Creating an Empty Array

To create an empty array use NSNull:

NSNull *myNull = [NSNull null];
NSMutableArray *arr = [NSMutableArray arrayWithObjects: myNull, myNull, nil];


Then to compare use something like:

if ([arr objectAtIndex:index]  == (id)[NSNull null]) {

  //do something
}


nil cannot be added into an Array or collection similar to how an int cannot be added.  NSNull is the helper object to get nil into the collection.  In Objective C there's nil, NSNull and Null.

NSNull, nil, null differences

Found a great answer on stack overflow.  Here's the gist of it:

nil is like (NSObject *)0
NULL is like (void*)0
Both are pointers with an integer value of zero.

NSNull is a helper object to add nil to NSArray or container classes requiring an object.  Similar to how NSNumber is needed to add an int.  [NSNull null] can be used instead of nil.  You can't compare NSNull to nil, but you can check if something is a NSNull.

How to check if bool is null.

A handy way to check if a stored bool value does not exist.  originally from http://stackoverflow.com/questions/2760112/how-to-check-if-a-bool-is-null

You can test first and assign then conditionally, e.g. something like the following:

if (NSValue* val = [results objectForKey:@"current_user_following"]) {
    mySTUser.current_user_following = [val boolValue];
}

This:

• avoids calling objectForKey: twice by storing the result in a variable
• limits the scope of the variable to the if statement
• works because nil is is equivalent to 0
• thus only executes the assignment statement if val is not nil

To additionally check for the value being NSNull you'd have to add another test as given by ChristopheD, but i question wether NSNull is really needed here - YES/NO should be sufficient for a description like "is following".
If you have no useful value for a key, you could simply remove it from the dictionary or not insert it in the first place.