Delete row from tableView - iphone

Question

I am a bit lost on what should be a simple action of deleting a row from a tableview. I have an sqlite database with a table called SUBJECT and I want to delete a record for this table, delete the member from the array and delete the entry in the tableview. I don't know what sequence is best and I keep getting errors.
Here is the code:
- (void)tableView:(UITableView *)tv commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath {
if(editingStyle == UITableViewCellEditingStyleDelete) {
QuotesAppDelegate *appDelegate = (QuotesAppDelegate *)[[UIApplication sharedApplication] delegate];
//Get the object to delete from the array.
Subject *sub = [self.subjects objectAtIndex:indexPath.row];
// check if there are any quoteMaps for this subject and if so do not do it
NSArray *qm = [appDelegate getQuoteMapsFromSubId:sub.subject_id];
if (qm.count==0){
//Delete the object from the table.
[self.subjects removeObjectAtIndex:indexPath.row];
[tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
[sub deleteSubject];
} else {
//DON'T DO IT BUT GIVE A WARNING INSTEAD
}
}
}
I get the following error on the self.subjects removeObject.... line:
-[__NSArrayI removeObjectAtIndex:]: unrecognized selector sent to instance 0x6b2a8f0
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason:
'-[__NSArrayI removeObjectAtIndex:]: unrecognized selector sent to instance 0x6b2a8f0'
The error message says my self.subjects is an NSArray yet my header assigns it as NSMutableArray:
#interface SubjectViewController : UITableViewController <UIAlertViewDelegate> {
NSMutableArray *subjects;
}
#property (nonatomic, retain) NSMutableArray *subjects;
Why is it changing to NSArray???
I checked and found that the Parent ViewController is assigning the following:
NSPredicate *catFilter = [NSPredicate predicateWithFormat:#"category_title == %#", cat.category_title];
NSArray *subs = [appDelegate.subjects filteredArrayUsingPredicate:catFilter];
subViewController.subjects = (NSMutableArray *) subs;
Looks like the problem is there.
I get the following error if I comment out the self.subjects removeObject and try to run the next line of "tv deleteRowsAtIndexPaths...":
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException',
reason: 'Invalid update: invalid number of rows in section 0. The number of rows
contained in an existing section after the update (1) must be equal to the number
of rows contained in that section before the update (1), plus or minus the number
of rows inserted or deleted from that section (0 inserted, 1 deleted) and plus or
minus the number of rows moved into or out of that section (0 moved in, 0 moved out).'
So far this was narrowed down to being a problem in the prior controller view where I am using an NSArray out of a NSPredicate because I couldn't get NSPredicate to work with NSMutableArray. So now that is the issue trying to be resolved.
Or converting/assigning an NSArray as an NSMutableArray...

Answer 1

NSArrays are inmmutable, that means you can't add or remove elements, hence your object doesn't have a removeObjectAtIndex: method.
Check your property to see if its an NSArray instead of an NSMutableArray.
Check that the object in that pointer is in fact an NSMutableArray.
To assign an NSArray to your variable first create an NSMutableArray from it using the arrayWithArray: method.

Answer 2

first your subjects array is immutable. use NSMutableArray class for subject array.
also try to use beginUpdates - endUpdates block...
[tv beginUpdates];
[tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
[tv endUpdates];

Answer 3

The error is being generated because your datasource, always has to be consistent with the rows added/deleted, which means if the previous call to the method:
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
Returned x, and you deleted n rows, then the next call to the method after the delete animation MUST return (x-n)
Additionally if you are using an NSArray as your datasource and you will be adding/deleting rows then it MUST be immutable, as you have to keep your Array consistent with the adding/deletion. Therefore you need to use NSMutableArray.