We have already seen a piece of Swift program while setting up the environment. Let's start once again with the following
Hello, World! program created for OS X playground, which includes
import Cocoa as shown below −
import Cocoa
/* My first program in Swift */
var myString = "Hello, World!"
println(myString)
If you create the same program for iOS playground, then it will include
import UIKit and the program will look as follows −
import UIKit
var myString = "Hello, World!"
println(myString)
When we run the above program using an appropriate playground, we will get the following result −
Hello, World!
Let us now see the basic structure of a Swift program, so that it
will be easy for you to understand the basic building blocks of the
Swift programming language.
Import in Swift
You can use the
import statement to import any Objective-C framework (or C library) directly into your Swift program. For example, the above
import cocoa statement makes all Cocoa libraries, APIs, and runtimes that form the development layer for all of OS X, available in Swift.
Cocoa is implemented in Objective-C, which is a superset of C, so it is easy to mix C and even C++ into your Swift applications.
Tokens in Swift
A Swift program consists of various tokens and a token is either a
keyword, an identifier, a constant, a string literal, or a symbol. For
example, the following Swift statement consists of three tokens −
println("test!")
The individual tokens are:
println (
"test!"
)
Comments
Comments are like helping texts in your Swift program. They are
ignored by the compiler. Multi-line comments start with /* and terminate
with the characters */ as shown below −
/* My first program in Swift */
Multi-line comments can be nested in Swift. Following is a valid comment in Swift −
/* My first program in Swift is Hello, World!
/* Where as second program is Hello, Swift! */
Single-line comments are written using // at the beginning of the comment.
// My first program in Swift
Semicolons
Swift does not require you to type a semicolon (;) after each
statement in your code, though it’s optional; and if you use a
semicolon, then the compiler does not complain about it.
However, if you are using multiple statements in the same line, then
it is required to use a semicolon as a delimiter, otherwise the compiler
will raise a syntax error. You can write the above Hello, World!
program as follows −
import Cocoa
/* My first program in Swift */
var myString = "Hello, World!"; println(myString)
Identifiers
A Swift identifier is a name used to identify a variable, function,
or any other user-defined item. An identifier starts with an alphabet A
to Z or a to z or an underscore _ followed by zero or more letters,
underscores, and digits (0 to 9).
Swift does not allow special characters such as @, $, and % within identifiers. Swift is a
case sensitive programming language. Thus,
Manpower and
manpower are two different identifiers in Swift. Here are some examples of acceptable identifiers −
Azad zara abc move_name a_123
myname50 _temp j a23b9 retVal
To use a reserved word as an identifier, you will need to put a backtick (`) before and after it. For example,
class is not a valid identifier, but
`class` is valid.
Keywords
The following keywords are reserved in Swift. These reserved words
may not be used as constants or variables or any other identifier names,
unless they're escaped with backticks −
Keywords used in declarations
class |
deinit |
enum |
extension |
func |
import |
init |
internal |
let |
operator |
private |
protocol |
public |
static |
struct |
subscript |
typealias |
var |
|
|
Keywords used in statements
break |
case |
continue |
default |
do |
else |
fallthrough |
for |
if |
in |
return |
switch |
where |
while |
|
|
Keywords used in expressions and types
as |
dynamicType |
false |
is |
nil |
self |
Self |
super |
true |
_COLUMN_ |
_FILE_ |
_FUNCTION_ |
_LINE_ |
|
|
|
Keywords used in particular contexts
associativity |
convenience |
dynamic |
didSet |
final |
get |
infix |
inout |
lazy |
left |
mutating |
none |
nonmutating |
optional |
override |
postfix |
precedence |
prefix |
Protocol |
required |
right |
set |
Type |
unowned |
weak |
willSet |
|
|
Whitespaces
A line containing only whitespace, possibly with a comment, is known as a blank line, and a Swift compiler totally ignores it.
Whitespace is the term used in Swift to describe blanks, tabs,
newline characters, and comments. Whitespaces separate one part of a
statement from another and enable the compiler to identify where one
element in a statement, such as int, ends and the next element begins.
Therefore, in the following statement −
var age
there must be at least one whitespace character (usually a space) between
var and
age for the compiler to be able to distinguish them. On the other hand, in the following statement −
int fruit = apples + oranges //get the total fruits
no whitespace characters are necessary between fruit and =, or
between = and apples, although you are free to include some for better
readability.
Literals
A literal is the source code representation of a value of an integer,
floating-point number, or string type. The following are examples of
literals −
92 // Integer literal
4.24159 // Floating-point literal
"Hello, World!" // String literal
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