Question

I have two methods, the generateRandomCard method gets called within the testMethod, where there is a for loop that runs 100 times. That way it works perfect, but if I set the for loop limit to 1000 or any other number greater than 100 it crashes. Can you see what’s wrong??

- (void)testMethod {

    Globals *myGlobals = [Globals sharedInstance];
    int rankOfFirst = 0;
    int rankOfSecond = 0;
    int playerOneWin = 0;
    int playerTwoWin = 0;
    int ties = 0;
    float firstPercent = 0;
    float secondPercent = 0;
    float tiePercent = 0;
    FiveEval *evaluator = [FiveEval theEvaluator];

    for (int i = 0; i < 100; i++) {
        short fPF = [self generateRandomCard];
        short fPS = [self generateRandomCard];
        short sPF = [self generateRandomCard];
        short sPS = [self generateRandomCard];
        short fFlop = [self generateRandomCard];
        short sFlop = [self generateRandomCard];
        short tFlop = [self generateRandomCard];
        short tur = [self generateRandomCard];
        short riv = [self generateRandomCard];

        rankOfFirst = [evaluator getRankOfSeven:fFlop 
                                               :sFlop 
                                               :tFlop
                                               :tur 
                                               :riv 
                                               :fPF 
                                               :fPS];

        rankOfSecond = [evaluator getRankOfSeven:fFlop 
                                             :sFlop 
                                             :tFlop 
                                             :tur 
                                             :riv 
                                             :sPF 
                                             :sPS];

        if (rankOfFirst > rankOfSecond) {
            playerOneWin++;
        } else if (rankOfSecond > rankOfFirst) {
            playerTwoWin++;
        } else {
            ties++;
        }

        [myGlobals.alreadyPickedCards removeAllObjects];
    }

    firstPercent = ((float)playerOneWin/(float)10000)*100;
    secondPercent = ((float)playerTwoWin/(float)10000)*100;
    tiePercent = ((float)ties/(float)10000)*100;

    NSLog(@"First Player Equity: %f", firstPercent);
    NSLog(@"Second Player Equity: %f", secondPercent);
    NSLog(@"Tie Equity: %f", tiePercent);
}


- (short)generateRandomCard {

    Globals *myGlobals = [Globals sharedInstance];
    short i = arc4random()%51;
    for (int j = 0; j < [myGlobals.alreadyPickedCards count]; j++) {
        if (i == [[myGlobals.alreadyPickedCards objectAtIndex:j] shortValue]) {
            [self generateRandomCard];
        }
    }
    [myGlobals.alreadyPickedCards addObject:[NSNumber numberWithShort:i]];
    return i;
}

Answer 1

You’re probably overflowing your stack in the recursive call to -generateRandomCard. If you generate a card that’s already been picked, you call yourself recursively (and ignore the result, which is a different bug). So, if your random number stream gave you an unlucky sequence that kept returning cards you’ve already picked, then you’ll recurse infinitely until the stack overflows.

Change your card selection algorithm so that instead of using rejection sampling with the potential for infinite looping/recursion, it uses an algorithm with a bounded runtime such as the Fisher-Yates shuffle.