how to add nil to nsmutablearray?

问题:

NSArray *array = [[NSArray alloc] initWithObjects:@"ΕΛΤΑ",
                      @"ΕΛΤΑ COURIER", @"ACS", @"ACS ΕΞΩΤΕΡΙΚΟ", 
                      @"DHL", @"INTERATTICA", @"SPEEDEX", 
                      @"UPS", @"ΓΕΝΙΚΗ ΤΑΧΥΔΡΟΜΙΚΗ", @"ΜΕΤΑΦΟΡΙΚΕΣ ΕΞΩΤΕΡΙΚΟΥ", nil];

This is working because it has nil at the end.

But I add objects like this: addObject:name etc... So at the end I have to add nil I do this addObhect:nil but when I run the app it still crashes at cellForRowAtIndexPath:

how can I do this work?

Ok, I dont have to add nil

What is the reason that my app crashes then?

回答1:

You can't add nil when you're calling addObject.

回答2:

If you must add a nil object to a collection, use the NSNull class:

The NSNull class defines a singleton object used to represent null values in collection objects (which don’t allow nil values).

Assuming "array" is of type NSMutableArray:

....
[array addObject:[NSNumber numberWithInt:2];
[array addObject:@"string"];
[array addObject:[NSNull null]];

回答3:

You don't need to call [addObject:nil]

The nil in initWithObjects: is only there to tell the method where the list ends, because of how C varargs work. When you add objects one-by-one with addObject: you don't need to add a nil.

回答4:

If you really want a Null-ish item in your collection, NSNull is there for that.

回答5:

nil is used to terminate the array

回答6:

You need to add NSNull class and the best way to do it is like this:

NSArray *array = @[ @"string", @42, [NSNull null] ];

I personally recommend to use a specific value like 0 instead of null or nil in your design of your code, but sometimes you need to add null.

There is a good explanation from this Apple reference.

回答7:

nil is not an object that you can add to an array: An array cannot contain nil. This is why addObject:nil crashes.

回答8:

You can't add an object to an NSArray because that class is immutable. You have to use NSMutableArray if you want to change the array after it is created.

回答9:

pass your object through this method when adding to array to avoid attempt to insert nil object from objects crashes.

-(id) returnNullIfNil:(id) obj  {
    return (obj == nil) ? ([NSNull null]) : (obj);
}

[NSNull null] returns an null object which represents nil.