iOS 5 Programming Cookbook

1.23. Allocating and Making Use of Arrays

Problem

You want to store a series of objects into another object for later use.

Solution

Use NSArray and NSMutableArray classes to store objects into arrays that are fixed and that you can change, respectively.

Discussion

An object of type NSArray or any of its subclasses has the capability to store n number of other objects. These objects can then be accessed using their index. For instance, let’s say you have 10 pairs of socks. Now imagine placing them all on a flat surface from left to right and calling them socks 1, socks 2, socks 3, etc. So the leftmost sock is now addressed as socks 1, the one next to it is called socks 2, and so on. Isn’t that easier than saying something like “the blue socks next to my red socks”? That’s exactly what arrays do: they make arranging items much easier.

Note

You can place any object of type NSObject or any of its subclasses into an array of type (or subclasses of) NSArray.

The primary difference between NSArray and NSMutableArray is that a mutable array can be changed/modified after it has been allocated and initialized, whereas an immutable array, NSArray, cannot.

Let’s have a look at an example. Let’s create an instance of NSString and two instances of NSNumber and place them in an immutable array:

NSString *stringObject = @"My String"; NSNumber *signedNumber = [NSNumber numberWithInteger:-123]; NSNumber *unsignedNumber = [NSNumber numberWithUnsignedInteger:123]; NSArray *array = [[NSArray alloc] initWithObjects: ...