Objective-C Fundamentals

Arrays
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4.1.4 Iterating through arrays
Using the count and objectAtIndex: messages of the NSArray class, you can loop
over each element in an array with a code snippet similar to the following:
int i;
for (i = 0; i < [pets count]; i++) {
NSString *pet = [pets objectAtIndex:i];
}
NSLog(@"Pet: %@", pet);
This code may not be the most efficient or cleanest of techniques. Each iteration
through the loop, when it’s time to evaluate the condition i < [pets count], you are
asking the array to recalculate its length; and depending on how the array is internally
stored (a dynamic array or linked list), repeatedly calling objectAtIndex: may not be
efficient because of repeated walks through the data structure to reach the required
element. One solution Objective-C provides for more efficient iteration through a
data structure is the enumerator.
NSENUMERATOR
An enumerator is an object that allows you to step through a sequence of related items in
an efficient manner. In Objective-C an enumerator is represented by the NSEnumerator
class. Because an enumerator must step through a set of data, you typically ask a data
structure such as an array to create a suitable enumerator on your behalf rather than
create an NSEnumerator instance directly.
As an example, the following code snippet uses NSArray’s objectEnumerator message
to obtain an NSEnumerator instance that enables you to step through each type of
pet stored in the pets array:
NSArray *pets = [NSArray arrayWithObjects:@"Cat", @"Dog", @"Rat", nil];
NSEnumerator *enumerator = [pets objectEnumerator];
NSString *pet;
while (pet = [enumerator nextObject]) {
NSLog(@"Pet: %@", pet);
}
The objectEnumerator method returns an instance of the NSEnumerator class that’s
suitable for enumeration of each element in the specified array.
NSEnumerator is a simple class that provides a single method called nextObject.
This method, as its name suggests, returns the next object in the sequence the enumerator
is enumerating over. By placing it in a while loop, you’ll eventually be provided
with the value of each element in the array. Once you reach the end of the
sequence, nextObject returns nil to indicate the end of the sequence (another reason
NSArray can’t store the value nil). Executing the code snippet on the array of pet
types results in output similar to the following:
Pet: Cat
Pet: Dog
Pet: Rat