Adding null object to SQLite - ios

Question

I implemented SQLite in xcode. I have 2 columns. When the first column has a row, then the second one is set to null, and vice versa. I'm trying to go through both columns and check if it has an object at that index.
NSLog(#"%# and %#", firstArray, secondArray); // It gives the right risults, but there aren't any null objects in any of them.
for (int i = 0; i < [firstArray count]; i++)
{
if (![[firstArray objectAtIndex:i] length] && [[secondArray objectAtIndex:i] length]) {
...
}
}
I have two issues.
First: There aren't any null objects in the rows, even though I set some to null. So what should I do to check if there is a null object, or if there's another way to add some kind null object that will actually add to the tables row?
Second: When I compile and run the app, it crashes with the error below. But when I remove the if statement, it works flawlessly.
-[__NSArrayM length]: unrecognized selector sent to instance 0x7fbfeb53b0a0
2015-03-30 11:25:04.731 MyApp [610:15600] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayM length]: unrecognized selector sent to instance 0x7fbfeb53b0a0'
*** First throw call stack:
(
0 CoreFoundation 0x0000000110874f35 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x000000011050dbb7 objc_exception_throw + 45
2 CoreFoundation 0x000000011087c04d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
3 CoreFoundation 0x00000001107d427c ___forwarding___ + 988
4 CoreFoundation 0x00000001107d3e18 _CF_forwarding_prep_0 + 120
5 MyApp 0x000000010fec3b59 -[ViewController ingredientAction] + 889
6 MyApp 0x000000010feba253 -[ViewController menuAction:] + 195
7 UIKit 0x0000000110ea58be -[UIApplication sendAction:to:from:forEvent:] + 75
8 UIKit 0x0000000110fac410 -[UIControl _sendActionsForEvents:withEvent:] + 467
9 UIKit 0x00000001110269ea -[UISegmentedControl _setSelectedSegmentIndex:notify:animate:] + 570
10 UIKit 0x0000000111028a0f -[UISegmentedControl touchesEnded:withEvent:] + 143
11 UIKit 0x0000000111252540 _UIGestureRecognizerUpdate + 9487
12 UIKit 0x0000000110eeaff6 -[UIWindow _sendGesturesForEvent:] + 1041
13 UIKit 0x0000000110eebc23 -[UIWindow sendEvent:] + 667
14 UIKit 0x0000000110eb89b1 -[UIApplication sendEvent:] + 246
15 UIKit 0x0000000110ec5a7d _UIApplicationHandleEventFromQueueEvent + 17370
16 UIKit 0x0000000110ea1103 _UIApplicationHandleEventQueue + 1961
17 CoreFoundation 0x00000001107aa551 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
18 CoreFoundation 0x00000001107a041d __CFRunLoopDoSources0 + 269
19 CoreFoundation 0x000000011079fa54 __CFRunLoopRun + 868
20 CoreFoundation 0x000000011079f486 CFRunLoopRunSpecific + 470
21 GraphicsServices 0x0000000114c759f0 GSEventRunModal + 161
22 UIKit 0x0000000110ea4420 UIApplicationMain + 1282
23 MyApp 0x000000010fec11b3 main + 115
24 libdyld.dylib 0x00000001122ad145 start + 1
25 ??? 0x0000000000000001 0x0 + 1
Edit
I put #"" instead of null into the row. Then I change the if statement (in the for loop) to the following:
NSStirng *objectAtIndexFromArray = [NSString stringWithFormat:#"%#", [firstArray objectAtIndex:i]];
if ([objectAtIndexFromArray isEqualToString:#""])
{
NSLog(#"Hello");
}
I don't get any NSLogs of Hello.

Answer 1

I'm pretty new to ios development, but I know that if you want to add null object to you data collection you have to use NSNULL class.
NSArray can't hold nils, if you want to check if null, please use this code
id object = myArray[0];// similar to [myArray objectAtIndex:0]
if(![object isEqual:[NSNull null]])
{
//do something if object is not equals to [NSNull null]
}