Objective-C Fundamentals

Dictionaries
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new entries added, removed, or updated. To create an empty dictionary, you use the
dictionary factory message:
NSDictionary *myDetails = [NSDictionary dictionary];
This message is primarily of use only with the NSMutableDictionary subclass. Otherwise,
your empty dictionary will forever stay empty because it’s immutable!
A message called dictionaryWithObject:forKey: enables you to create a dictionary
that initially consists of a single key/value pair:
NSDictionary *myDetails = [NSDictionary dictionaryWithObject:@"Christopher"
forKey:@"Name"];
This code creates a new dictionary containing a single entry consisting of the key
"Name" with a value of "Christopher", both of which are strings. More than likely,
however, you’ll want to initialize a dictionary with multiple key/value pairs. A similarly
named dictionaryWithObjects:forKeys: message allows for doing so:
NSArray *keys = [NSArray arrayWithObjects:@"Name", @"Cell", @"City", nil];
NSArray *values = [NSArray arrayWithObjects:@"Christopher", @"+643123456",
@"Christchurch", nil];
NSDictionary *myDetails = [NSDictionary dictionaryWithObjects:values
forKeys:keys];
In this example, you create a new dictionary with details about a particular person.
The dictionaryWithObjects:forKeys: message expects to be provided with two
arrays of equal length. The first value from the keys array is matched up with the first
value from the values array, and so on, to create the key/value pairs that will populate
the dictionary.
The creation of temporary arrays can be tiresome, especially if your only intent is
to populate a dictionary and you don’t need the arrays for other purposes. Naturally,
the designers of the NSDictionary class considered this scenario, and they provided a
more convenient factory method to allow you to specify multiple key/value pairs without
creating temporary arrays:
NSDictionary *myDetails = [NSDictionary dictionaryWithObjectsAndKeys:
@"Christopher", @"Name",
@"+643123456", @"Cell",
@"Christchurch", @"City",
nil];
This message expects a variable number of parameters to be provided to it. The parameters
alternate between being interpreted as a value or a key and are matched up into
pairs until a nil value is detected to indicate the end of the list. Other than providing an
alternative way to specify the list of key/value pairs, dictionaryWithObjectsAndKeys:
and dictionaryWithObjects:forKeys: perform identical functionality.
A number of other factory methods are also available on the NSDictionary and
NSMutableDictionary classes. For example, dictionaryWithContentsOfURL: performs
a function similar to NSArray’s arrayWithContentsOfURL: and enables a dictionary
to easily be populated with contents of a file located on a website.