Remove NSNull values in a KVC generated NSMutableArray - ios

Question

Here's a little test:
NSDictionary * test = # { #"centers" : #[
#{ #"id":#"12345", #"favorite":#NO },
#{ #"id":#"2345", #"favorite":#NO },
#{ #"id":#"345", #"favorite":#YES },
#{ #"favorite":#NO }
]};
NSMutableArray * test2 = [test mutableArrayValueForKeyPath:#"centers.id"];
NSLog(#"%#",test2);
The log gives:
(
12345,
2345,
345,
"<null>"
)
That's great, now, I wish to get rid of the NSNull values... According to How will I be able to remove [NSNull Null] objects from NSMutableArray? this is the way:
[test2 removeObjectIdenticalTo:[NSNull null]];
NSLog(#"%#",test2);
Unfortunately, it crashes:
*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSDictionaryI 0x7fe2ee01e970> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key id.'
*** First throw call stack:
(
0 CoreFoundation 0x000000010e47df35 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x000000010e116bb7 objc_exception_throw + 45
2 CoreFoundation 0x000000010e47db79 -[NSException raise] + 9
3 Foundation 0x000000010dcb37b3 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 259
4 Foundation 0x000000010dd44be0 -[NSArray(NSKeyValueCoding) setValue:forKey:] + 186
5 Foundation 0x000000010dd471cc -[NSKeyValueSlowMutableArray removeObjectAtIndex:] + 89
6 CoreFoundation 0x000000010e3d75a3 -[NSMutableArray removeObjectIdenticalTo:] + 131
7 TestImageSizeReduction 0x000000010dbe6d35 -[ViewController viewDidLoad] + 1397
8 UIKit 0x000000010ebd2a90 -[UIViewController loadViewIfRequired] + 738
9 UIKit 0x000000010ebd2c8e -[UIViewController view] + 27
10 UIKit 0x000000010eaf1ca9 -[UIWindow addRootViewControllerViewIfPossible] + 58
11 UIKit 0x000000010eaf2041 -[UIWindow _setHidden:forced:] + 247
12 UIKit 0x000000010eafe72c -[UIWindow makeKeyAndVisible] + 42
13 UIKit 0x000000010eaa9061 -[UIApplication _callInitializationDelegatesForMainScene:transitionContext:] + 2628
14 UIKit 0x000000010eaabd2c -[UIApplication _runWithMainScene:transitionContext:completion:] + 1350
15 UIKit 0x000000010eaaabf2 -[UIApplication workspaceDidEndTransaction:] + 179
16 FrontBoardServices 0x0000000111f582a3 __31-[FBSSerialQueue performAsync:]_block_invoke + 16
17 CoreFoundation 0x000000010e3b353c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
18 CoreFoundation 0x000000010e3a9285 __CFRunLoopDoBlocks + 341
19 CoreFoundation 0x000000010e3a9045 __CFRunLoopRun + 2389
20 CoreFoundation 0x000000010e3a8486 CFRunLoopRunSpecific + 470
21 UIKit 0x000000010eaaa669 -[UIApplication _run] + 413
22 UIKit 0x000000010eaad420 UIApplicationMain + 1282
23 TestImageSizeReduction 0x000000010dbe81f3 main + 115
24 libdyld.dylib 0x0000000110c50145 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
So, I know many ways of removing NSNull instances from the array (predicates, indexesPassingTests, etc.) but I really want to understand why this fails with so little explanations.
Thanks in advance.
Edit:
To prevent the proxy problem given in Ian's answer the simplest solution is:
NSMutableArray * test2 = [[test valueForKeyPath:#"centers.id"] mutableCopy];

Answer 1

The NSMutableArray returned from mutableArrayValueForKeyPath is not a copy of the values, but instead a proxy mutable array that does a lookup to the original object on each access.
Telling it to remove objects will act upon your original object. Telling it to remove objects whose centers.id is [NSNull null] causes a failure because those objects do not respond to the key-value mapping for id. It's strange to think that it would have given you any result in the first place, but you cannot use this [NSNull null] result in the way that you want.
If you just want the values and you don't want any operations available to allow modification of the original object, I suggest that you construct your array in a different way to begin with; perhaps with a predicate.

Answer 2

NSNull is a null object type which allows to add NULL object to array because array doesn't accept nil value as input. If anything as a NULL we have to feed into array we use [NSNull null] singleton type object.
In your case is generated because of empty memory location it is not [NSNull null] object type. You cannot remove it using [test2 removeObjectIdenticalTo:[NSNull null]];
It can be removed by using NSPredicate Search
Try this Use NSPredicate Search to filter NSArray. You can remove null by using NSPredicate
NSDictionary * test = # { #"centers" : #[
#{ #"id":#"12345", #"favorite":#NO },
#{ #"id":#"2345", #"favorite":#NO },
#{ #"id":#"345", #"favorite":#YES },
#{ #"favorite":#NO }
]};
NSMutableArray * test2 = [test mutableArrayValueForKeyPath:#"centers.id"];
NSLog(#"%#",test2);
NSPredicate *predicate=[NSPredicate predicateWithFormat:#"SELF!=nil"];
NSArray *test3=[test2 filteredArrayUsingPredicate:predicate];
NSLog(#"%#",test3);