Test if my array is nil at a given index?

Question

I'm trying to create an NSMutableArray containing the contents of another at a given index.

I'm initializing the new array with initWithArray: if the previous array has anything at the index, otherwise with plain init

This code doesn't work but you should be able to get the idea from it:

NSMutableArray *typeCourseCutId;
if([self.typeCourseCut objectAtIndex:typeCourseIndex] != nil){
    typeCourseCutId = [[NSMutableArray alloc]initWithArray:[self.typeCourseCut objectAtIndex:typeCourseIndex]];
} else {
    typeCourseCutId = [[NSMutableArray alloc]init];
}

Is there a solution for this kind of test?

objectAtIndex: will never return nil for any index. Cocoa arrays can't contain nil, and if you try to access an index outside the array's bounds, an exception will be raised. Trying to put nil into an array will also cause an exception. — jscs
– jscs, Commented Aug 23, 2012 at 19:50

Sergey Kalinichenko · Accepted Answer · 2012-08-23 18:57:25Z

6

There is a special class called NSNull that is created specifically to use as null indicator in collections:

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

if([self.typeCourseCut objectAtIndex:typeCourseIndex] != [NSNull null]) ...

answered Aug 23, 2012 at 18:57

Sergey Kalinichenko

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jscs · Accepted Answer · 2012-08-23 19:52:34Z

3

You can check the count of the array. If the count is greater than the index you are fetching, then it means that an object is present at that index.

Also make sure your self.typeCourseCut is an array of NSArray objects, otherwise your initWithArray: will crash.

Also you can add an empty object to an array as [yourArray addObject:[NSNull null]]; And most importantly, without having an object at index 1, you cannot add an object to index 2.

edited Aug 23, 2012 at 19:52

jscs

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answered Aug 23, 2012 at 19:03

Sj.

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2 Comments

jscs Over a year ago

This is the correct answer; if the index is in the correct range, there's an object there. The note about initWithArray: is also important.

Tim Over a year ago

Thank you, i did though about this [... count] option, but didn't go thru... i'll try to get an update on the NSNull doc from Apple.

The Lazy Coder · Accepted Answer · 2012-08-23 18:58:25Z

2

An array should not be nil at any index. the way an array is built is that as soon as it reaches a nil object. it is considered the end of the array.

You cannot add a nil object nor can you update an existing object to nil.

If you must set something to a "Null" style object you can use NSNull in its place. THis is what most of the JSON parsers do for objects that they want to show up, but be null.

answered Aug 23, 2012 at 18:58

The Lazy Coder

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Comments

Frank · Accepted Answer · 2018-07-19 11:52:20Z

0

If this can help

   if([array objectAtIndex:0] != [NSNull null])
    {
       NSLog(@"INDEX 0);
    }
    else if([array objectAtIndex:1] != [NSNull null])
    {
         NSLog(@"INDEX 1);
    }
    else if([array objectAtIndex:2] != [NSNull null])
    {
         NSLog(@"INDEX 2);
    }

answered Jul 19, 2018 at 11:52

Frank

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