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 *-sortedArrayUsingSelector:* method in your example calls the *-compare:* method on the objects in the array. For some objects Apple has already implemented a *-compare:* method, for example if you read through [the NSString documentation](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html#//apple_ref/doc/uid/20000154-compare_), you'll find a *-compare:* method implemented. You can also call custom comparison methods on your own custom objects if you've implemented a comparison method in these objects. Please note the comparison method doesn't have to be called *-compare:*, of importance is only the return value (NSComparisonResult) and the object the method receives.
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.