Objective-C Fundamentals

48 CHAPTER 2 Data types, variables, and constants
2.4.3 Arrays
Like variables of primitive data types, structure variables can be initialized with an initial
value. You can achieve this through two forms of syntax. The first is to provide a
value for each field in a set of curly braces:
struct box a = { 10, 20, 30 };
This line specifies a box 10 wide 20 high 30 deep. The values in the curly braces are
specified in the same order as the fields were defined in the structure declaration. If
you don’t specify enough values, any unspecified fields are left uninitialized. An alternative
syntax explicitly specifies the name of each field being initialized, as follows:
struct box a = { .width=10, .height=20, .depth=30 };
This alternative syntax can be clearer in intent and will also cope if the order of the
fields in the structure declaration changes. It’s even possible to initialize only select
fields in the structure when using this syntax:
struct box a = { .depth=30 };
This last example sets the depth field to 30 and leaves the width and height fields
uninitialized. In this sense, uninitialized means the field will have the value 0 or its
equivalent, such as nil, although this depends on the storage class of the variable.
Structures are a handy way to group related sets of variables into easier-to-manage
chunks, but they don’t solve all of the problems you may encounter with storing
details about multiple highly similar objects. For example, you may need to calculate
the total width of all the boxes in your application. To do this, you could come up with
an expression similar to the following:
struct box a, b, c, d, e, f;
int totalWidth = a.width + b.width + c.width + d.width + e.width;
Although this is manageable with only five boxes, imagine how torturous the expression
would be to write if the application instead needed to store details of 150 different boxes.
There would be a lot of repetitive typing involved, and it would be easy to miss a box or
include a box multiple times. Each time you change the number of boxes your application
needs to store, you’d also need to update this calculation. That sounds like a lot of
pain! What you want the expression to say is “add the width of every box” and have the
statement work without modification no matter the number of boxes your application
currently keeps track of. Not surprisingly, Objective-C has a data type to help you out.
Another common scenario is the need to store multiple values of the same data type.
C provides a handy data structure called an array to make this task easier. An array is a
data structure that stores a list of items. Each item (or element) is stored consecutively,
one after the other, in memory and can only be accessed by its relative position.
This position is often called the item’s index. To declare an array capable of storing
details of 150 boxes, you could write a variable declaration similar to this:
struct box boxes[150];