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 CoreDat

Answer 1

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

Answer 2

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.

Answer 3

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.