Whats is this Objective C code doing?

I am learning Objective C . I was given a code to fix that I have fixed but not entirely sure what was going on though. Can some please explain what is following code doing.

 //in header file  there is this line
 @property (retain) NSMutableArray *anArray;  

  // 开发者_StackOverflowIn implementation file in a method
  self.anArray = [NSMutableArray array];
  //This assigns a large value to index . What is this value. Does NSIteger needs initialization I think default is 0 
  NSInteger _nextIndex = (NSInteger)[self.anArray];

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That code is invalid, it will fail to parse [self.anArray].

The square brackets are used to call methods, but there is no method you are calling. It seems like what you wan to do is NSInteger _nextIndex = (NSInteger)[self.anArray count]; which will assign to _nextIndex the number of elements in the array, which is the position of the next index.

An array is a list of items, starting by 0. So, if the list doesn't have anything the count method will return 0, which is the first position. If the list has 100 items, they will use indexes from 0 to 99, then the count will return 100 and the next item position will be 100.

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You're typecasting the pointer pointing to self.anArray to an NSInteger. In other words, _nextIndex contains the address at which self.anArray is stored.