NSUserDefaults set the default value of integer variable to empty - objective-c

Question

I used NSUserDefaults to store an integer value. When I run the project there's a default value of 0 in it. How do I remove the 0 value? I just want it to be empty until the user put something in it.
Here is my code:
- (IBAction)save:(id)sender{
int port = [[portField text] integerValue];
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults setInteger:port forKey:#"port"];
[defaults synchronize];
}
- (void)viewDidLoad {
int port = [defaults integerForKey:#"port"];
NSString *portString = [NSString stringWithFormat:#"%lu",(unsigned long)port];
portField.text = portString;
[super viewDidLoad];
}

Answer 1

Well, this line:
int port = [defaults integerForKey:#"port"];
will always assign some value to port.
Also, this:
NSString *portString = [NSString stringWithFormat:#"%lu",(unsigned long)port];
will always produce a non-empty string. There's no value you could pass for a %lu specifier that will produce an empty string.
So, what you want to do is ask for the object that's stored in the preferences, without converting it to a integer value:
id portObject = [defaults objectForKey:#"port"];
if (portObject)
{
NSInteger port;
if ([portObject respondsToSelector:#selector(integerValue)])
port = [portObject integerValue];
else
{
// portObject is some unexpected class of object, such as an array or dictionary. Take some appropriate action. Or:
port = 0;
}
portField.text = [NSString stringWithFormat:#"%ld", (long)port];
}
else
{
// There was no value either stored or registered for the key "port"
portField.text = #"";
}
Note that for this to work, you must not have registered a default value for the "port" key using -[NSUserDefaults registerDefaults:].

Answer 2

As far as I know, NSUserDefaults always return default values for primitive types when they are not exists.
If the default value is a valid value you use, I suggest to put a blank object or just NSString object to determine if the associated value is valid. e.g.
NSObject *objValid = [userDefaults objectForKey:#"integerIsValid"];
if(objValid != nil) {
// User stored integer value exists.
NSInteger i = [userDefaults integerForKey:#"SomeInteger"];
} else {
// Have not saved integer value.
}
When user save the integer value, also save the associated object:
[userDefaults setObject:[[NSObject alloc]init] forKey:#"integerIsValid"];
[userDefaults setInteger:i forKey:#"SomeInteger"];
UPDATE
If your value range can transform to another value range by some rules, then you can do it another way.
All values are in range [0, 100) for example, you can store in range [1, 101) and retrieve the real value by subtracting 1.