About objectAtIndex's return value

    NSLog(@" --- object id = %ld --- ", 
          (long) [mp_list objectAtIndex : 0]);     

    target_coordinate_2D = [[mp_list objectAtIndex : 0] coordinate];

    // Some test code here which verifies that target_coordinate_2D   
    // gets assigned a valid "coordinate" value.

    [mapView addAnnotation : [mp_list objectAtIndex : 0]];

Consider the above 3 statements :

According to documentation, the NSArray's method "objectAtIndex" should return an object. My understanding is the value returned is a numeric pointer to the object.

But what I got is :

--- object id = 0 ---
'NSInvalidArgumentException', reason: '-[... addObject:]: attempt to insert nil'

My questions are :

1

Why "object id" is nil ? Since target_coordinate_2D got assigned a valid value, [mp_list objectAtIndex : 0] should indeed be a valid object with a valid "coordinate" property.

2

On the last statement, I wish to add the object in mp_list to mapView. The NS Exception thrown by t开发者_如何学编程he simulator is consistent of the fact that "[mp_list objectAtIndex : 0]" is indeed nil. So there must be something I have missed about NSArray. What should be done in order to assign an object to an array from another array ?

Hope that somebody knowledgable in this area can help ...

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Foundation/Classes/NSArray_Class/NSArray.html

It sounds like mp_list itself is nil, which is why [mp_list objectAtIndex:0] is returning nil (because messaging nil returns nil).

Not sure if this is right, but in your NSLog, you're printing out the object as a long? From what I know, you can only put objects in NSArray, you can't put primitive types in them. So it might be something about the data you're putting in your array.