardArray=%@", cardArray);
- (IBAction)getCard:(id)sender {
// check the array
int count = [cardArray count];
assert(count <= N_DECK);
if (count < 1) {
NSLog(@"getCard: Empty deck");
return;
NSLog(@"getCard: count=%d", count);
// select a card from the array
int index = arc4random() % count;
NSNumber *selected = [cardArray objectAtIndex:index];
NSLog(@"getCard: index=%d selected=%@", index, selected);
// print the value of the selected card
int card = [selected intValue];
NSLog(@"getCard: card=%d", card);
// remove the selected card from the array
[cardArray removeObjectAtIndex:index];
// note selected NSNumber object has been released
// NSLog(@"getCard: cardArray=%@", cardArray);
@end
For extra credit you might want to wrap the above in a custom Deck class.
Hope that helps!
- Ray