Objective-C Fundamentals

Sample application
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To append one string onto the end of another, you can use the stringByAppending-
String: message:
NSString *str2 = [str1 stringByAppendingString:@"Cool!"];
This statement appends "Cool!" to the end of str1 and stores the result in str2. You
can’t state str2 = str1 + @"Cool!" for reasons that will soon become clear.
Now that you know how to create and manipulate string objects, it’s only natural to
want to compare one against another to determine if they’re identical.
3.5.4 Comparing strings
To compare two string variables against each other, you may come up with a code snippet
such as the following based on what you’ve learned about pointers in this chapter:
NSString *str1 = [NSString stringWithFormat:@"Hello %@", @"World"];
NSString *str2 = [NSString stringWithFormat:@"Hello %@", @"World"];
if (str1 == str2) {
NSLog(@"The two strings are identical");
}
Surprisingly, when this code is executed, you’ll notice it indicates that the two freshly
created strings aren’t equal! To understand why, you need to realize that the two variables
str1 and str2 are both simple C-style pointers. With its C-based roots, the ==
operator checks for equality by determining if both variables point to the same piece
of memory. Because you create two separate string instances, the two pointers point to
distinct locations, and the expression evaluates to false. That both strings contain an
identical sequence of characters is irrelevant because the == operator only considers
the string’s memory location.
To work around this problem, Objective-C, like most object-oriented languages
with pointers, provides a message to compare the contents of an object instead of its
location in memory. In Objective-C this message is called isEqual:
if ([str1 isEqual:str2]) {
NSLog(@"The two strings are identical");
}
It’s for a similar reason that the + operator can’t be used for string concatenation: it’s
already used with C-style pointers for pointer arithmetic.
3.6 Sample application
To put the concepts you’ve learned in this chapter into practice, let’s expand the Rental
Manager application to make greater use of the services provided by NSString to search
and modify string content. Figure 3.5 shows how the application will look once you finish
this round of modifications. Each rental property has an image beside it that categorizes
the property’s location, such as near the sea, in the city, or in an alpine setting.
To display images in your iPhone application, the obvious first step is to include
some image resources in your project. To add images to the Xcode project, drag and