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];

Are you sure that dataset is a NSMutableArray? If it isn't responding to removeObjectAtIndex: it might just be a NSArray. — Tom H, Jul 12 '10 at 17:47
It isn't a `NSMutableArray` if it doesn't respond to that selector. You might not create it correctly or mistakenly use a [`copy` property](http://stackoverflow.com/questions/3220120/nsmutablearray-addobject-nsarrayi-addobject-unrecognized-selector-sent-to/3220137#3220137). If you have doubts try testing for it using `-isKindOfClass:`. — Georg Fritzsche, Jul 12 '10 at 17:57
umble, it is not a NSMutableArray, but: @interface ListOfVideo : UITableViewController { NSMutableArray *dataSet; } @property (nonatomic, retain) NSMutableArray *dataSet; ... // In the .m file @synthesize dataSet; — IssamTP, Jul 12 '10 at 19:44
// 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]; — IssamTP, Jul 12 '10 at 22:22
Please update your question next time, code in comments is hard to read. — Georg Fritzsche, Jul 12 '10 at 22:55

Georg Fritzsche · Accepted Answer · 2010-07-12T22:46:16.903

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.

score 5 · Answer 2 · answered Jul 12 '10 at 17:53

5

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

answered Jul 12 '10 at 17:53

jdot

801
1
6
9

Mumble, it is not a NSMutableArray, but: @interface ListOfVideo : UITableViewController { NSMutableArray *dataSet; } @property (nonatomic, retain) NSMutableArray *dataSet; ... // In the .m file @synthesize dataSet; – IssamTP Jul 12 '10 at 19:42
2

The property being for a mutable array doesn't make the array magically become mutable. – Joshua Weinberg Jul 12 '10 at 21:33
@Joshua, while it doesn't "magically" make it mutable, it will prevent a mutable array from being mutated it it isn't properly defined. – BlackHatSamurai May 18 '13 at 22:51

score 1 · Answer 3 · answered Aug 11 '14 at 18:08

jdot is correct dataSet must be NSMutableArray..

you must do it this way..

  dataSet = [NSMutableArray arrayWithArray:[[NewsFetcher sharedInstance] 
                               fetchManagedObjectsForEntity:@"News" 
                               withPredicate:predicate
                               withDescriptor:@"Titolo"]];

Now the dataSet is a mutable instance of the array you got from NewsFetcher and it won't crash on removing objects.

NSMutableArray removeObjectAtIndex: throws invalid argument exception

3 Answers3

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