Accessing properties of an object inside array of objects

Question

I have a list of items showing up on a table view. Every item has its properties such as name, pic, rank etc'. My goal is, every time the user selects a row the item with its properties will be added to a new list.

I've created a new list called listOfBugs and because i want it to be global i've allocated and initialized it inside viewDidLoad. (Is that a proper thing to do?)

Here is my code:

MasterViewController.h

@interface MasterViewController : UITableViewController
{
    NSMutableArray *listOfBugs;
}
@property (strong) NSMutableArray *bugs;

MasterViewController.m

- (void)viewDidLoad
{
    [super viewDidLoad];
    listOfBugs = [[NSMutableArray alloc]init];
    self.title = @"Scary Bugs";
}
...
...
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    ScaryBugDoc *bug = [self.bugs objectAtIndex:indexPath.row];

    UIAlertView *messageAlert = [[UIAlertView alloc]
                                 initWithTitle:@"Row Selected" message:bug.data.title delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

    [messageAlert show];
    [listOfBugs addObject:bug];
    NSLog(@"this is %@",listOfBugs);

}

Using NSLog I can see that the objects are added:

ScaryBugs[1195:11303] this is <ScaryBugDoc: 0x75546e0>
2012-12-05 17:45:13.100 
ScaryBugs[1195:11303] this is <ScaryBugDoc: 0x75546e0>

I have a few questions.

1.How can I access the properties of the objects inside of the array listOfBugs ?

Update: This worked for me:

NSLog(@"this is %@",((ScaryBugDoc *)[listOfBugs objectAtIndex:0]).data.title);

But I can't access the listOfBugs from another class.

I turned it into a property as suggested to make my life easier but still can't access it from another class. For example in listOfBugsViewController.m return [_listOfBugs count]; will give me the error Use of undeclared identifier '_listOfBugs'

2.I want to be abale to populate a table view with the customized list, how can i do that?

After accomplishing that I would like to save the list as a plist and also add and remove objects from it at ease so I need to take that under consideration.

This is the code that I'm based on, I only made a few adjustments to create the new list

Can't you just create a temp object that is the type of object you want, and set the value to be objectAtIndex: and use the properties there? — CBredlow
– CBredlow, Commented Dec 5, 2012 at 16:18
I'm not sure I'm following you but the idea is to have a list of music albums, every album has a list of songs, every song got its properties. The user will be able to select songs from any album to create a new list of songs. — Segev
– Segev, Commented Dec 5, 2012 at 16:22
Edited the first post, Sorry about that. Error: Property 'data' not found on object of type 'id' — Segev
– Segev, Commented Dec 5, 2012 at 17:39
Ah...that's just a case of needing to tell the compiler what kind of object you are getting from the array. Something like: ((ScaryBugDoc *)[newList objectAtIndex:0]).data.title. — Phillip Mills
– Phillip Mills, Commented Dec 5, 2012 at 17:50

lnafziger · Accepted Answer · 2012-12-05 18:47:14Z

2

This is really two questions:

1) How do I make my property a public property which can be accessed by other classes?

You do this just like you did with your bugs property. Add this to your .h file:

@property (strong) NSMutableArray *newList;

Note that if you aren't using different threads, you can make it a little more efficient by using the nonatomic property as well (@property (nonatomic, strong)).

Once you do that, you don't need your iVar declaration because it will automatically be generated for you. (i.e. you can remove NSMutableArray *newList;.)

2) How do I access an object in an array?

Objects in an array are stored as an id object, meaning that it is a "generic" object. If you know what type of object is stored, then you need to tell the compiler what it is so that it knows what properties and methods are appropriate for that class. You do this by casting the variable to the proper type:

ScaryBugDoc *bug = (ScaryBugDoc *)[self.newList objectAtIndex:0];

Then, you can access the properties of the object, assuming that they are public (as covered in point 1 above) like this:

NSLog(@"this is %s", bug.data.tile);

answered Dec 5, 2012 at 18:47

lnafziger

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4 Comments

Segev Over a year ago

For some reason using newList as a property gives me the error "Semantic Issue: Property's synthesized getter follows Cocoa naming convention for returning 'owned' objects." so i'll refer to it as listOfBugs from now on. I turned it into a property but I still can't access it from another class. For example in listOfBugsViewController.m return [_listOfBugs count]; will give me the error Use of undeclared identifier '_listOfBugs'

Segev Over a year ago

I also didn't understood what you meant when you said "You don't need your iVar declaration...i.e. you can remove NSMutableArray *listOfBugs;" What line of my code I can remove?

lnafziger Over a year ago

Yeah, sorry, it can't start with new unless it creates a new object and returns it. Try using listOfBugs instead of _listOfBugs. And the line that you can remove is the second line of your .h file (not counting braces).

Segev Over a year ago

I'm still getting Use of undeclared identifier 'listOfBugs' when trying to access it from another .m file

CBredlow · Accepted Answer · 2012-12-05 18:16:30Z

0

Okay, so based from the comments, this should work:

Album* tempAlbum = [albumList objectAtIndex:i];
//now you can access album's properties
Song* tempSong = [album.songs objectAtIndex:j];
//now you can access song's properties

This can be simplified down to:

Song* someSong = [((Album)[albumList objectAtIndex:i]).songs objectAtIndex:j];

When returning an object from an NSArray, or a collection object like that it will return a generic id object. This will need to be typecasted to the expected object so you can access the right properties.

edited Dec 5, 2012 at 18:16

answered Dec 5, 2012 at 16:58

CBredlow

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