iOS Core Data NSRangeException - ios

Question

I have an issue I cant solve. Im creating an iOS application with Core Data framework. Everything was going just fine until I got NSRangeException. This exception I get in function which checks if Application data needs to be updated.
My code is:
- (BOOL) isTimetableUpdatedWithJSONData:(NSMutableDictionary*) data
{
appDelegate = [[UIApplication sharedApplication] delegate];
managedObjectContext = [appDelegate managedObjectContext];
//Do database update check logic here
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:#"MobileTimetableDataHash" inManagedObjectContext:managedObjectContext]];
NSError *error = nil;
NSArray *results = [managedObjectContext executeFetchRequest:request error:&error];
if (error != nil)
{
[self printErrorAlert];
}
//THERE IS ERROR HERE
NSManagedObject* changesGrabbed = [results objectAtIndex:0];
NSString *changesFromDatabase = [changesGrabbed valueForKey:#"changes"];
NSString *changesFromService = [data valueForKeyPath:#"changes"];
if ([changesFromService isEqualToString:changesFromDatabase])
{
return YES;
}
else
{
[changesGrabbed setValue:changesFromService forKey:#"changes"];
[managedObjectContext save:&error];
if (error != nil)
{
[self printErrorAlert];
}
return NO;
}
}
Exception:
2014-06-12 21:36:12.034 MIF[437:60b] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayI objectAtIndex:]: index 0 beyond bounds for empty array'
*** First throw call stack:
(
0 CoreFoundation 0x0000000101cb3495 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x0000000101a1299e objc_exception_throw + 43
2 CoreFoundation 0x0000000101c6be3f -[__NSArrayI objectAtIndex:] + 175
3 MIF 0x0000000100001942 -[MIFTimetableService isTimetableUpdatedWithJSONData:] + 514
4 MIF 0x000000010000219d -[MIFAppDelegate synchrinizeTimetableService] + 157
5 MIF 0x0000000100001dc2 -[MIFAppDelegate application:didFinishLaunchingWithOptions:] + 114
6 UIKit 0x00000001005ba3d9 -[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 264
7 UIKit 0x00000001005babe1 -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1605
8 UIKit 0x00000001005bea0c -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 660
9 UIKit 0x00000001005cfd4c -[UIApplication handleEvent:withNewEvent:] + 3189
10 UIKit 0x00000001005d0216 -[UIApplication sendEvent:] + 79
11 UIKit 0x00000001005c0086 _UIApplicationHandleEvent + 578
12 GraphicsServices 0x0000000103ce671a _PurpleEventCallback + 762
13 GraphicsServices 0x0000000103ce61e1 PurpleEventCallback + 35
14 CoreFoundation 0x0000000101c35679 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
15 CoreFoundation 0x0000000101c3544e __CFRunLoopDoSource1 + 478
16 CoreFoundation 0x0000000101c5e903 __CFRunLoopRun + 1939
17 CoreFoundation 0x0000000101c5dd83 CFRunLoopRunSpecific + 467
18 UIKit 0x00000001005be2e1 -[UIApplication _run] + 609
19 UIKit 0x00000001005bfe33 UIApplicationMain + 1010
20 MIF 0x0000000100001d23 main + 115
21 libdyld.dylib 0x000000010234b5fd start + 1
22 ??? 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)
Guys, can anyone help me please? I realy need your help. Thank you.

Answer 1

Your troublemaker seems to be
NSManagedObject* changesGrabbed = [results objectAtIndex:0];
If results is an empty array, calling objectAtIndex:0 will cause a NSRangeException, because there is no object at index 0. You better call [results firstObject], which won't crash with an error if results is empty array. firstObject() just returns nil in this case.

Answer 2

You are attempting to retrieve an object from an empty array. You should first check to see if the array contains any objects. Use the count property:
if (results.count > 0) {
NSManagedObject* changesGrabbed = [results objectAtIndex:0];
//continue...
} else {
//Array is empty, object 0 will be beyond bounds
}

Answer 3

Just before
NSManagedObject* changesGrabbed = [results objectAtIndex:0];
Add a branch based on the returned results. Something like the following:
if (results && [results count] > 0) {
NSManagedObject* changesGrabbed = [results objectAtIndex:0];
...
}
Your array is empty. The stacktrace pretty much spells it out for you --
reason: '* -[__NSArrayI objectAtIndex:]: index 0 beyond bounds for
empty array'