NSMutableArray removeObjectAtIndex: throws invalid argument exception

Question

I'm writing an application to show some news from a portal. The news are fetched using a JSON file from the Internet and then stored into a NSMutableArray using the CoreData Model. Obviously a user can't delete the news from the JSON file on the Internet, but he can hide them locally. The problems come out here, where I have the following code:

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath {    
if (editingStyle == UITableViewCellEditingStyleDelete) {
    if( !moc ){
        moc = [[NewsFetcher sharedInstance] managedObjectContext];
    }
    [[dataSet objectAtIndex:indexPath.row] setEliminata:[NSNumber numberWithBool:YES]];
    NSError *error;
    if( ![moc save:&error] ){
        NSLog( @"C'è stato un errore!" );
    }
    [dataSet removeObjectAtIndex:indexPath.row];
    [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:YES];
}   

The line:

[dataSet removeObjectAtIndex:indexPath.row];

cause my apps to crash with the following error:

2010-07-12 19:08:16.021 ProvaVideo[284:207] * -[_PFArray removeObjectAtIndex:]: unrecognized selector sent to instance 0x451c820 2010-07-12 19:08:16.022 ProvaVideo[284:207] * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[_PFArray removeObjectAtIndex:]: unrecognized selector sent to instance 0x451c820'

I'm trying to understand why it doesn't work but I can't. If I re-launch the app, the new is correctly logically cancelled. Any suggestions?? Thanks in advance.

---

Interface:

@interface ListOfVideo : UITableViewController <NSFetchedResultsControllerDelegate> { 
    NSMutableArray *dataSet;
} 
@property (nonatomic, retain) NSMutableArray *dataSet;

// In the .m file:
@synthesize dataSet;

Initialization in viewDidLoad:

dataSet = (NSMutableArray *) [[NewsFetcher sharedInstance] 
                                fetchManagedObjectsForEntity:@"News" 
                                withPredicate:predicate
                                withDescriptor:@"Titolo"]; 
[dataSet retain]; 

updateDatabase ... this is when checking for new news from the net, I add them in the MutableArray:

[dataSet addObject:theNews];

Answer 1

Your NewsFetcher returns you an immutable array, not a mutable instance. Use the following instead for initialization:

NSArray *results = [[NewsFetcher sharedInstance] 
                     fetchManagedObjectsForEntity:@"News" 
                     withPredicate:predicate
                     withDescriptor:@"Titolo"];
dataSet = [results mutableCopy];

An expression like A *a = (A*)b; only casts the pointer to a different type - it doesn't convert/change the actual type of the instance it points to.

Answer 2

Verify that dataSet is an NSMutableArray. The exception is getting thrown because it doesn't respond to removeObjectAtIndex.