A Swifr Tour

Tradition suggests that the first program in a new language should print the words "Hello ,world！" on the screen. In Swift , this can be done in a single line :  print("Hello world")

If you gave written code in C otr Objective - C , this syntax looks famiiar to you --- in Swift . this line of code is a complete program. Youdon't need to import a separate library for functionality like input /output or string handling . Code written at global scope is used sa the entry point for the program , so you don't need a main () function . You also don't need to write semicolons at the end of every statement .

This tour gives you enough information to start writing code in Swift by showing you how to accomplish a variety of programming tasks . Don't worry if you don't understand something --everything introduced in this tour is explained in detail in the rest of this book.

Simple Values

Use ler to make a constant and var to make a variable . The value of a constant doesn't need to be known at compile time , but you must assign it a value exactly once . This means you can use constants to name a value that you determine once but use in many places .

var myVariable = 42

　　　　　　　　　myVariable  = 50

　　　　　　　　　let myConstant = 42

A constant or variable must have the same type as the value you want to assign to it. However, you don't always have to write the type explictly.Providing a vakue when you creare a constant or variable lets the conpiler infer its type . In the exmple  above , the compiler infers  that myVariable is  an integer because its initial value is an integer.

If the  initial value doesn't provide enough information (or if there is no initial value ), specify the type by writing it after the variable , separated by a colon.

let implicitInterger = 70

　　　　　　　　　　　　　　　let impicitDouble = 80.0

　　　　　　　　　　　　　　　let explisitDouble : Double = 70

Values are never implicitly converted to  another type . If you need to convert a value to a different type, explicitly make an instance of the desired type .

　　　　　　let label = " The width is "

　　　　　　let width = 94

　　　　　　let widthLable = lable + String (width)

There's an even simpler way to include balues in string : write the value in parentheses, and write a backslash (\) before th parentheses . For example: 　　　　　　let apples = 4

　　　　　　let oranges = 5

　　　　　　let appleSummary = " I have \ (apples) apples."

　　　　　　let fruitSummary = " I have \ (apples  + oranges ) piceces of fruit ."

Create arrays  and dictionaries using brackets ([]), and access their elements by writing the index or key in brackets . A comma is allowed is allowed after the last element.

var shoppingList  = ["catfish","water"]

　　　shoppingList [1] = "bottle of water"

　　　var occupations = ["Malcolm":"Captain"]

　　　　occupations ["Jayne"] = "Public Relations"

To create an empty array or dictionary, use the initializer syntax.

　　　　let emptyArray = [String] ()

　　　　let emptyDictionary = [String : float]()

If type information can be inferred , you can write an empty array as [] and an empty dictionary as [:] -- for example , when you set a new value for a variable or pass ab argument to a function.

　　　　shoppingList = []

　　　　occupation = [:]

Control Flow

Use if and switch to make conditionals , and use for - in , while ,and repeat- while to make loops . Paretheses around the condition or loop variable are optional . Braces around the body are required.

　　　　let indviidualScores = [75, 43 , 103, 87 12]

　　　　var teamScore = 0

　　　　for score in individualScores {

if score > 50{

　　teamScore += 3

}

　　else {

　　teamScore += 1

}

}

print(teamScore)

In a if statement, the conditional must be a Boolean expression --- this means that code such as if score {...} is an error, not an implicit comparison to zero.

You can use if and let together ro work with values that might be missing . These values are represented as optionals. An optional value either contains a value or contains nil to indicte that a value a is missing . Write a question mark (?) after the type of a value to mark the value as optional .

　　 var optionalString :String? = "Hello"

　　print(optionalString == nil )

　　var optionalName :string? = "john appleseed"

　　var greeting = "Hello!"

　　if let name = optionalName {

　　greetiing = "Hello, \(name)"

}

If the optional value is nil , the conditional is false and the code in braces is skipped . Otherwise , the optional value is unwrapped and assigned to the constnat after let , which makes the unwrapped value available inside the block of code .

Switches support any kind of data and a wide variety of comparison operations -- they aren't limited to integers and tests for equality.