iPhone App Development: The Missing Manual

Collections

When you're dealing with objects, you're often dealing with many of them. It's essential to stay organized, and the easiest way to do this is by grouping similar objects into collections.

For example, when you look at a scrolling list on your iPhone, the data in each row is represented by an object. And if that row contains a person's name and age, you're likely to have other objects for those personal details.

NSArray is a natural collection for this kind of list view. You can specify the order within the collection so the first row in the list is the first object in the array. This array class also provides mechanisms for sorting and filtering the data, making it easy to model that behavior in a user interface.

When you want to collect objects by using a unique key, use the NSDictionary class. It implements an associative array that lets you look up objects (values) associated with a key.

Note

If you've used other languages, associative arrays are called a lot of different things: hash tables, hashes, maps, and containers are but a few. Regardless of name, they all let you find a value with a key.

You might use an NSDictionary to store the data for each row in your list. Since you can store NSString, NSDate, and NSNumber in a collection, it's easy to create a collection for that information:

NSDictionary *personalDetails = [NSDictionary dictionaryWithObjectsAndKeys:
    @"Craig", @"name", [NSNumber numberWithInt:50], @"age"];

You can then query the dictionary collection whenever ...