How to access a string in a class stored in an array?

Question

I have a string (titleName) stored in a class (newNoteBook) stored in an array (myLibrary). I was trying to access it, but I only get a (null) printed in the log.

What am I doing wrong?

-(void) setupLibrary {

    NoteBook *newNoteBook = [[NoteBook alloc] init];

    newNoteBook.titleName = @"TEST";
    NSLog(@"titleName:%@", newNoteBook.titleName); // this prints TEST in the log
    [myLibrary addObject:newNoteBook];
    NSLog(@"titleName:%@", [[self.myLibrary objectAtIndex:0] titleName]); // this prints (null) in the log)
}

There is nothing fancy in my class... simply:

@interface NoteBook : NSObject {

NSString *titleName; }


@property (nonatomic, retain) NSString *titleName;
@end

@implementation NoteBook
@synthesize titleName;

Aaron · Accepted Answer · 2011-04-30 09:27:31Z

2

Try this

NSLog(@"titleName:%@", ((NoteBook *)[self.myLibrary objectAtIndex:0]).titleName);

answered Apr 30, 2011 at 9:27

Aaron

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1 Comment

user557219 Over a year ago

There’s no need for the type cast since the return type of -objectAtIndex: is id.

score 1 · Accepted Answer · 2011-04-30 09:37:13Z

1

Possible reasons:

myLibrary (the instance variable) is nil;
self.myLibrary is nil or its backing instance variable isn’t myLibrary;
[self.myLibrary objectAtIndex:0] is not the same object as newNoteBook because there was at least one other element in self.myLibrary.

Edit: you need to create a new mutable array and assign it to your property/instance variable myLibrary:

self.myLibrary = [NSMutableArray array];

or

myLibrary = [[NSMutableArray alloc] init];

Where you should this depend on how your class is used. If an instance of your class should always have valid myLibrary, a good place to do that is in -init:

- (id)init {
    self = [super init];
    if (self) {
        myLibrary = [[NSMutableArray alloc] init];
    }
    return self;
}

Alternatively, if you want to lazily create myLibrary only when -setupLibrary is executed, create it in that method:

-(void) setupLibrary {
    self.myLibrary = [NSMutableArray array];

    NoteBook *newNoteBook = [[NoteBook alloc] init];
    …
}

Don’t forget to release it in your -dealloc method:

- (void)dealloc {
    [myLibrary release];
    [super dealloc];
}

edited Apr 30, 2011 at 9:37

answered Apr 30, 2011 at 9:25

user557219

5 Comments

n.evermind Over a year ago

Thanks, but how do I achieve that myLibrary or self.myLibrary is not nil? All I have done so far with myLibrary is that I've declared it in my header as NSMutableArray *myLibrary; @property (nonatomic, retain) NSMutableArray *myLibrary; and implementation @synthesize myLibrary;. As for the third possible reason: this is the first object added to myLibrary, so I guess there can't be another one. Sorry if this is basic, but I don't get it. Thanks so much for your help!

n.evermind Over a year ago

Just solved it... it must be myLibrary = [[NSMutableArray alloc] init]; ... I should have known!

n.evermind Over a year ago

Thanks also about the clarification about the not needing to typecast... it looked way too complicated.

user557219 Over a year ago

@never Great; I’ve edited my answer anyway. Don’t forget to release the array in your -dealloc method!

n.evermind Over a year ago

Thanks every so much! This really helped a lot. Slightly off-topic: Good luck with your travels and do let me know when you're down in Oxford.

saadnib · Accepted Answer · 2011-04-30 09:27:47Z

1

I think you are not type casting object from array -

NSLog(@"titleName:%@", [(NoteBook*)[self.myLibrary objectAtIndex:0] titleName]);

and you should alloc your array before adding object to it -

myLibrary = [[NSMutableArray alloc] init];

answered Apr 30, 2011 at 9:27

saadnib

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2 Comments

user557219 Over a year ago

There’s no need for the type cast since the return type of -objectAtIndex: is id.

n.evermind Over a year ago

thanks, I didn't alloc myLibrary of course! Thanks for your help. As for the type casting, NSLog(@"titleName:%@", [[self.myLibrary objectAtIndex:0] titleName]); works fine and looks (in my beginner's eyes) a little bit less complicated than what you suggested?

William A · Accepted Answer · 2011-04-30 10:21:25Z

1

NSLog(@"titleName:%@", [self.myLibrary objectAtIndex:0].titleName);

Is correct as they said before you don't need to cast.

answered Apr 30, 2011 at 10:21

William A

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