Programming in Objective-C, Sixth Edition

Dictionary Objects

A dictionary is a collection of data consisting of key-object pairs. Just as you would look up the definition of a word in a dictionary, you obtain the value (object) from an Objective-C dictionary by its key. The keys in a dictionary must be unique, and they can be of any object type, although they are typically strings. The value associated with the key can also be of any object type, but it cannot be nil.

Dictionaries can be mutable or immutable; mutable ones can have entries dynamically added and removed. Dictionaries can be searched based on a particular key, and their contents can be enumerated. Program 15.16 sets up a mutable dictionary to be used as a glossary of Objective-C terms and fills in the first three entries. ...