iOS 6 Programming Cookbook

1.25. Allocating and Making Use of Sets

Problem

You would like to store an array of objects but you don’t want any one object to appear more than once in the array.

Solution

Use sets instead of arrays.

Discussion

Sets are very similar to arrays. The big difference is that sets allow objects to be added only once. The second time you try to add the same object, it will be rejected by the set. We use NSSet for immutable and NSMutableSet for mutable sets. Let’s have a look at an example of an immutable set:

NSString *hisName = @"Robert";
NSString *hisLastName = @"Kiyosaki";

NSString *herName = @"Kim";
NSString *herLastName = @"Kiyosaki";

NSSet *setOfNames = [[NSSet alloc] initWithObjects:
                     hisName,
                     hisLastName,
                     herName,
                     herLastName, nil];

NSLog(@"Set = %@", setOfNames);

We created an immutable set and passed 4 string objects to its initializer method. So let’s see what gets printed out to the console window with our NSLog:

Set = {(
    Kim,
    Robert,
    Kiyosaki
)}

You can see that the last name Kiyosaki was added only once to the list. Our set rejected the second addition of the same object to the list. It is very important to understand that a set doesn’t just do a comparison on where in memory an object sits, but it actually looks into its contents. hisLastName and herLastName are two separate variables, and they will sit in two different places in the memory. Our set, however, managed to understand that we are passing instances of NSString to it and did a comparison on the contents of these strings ...