Objective-C Fundamentals

86 CHAPTER 4 Storing data in collections
Finally, it’s possible to empty the dictionary completely by calling the removeAllObjects
method, which, as its name suggests, deletes every key/value pair from the dictionary:
[myDetails removeAllObjects];
NSLog(@"There are %d details in the dictionary", [myDetails count]);
This code snippet results in the string "There are 0 details in the dictionary"
because the call to removeAllObjects emptied it completely.
One message that can easily be confused with setObject:forKey: is the message
setValue:forKey:. Although these two messages have similar names, their behavior is
subtly different with regard to how they handle the special value nil, which, as you
may recall, typically represents the absence of a value.
As the following code snippet demonstrates, both setObject:forKey: and set-
Value:forKey: can be used to update the value stored in a dictionary for a particular key:
[myDetails setObject:@"Australian" forKey:@"Nationality"];
[myDetails setValue:@"Melbourne" forKey:@"City"];
These two messages diverge in how they handle a nil value being specified for the
key’s value. Sending setObject:forKey: with the object parameter specified as nil
results in an exception that crashes your application because nil isn’t a valid value to
store in a dictionary. Passing nil to setValue:forObject: is acceptable, however,
because it is interpreted as though you had instead called
[myDetails removeObjectForKey:@"Nationality"];
Although it hardly makes a difference in the example code snippets (and arguably
makes the code harder to understand), the benefits of using setValue:forKey: to
remove a key/value pair really come to light when cleaning up a code snippet such as
the following:
NSString *myNewValue = ...get new value from somewhere...
if (myNewValue == nil)
[myDetails removeObjectForKey:@"Nationality"];
else
[myDetails setObject: myNewValue forKey:@"Nationality"];
With setValue:forKey:, this entire code snippet can be replaced with a single line of
source code. setValue:forKey: will be discussed in greater detail in chapter 11,
which introduces the concept of Key-Value Coding. Throughout the rest of the book,
we come back to the subject of dictionaries, but let’s round off this discussion with a
look at how to list all entries in a dictionary, similar to an index or table of contents in
a book.
4.2.4 Enumerating all keys and values
Like arrays, dictionaries can be enumerated to list all of the key/value pairs contained
in them. But because dictionaries don’t retain any ordering, the order in which key/
value pairs are enumerated may not match the sequence in which they were added to
the dictionary.