NSMutableArray removeObjectAtIndex causing unexpected SIGABRT

Question

I have looked at numerous posts which state various ways in which to remove an object from an array correctly, but I am not sure which method is best to use in my instance. I am loading a dictionary from a plist, this dictionary contains numerous arrays, and these arrays contain another dictionary. So I have 3 storage devices setup, 1 to hold the overall dictionary, another for an array, and finally a dictionary to hold the object from the array:

Header:

NSMutableDictionary *questionsDictionary;
NSMutableArray *questionsArray;
NSDictionary *currentQuestion;

@property (nonatomic, retain) NSMutableDictionary *questionsDictionary;
@property (nonatomic, retain) NSMutableArray *questionsArray;
@property (nonatomic, retain) NSDictionary *currentQuestion;

So my first question is to do with the above, are (nonatomic, retain) the right things to use for the following code.

Next I load in my dictionary from the plist:

.m:

NSString *path = [[NSBundle mainBundle] bundlePath];
NSString *finalPath = [path stringByAppendingPathComponent:@"MultipleChoice.plist"];
self.questionsDictionary = [[NSDictionary dictionaryWithContentsOfFile:finalPath] retain];

I then setup my question array based upon type based upon my question type:

- (void)setupQuestionType : (NSString *)qType
{
if ([self.questionsDictionary objectForKey:qType])
{
    self.questionsArray = [self.questionsDictionary objectForKey:qType];

    [self pickRandomQuestion];
}
}

Finally (this is where I get the error), I want to grab the a question at random from this question category:

// Pick a random question number based upon amount of questions
int randomQuestionNum = [[NSNumber numberWithInt:(arc4random() % [self.questionsArray count])] intValue];

// Grab the dictionary entry for that question
currentQuestion = [self.questionsArray objectAtIndex:randomQuestionNum];

// Remove the question from the available questions
[self.questionsArray removeObjectAtIndex:randomQuestionNum]; (Error here)

// Set the question text
self.question.text = [currentQuestion objectForKey:kQuestionkey];

Now if I comment out the removeObjectAtIndex line then the code runs fine, and my question is displayed on the screen. This leads me to believe that it isn't a null pointer. So the logical answer points to the fact that self.questionsArray isn't a NSMutableArray. So I tried the following when setting the array:

- (void)setupQuestionType : (NSString *)qType
{
if ([self.questionsDictionary objectForKey:qType])
{
    NSMutableArray *temp = (NSMutableArray *)[self.questionsDictionary objectForKey:qType];
    self.questionsArray = (NSMutableArray *)temp;

    [self pickRandomQuestion];
}
}

Purely to see if I could type_cast it but the error still occurs. Can anyone shed some light on what I'm doing wrong, or the best approach to take?

Answer 1

Don't typecast NSArray to NSMutableArray. Instead:

NSArray *temp = [self.questionsDictionary objectForKey:qType];
self.questionsArray = [NSMutableArray arrayWithArray:temp];
// code not tested.