replaceObjectAtIndex array problems

Question

Been searching for the answer to this for a while now and I think due to the nature of my array set up, I may be searching for the wrong answer!

I have a class which handles adding items to my array:

// Item.h
@interface Item : NSObject {
    NSString *name;
    NSNumber *seconds;
}

@property(nonatomic,copy) NSString *name;
@property(nonatomic,copy) NSNumber *seconds;

- (id)initWithName:(NSString *)n seconds:(NSNumber *)sec;

@end

and...

//item.m

@implementation Item
@synthesize name, seconds;


- (id)initWithName:(NSString *)n seconds:(NSNumber *)sec {
    self.name = n;
    self.seconds = sec;
    return self;
}
@end

So to add an item, I use

Item *item1 = [[Item alloc] initWithName:@"runnerA" seconds:[NSNumber numberWithInt:780]];

I have some code which allows a user to edit a textfield (runner name) and the time which is a UIdatepicker set to hours and minutes. In the save method, that's working fine. It's the UPDATE that I cannot get to work. I've tried alsorts! Here's the code at the moment...

mainAppDelegate *appDelegate = (mainAppDelegate *)[[UIApplication sharedApplication] delegate];    
Item *item = [[Item alloc] initWithName:inputName.text seconds:[NSNumber numberWithInt:secs]];
[appDelegate.arrItems replaceObjectAtIndex:rowBeingEdited withObject:item];

The above is simply adding a new item to the array (which is what I don't want). I'm not sure how to replace values. At the function, I have the row I need to update (rowBeingEdited) and the fields inputName.text and secs are both OK. (NSLog out confirms this).

How do I use the replaceObjectAtIndex to actually replace it with the values?! It's driving me mad now!!

Answer 1

1. Are you using NSArray or NSMutableArray?
2. Assuming you are using NSMutableArray, how did you initialize and populate the array in the first place?

For example, it's not enough to use -initWithCapacity: or +arrayWithCapacity: which only sets aside space. You have to use -addObject: for the first round of population, before you can use -replaceObjectAtIndex:withObject::

Note that NSArray objects are not like C arrays. That is, even though you specify a size when you create an array, the specified size is regarded as a “hint”; the actual size of the array is still 0. This means that you cannot insert an object at an index greater than the current count of an array. For example, if an array contains two objects, its size is 2, so you can add objects at indices 0, 1, or 2. Index 3 is illegal and out of bounds; if you try to add an object at index 3 (when the size of the array is 2), NSMutableArray raises an exception.

Answer 2

Since you are simply trying to edit a particular row, why not use those property accessors that you already have set up in Item? It would look something like this:

Item *item = (Item *)[appDelegate.arrItems objectAtIndex:rowBeingEdited];
[item setName:inputName.text];
[item setSeconds:[NSNumber numberWithInt:secs]];

An a side note, are you using garbage collection, or do you manually release the Item objects that you create when adding items to the array? If you are doing it manually, it should look like this:

Item *item1 = [[Item alloc] initWithName:@"runnerA"
                                 seconds:[NSNumber numberWithInt:780]];
[appDelegate.arrItems addObject:item1];
[item1 release];

This follows the rule of thumb: if you alloc, copy or retain anything, you must also release it. Note that this works because the array will retain the item when it is added.