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 cocoastatement 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 andmanpower 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 varand 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
