== What are Dictionary Objects? ==

== What are Dictionary Objects? ==

In the previous chapter we looked at using array objects to store collections of objects. Dictionary objects perform a similar purpose except each object stored in the dictionary has associated with it a unique key (to be precise, the key is unique to the particular dictionary object). The unique key can be of any object type though to be useful they are typically string objects.

In the previous chapter we looked at using array objects to store collections of objects. Dictionary objects fulfill a similar purpose except each object stored in the dictionary has associated with it a unique key (to be precise, the key is unique to the particular dictionary object). The unique key can be of any object type though to be useful they are typically string objects.

Objective-C dictionary objects are created using the NSDictionary and NSMutableDictionary classes. NSDictionary based objects are immutable (in other words once they have been created and initialized their contents cannot be modified). Mutable dictionaries are instantiated from the NSMutableDictionary class and may be modified after creation and initialization.

Objective-C dictionary objects are created using the NSDictionary and NSMutableDictionary classes. NSDictionary based objects are immutable (in other words once they have been created and initialized their contents cannot be modified). Mutable dictionaries are instantiated from the NSMutableDictionary class and may be modified after creation and initialization.

Each key-value pair contained within a dictionary is referred to as an entry. Once a relationship between a key and a value has been estabilshed that relationship cannot subsequently be modified.

Each key-value pair contained within a dictionary is referred to as an entry. Once a relationship between a key and a value has been estabilshed that relationship cannot subsequently be modified.