Objective-C Fundamentals

84 CHAPTER 4 Storing data in collections
Due to the use of the NSDictionary class, all of the dictionaries constructed by the
code samples in this section result in immutable dictionaries, which are read-only. If
you want to create a dictionary that can be modified after creation, you need to
replace the class name NSDictionary with NSMutableDictionary.
Now that you can construct dictionary instances, let’s proceed to determine how to
query a dictionary for details about the key/value pairs it contains.
4.2.2 Accessing dictionary entries
The methods for interacting with a dictionary are similar to those for an array. For
example, you can send the count message to determine how many entries are currently
contained in the dictionary:
int count = [myDetails count];
NSLog(@"There are %d details in the dictionary", count);
Rather than an objectAtIndex: message, which accesses an element by index position,
you’re provided with a similar message called objectForKey:, which enables you
to obtain the value associated with a given key:
NSString *value = [myDetails objectForKey:@"Name"];
NSLog(@"My name is %@", value);
This statement searches the dictionary to determine if a key named "Name" exists and
then returns the value associated with it. Unlike NSArray’s objectAtIndex: message,
it’s not an error to provide a key to objectForKey: that doesn’t exist in the dictionary.
In this case, the special value nil will be returned, indicating that the key was not
found in the dictionary.
One common use of dictionaries is to store related but flexible data about a
given item. For example, the dictionaries you constructed over the previous couple
of pages store various bits of information about a particular person, such as name,
cellphone number, and location. When interacting with such dictionaries, you’ll
likely want to query the value of multiple keys in quick succession. For example, if
you were printing address labels, you would probably want the person’s name and
location details. Although you could make multiple calls to objectForKey:, you can
also perform multiple lookups in a single statement via the objectsForKeys:not-
FoundMarker: message:
NSArray *keys = [NSArray arrayWithObjects:@"Name", @"City", @"Age", nil];
NSArray *values = [myDetails objectsForKeys:keys notFoundMarker:@"???"];
NSLog(@"%@ is located in %@ and is %@ years old",
[values objectAtIndex:0],
[values objectAtIndex:1],
[values objectAtIndex:2]);
objectsForKeys:notFoundMarker: expects to be provided with an array consisting of
the keys of the dictionary entries you want to query. It then returns an array with the values
of those keys. If a particular key you request isn’t present in the dictionary (such