Question: I am still new to objective-c and am trying to figure out what this statement is doing exactly.

```
[names allKeys] sortedArrayUsingSelector:@selector(compare:)];

```

I know that `allKeys` is getting all the keys from my dictionary. I know that `sortedArrayUsingSelector` is sorting my array im creating. Then im calling the compare method, that is where I am lost what is this doing? From the document on apple it says that "Returns an NSComparisonResult value that indicates whether the receiver is greater than, equal to, or less than a given number." I dont understand how it is sorting based of that method.

Answer: 
The new array contains references to the receiving array’s elements, not copies of them.  

The comparator message is sent to each object in the array and has as its single argument another object in the array.  

For example, an array of NSString objects can be sorted by using the caseInsensitiveCompare: method declared in the NSString class. Assuming anArray exists, a sorted version of the array can be created in this way:

```
NSArray *sortedArray =
     [anArray sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];

```
You should probably be using `caseInsensitiveCompare:` in this situation since you are most likely ordering an array of strings (keys from a dictionary). 

You've assumed that sortedArrayUsingSelector is somehow separate to the `compare:` part of the code. This is not the case. `compare:` *is* the selector that is used to sort the array. 

The sorting method gives you back an array where each element, when sent the specified selector and the next element in the array, gives the correct sort order.

How NSArray achieves this is not public but at root, the selector you define is used to compare pairs of objects from within the array, the result of which informs their placement in the sorted array.