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CHAPTER 3: Foundational APIs 59Remember that sets do not allow duplicate object references, so any objects that were alreadyin the set and are added to a mutable set will still only appear once in the set.Developers with a background in mathematics may find sets interesting; sets in the Foundationframework behave similarly to sets in mathematics. This brings us to the final methods forchanging the contents of a mutable set, shown in Table 3-6.Table 3-6. Mutable Set MethodsSet OperationIntersection of two setsDifference of two setsUnion of two setsInstance MethodinsertsectSet:minusSet:unionSet:Listing 3-29 demonstrates the power of mutable sets; while sets are not found in many APIs,they are an efficient and useful data structure to keep in your Objective-C toolbox.Listing 3-29. Mutable SetsNSMutableSet *mutableSet = [NSMutableSet setWithObjects:@"apple", @"bananas", @"milk", nil];[mutableSet intersectSet:[NSSet setWithObject:@"apple"]];//mutableSet contains only @"apple" now[mutableSet unionSet:[NSSet setWithObjects:@"bread", @"jam", @"apple", nil];//mutableSet now contains @"apple", @"bread", and @"jam"[mutableSet minusSet:[NSSet setWithObject:@"bread"]];// mutableSet now contains @"apple" and @"jam"[mutableSet minusSet:[NSSet setWithObject:@"pasta"]];// mutableSet is unchanged; it still contains @"apple" and @"jam"DictionariesThe final member of the collections library in Foundation is NSDictionary. A dictionary is a listof key-value pairs. Keys are typically NSString instances and the values are id. Dictionaries areoften used to pass a variable number of optional parameters to an API, such as Core Text. Theyare also used heavily with NSNotification and NSError.Keys within a dictionary must be unique. Creating a dictionary with two identical keys will causea runtime error.Creating a dictionary is possible with both the allocate-and-initialize and convenience classmethods. Table 3-7 details the common dictionary creation methods, although this is not acomplete list.www.it-ebooks.info