iOS 5 Programming Cookbook

1.24. Allocating and Making Use of Dictionaries

Problem

You want to store key-value data in an object, or you would like to retrieve objects from an array using a key into the array, but arrays won’t quite suffice for this purpose, as they do not facilitate finding objects inside the array using a key or a marker for that object.

Solution

Use NSDictionary and its mutable counterpart, NSMutableDictionary.

Discussion

A dictionary is a special container for objects in which each object is given a key, which itself is an object. That is one of the key differences between dictionaries and arrays. An array has a numeric index into each item/object that it holds whereas a dictionary holds a key to each item. I’ll show you what I mean.

Let’s say we want to store a person’s first name, last name, and age into an array and then into a dictionary. This is how we would store those values in an array:

NSArray *person = [[NSArray alloc] initWithObjects:
                   @"Anthony",
                   @"Robbins",
                   [NSNumber numberWithUnsignedInteger:51], nil];

NSLog(@"First Name = %@", [person objectAtIndex:0]);
NSLog(@"Last Name = %@", [person objectAtIndex:1]);
NSLog(@"Age = %@", [person objectAtIndex:2]);

You can see that we are using an index into the array to access each one of these values. With dictionaries, we give each value a key, which is an object, and then use that key to access those values. Let’s see the same example, using dictionaries. We have a "First Name" key with the value "Anthony" and so on:

NSNumber *age = [NSNumber ...