Pointer to NSMutableArray returns Null - ios

Question

I am trying to use a NSMutableArray that I have created in ViewController in an ViewController2. But it is just returning nil.
Here is my ViewController.h file:
#property (nonatomic, strong) NSMutableArray *total_hours;
Here is my ViewController.m file:
total_hours = [[NSMutableArray alloc] init];
I also add object. Use NSLog to display that this have actually been added, so that is working. But now I try to use NSLog to display them again in the other ViewController2.
Here is my ViewController2.h file:
#property(nonatomic, assign)NSMutableArray*total_hours_copy;
here is my ViewController2.m file:
#import "TimelisteViewController.h"
// some auto enabled code
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
TimelisteViewController *test = [[TimelisteViewController alloc]init];
NSMutableArray *total_hours_copy = test.total_hours;
NSLog(#"%#", [total_hours_copy objectAtIndex:0]);
}
Why is this not working?

Answer 1

Your question implies that you create the array in ViewController and want to later pass it to ViewController2. However, in ViewController2's viewDidLoad method, you create a new instance of ViewController. So that's a problem.
It would be easier to answer your question if :
You indicated in which method total_hours is initialized.
How control is transferred between the 2 controllers.

Answer 2

You are not initializing you array in the right place. viewDidLoad is only called when the view controller is shown, not at the initialization.
You could override init method in your view controller :
- (void)init{
[super init];
total_hours = [[NSMutableArray alloc] init];
}
However this is not a usual pattern, and i won't recommend it. I'm not sure what you're trying to do, but i believe it would be best to initialize you array in your viewcontroller2 and pass it after to your newly initialize controller.
TimelisteViewController *test = [[TimelisteViewController alloc]init];
NSMutableArray *total_hours = [[NSMutableArray alloc] init];
//Add your data in the array
test.total_hours = total_hours;

Answer 3

Based on the fact that Paul Lalonde has made good considerations, if you need to pass the array from a view controller to another you can follow two ways.
Create a singleton class that would share the array (in this way each controller can access to that singleton and hence to that array)
Inject the array from a controller to another (preferred way since it allows decoupling components and having less application rigidity)
So, following the second solution, from ViewController1 you inject the array like the following snippet. Now both controller will share the same array. Modifications made by one controller will be visible to the other and vice versa...
ViewController2 secondController = // alloc-init here…
secondController.sharedArray = [self sharedArray];
where ViewController2 would have a property like
#property (nonatomic, strong) NSMutableArray* sharedArray;
Then, for example, within its viewDidLoad or wherever you want you can say
[self.sharedController add…]
Said this, what it your application flow? For example, is ViewController2 a controller that is displayed after ViewController1 through a UINavigationController or something similar?

Answer 4

total_hours_copy is a local variable in your viewDidLoad method, not a property!. Change your code to
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view.
TimelisteViewController *test = [[TimelisteViewController alloc]init];
self.total_hours_copy = test.total_hours;
// or maybe self.total_hours_copy = [test.total_hours copy];
NSLog(#"%#", [total_hours_copy objectAtIndex:0]);
}