iOS How to check if valueForKey exist or not? - ios

Question

I have another question about checking the existence of valueForKey.
Here's my code:
id _jsonId = [NSJSONSerialization JSONObjectWithData:[_json dataUsingEncoding:NSUTF8StringEncoding] options:0 error:&_error];
Monday = [[NSMutableArray alloc] init];
if ([_jsonId valueForKey:#"Mon"]!= Nil) {
_keyMon = [_jsonId valueForKey:#"Mon"];
[Monday addObject:#"Monday"];
for (int i=0; i<5; i++) {
if ([_keyMon objectAtIndex:i]!=[NSNull null]) {
[Monday addObject:[_keyMon objectAtIndex:i]];
}
}
} else {
[_keyTues addObject:#"No Class For Today"];
[Monday addObject:[_keyMon objectAtIndex:1]];
}
The idea is, if the _jsonId valueForKey came back nothing, I can add this NSStringaddObject:#"No Class For Today incase XCode tells me that:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'
like it has been telling me now. Anyone have any solution?
I tried if ([_jsonId valueForKey:#"Mon"]!= Nil) didn't work.
I tried if ([_jsonId valueForKey:#"Mon"]!= nil) didn't work.
I tried if ([_jsonId valueForKey:#"Mon"]!= (id)[NSNull null]) didn't work.
I tried if ([_jsonId valueForKey:#"Mon"]!= [NSNull null) didn't work.
I know that _jsonId valueForKey is came back as a pointer, but I have no idea how to check if the pointer is null(or nil in Objective-C if I'm correct). Please help me.
UPDATE:
I have tried using objectForKey and here is a code(_jsonId objectForKey is nil, that I know to try testing the code)
if (_jsonId[#"Sun"]== nil) {
[_keySun addObject:#"No Class For Today"];
[Monday addObject:[_keySun objectAtIndex:1]];
}
else{
_keySun = [_jsonId objectForKey:#"Sun"];
[Monday addObject:#"Sunday"];
for (int i=0; i<5; i++) {
if ([_keySun objectAtIndex:i]!=nil) {
[Monday addObject:[_keySun objectAtIndex:i]];
}
}
}
Still, XCode telling me that I'm add a nil into NSArray.
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil'
This is driving me crazy...

Answer 1

_jsonId as defined by:
id _jsonId = [NSJSONSerialization JSONObjectWithData:[_json dataUsingEncoding:NSUTF8StringEncoding] options:0 error:&_error];
... will be either an array or a dictionary. Based on your code I think you're assuming that it's always a dictionary. So you probably want to add:
if (![_jsonId isKindOfClass:[NSDictionary class]]) ...
You then want to know what's in a dictionary, if anything. So use objectForKey:. Or, better, just _jsonId[#"Mon"]. You'll get an object or you'll get nil.
valueForKey: is a completely different mechanism, key-value coding. It is defined on NSObject so inherited by NSDictionary as well as just about everything else. It is defined to raise an exception for an unknown key. Since it raises an exception instead of returning a result, your attempts to compare a result are ineffective.

Answer 2

I think I figured it out. Let's look at the code:
if (_jsonId[#"Sun"]== nil) {
[_keySun addObject:#"No Class For Today"];
[Monday addObject:[_keySun objectAtIndex:1]];
}
else {
_keySun = [_jsonId objectForKey:#"Sun"];
...
If there is an object for the key "Sun" then you store it into _keySun. If there is no object, then you store nothing into _keySun. _keySun will be nil. You send an addObject message to _keySun - but any message sent to a nil object does nothing. You then call [_keySun objectAtIndex:1]. Again, sending a message to a nil object does nothing, and returns nil, which is why you add nil to the Monday object and get a crash.
If you had set _keySun to an empty NSMutableArray, you would have crashed for another reason: You try to add one object. You can read that one object with objectAtIndex:0. But to read objectAtIndex:1 you need two objects in the array.
BTW. You will get [NSNull null] from the JSON parser if the JSON document that you are reading contains an explicit null. So if your JSON contains "Sun": null then objectForKey:#"Sun" will give [NSNull null]. If your JSON doesn't contain the key "Sun" at all, objectForKey:#"Sun" will give nil.