Swift 语言基础(1)-The basics

常量与变量

letmaximumNumberOfLoginAttempts = 10
varcurrentLoginAttempt = 0

varx = 0.0,y = 0.0,z = 0.0

varwelcomeMessage:String
welcomeMessage= "Hello"

 letπ = 3.14159
 let你好 = "你好世界"
 let □□="dogcow"

 varfriendlyWelcome = "Hello!"
 friendlyWelcome= "Bonjour!"
 // friendlyWelcome is now "Bonjour!"

 letlanguageName = "Swift"
 languageName= "Swift++"
 // this is a compile-time error - languageName cannot be changed

 println(friendlyWelcome)
 // prints "Bonjour!"

 println("This is a string")
 // prints "This is a string"

println("The current value of friendlyWelcome is \(friendlyWelcome)")
 // prints "The current value of friendlyWelcome is Bonjour!"

注释

// this is a comment

/* this is also a comment,
 but written over multiple lines */

/* this is the start of the first multiline comment
/* this is the second, nested multiline comment */
this is the end of the first multiline comment */

分号

 letcat= "□";println(cat)
 // prints "□"

整型数

letminValue= UInt8.min// minValue is equal to 0, and is of type UInt8
let maxValue= UInt8.max// maxValue is equal to 255, and is of type UInt8

On a 32-bit platform,Int is the same size as Int32.
On a 64-bit platform, Int is the same size as Int64.
On a 32-bit platform, UInt is the same size as UInt32.
On a 64-bit platform, UInt is the same size as UInt64.

类型安全与类型推理

 letmeaningOfLife = 42
 // meaningOfLife is inferred to be of type Int

letpi = 3.14159
 // pi is inferred to be of type Double

 letanotherPi = 3 + 0.14159
 // anotherPi is also inferred to be of type Double

数字字面常量

letdecimalInteger = 17
 letbinaryInteger = 0b10001 // 17 in binary notation
 letoctalInteger = 0o21 // 17 in octal notation
 lethexadecimalInteger = 0x11 // 17 in hexadecimal notation

1.25e2means 1.25 × 102, or125.0.
1.25e-2 means 1.25 × 10-2, or 0.0125.

0xFp2means 15 × 22, or60.0.
0xFp-2 means 15 × 2-2, or 3.75.

 letdecimalDouble = 12.1875
 letexponentDouble = 1.21875e1
 lethexadecimalDouble = 0xC.3p0

 letpaddedDouble = 000123.456
 letoneMillion = 1_000_000
 letjustOverOneMillion = 1_000_000.000_000_1

数字类型转换

 letcannotBeNegative:UInt8 = -1
 // UInt8 cannot store negative numbers, and so this will report an error
 lettooBig:Int8 = Int8.max+ 1
 // Int8 cannot store a number larger than its maximum value,
 // and so this will also report an error

lettwoThousand:UInt16 = 2_000
letone:UInt8 = 1
lettwoThousandAndOne = twoThousand + UInt16(one)

letthree = 3
letpointOneFourOneFiveNine = 0.14159
letpi = Double(three) +pointOneFourOneFiveNine
// pi equals 3.14159, and is inferred to be of type Double

 letintegerPi = Int(pi)
 // integerPi equals 3, and is inferred to be of type Int

类型别名

typealiasAudioSample = UInt16

varmaxAmplitudeFound = AudioSample.min
// maxAmplitudeFound is now 0

布尔型

letorangesAreOrange = true
 letturnipsAreDelicious = false

 ifturnipsAreDelicious {
println("Mmm, tasty turnips!")
 }else {
println("Eww, turnips are horrible.")
 }
// prints "Eww, turnips are horrible."

 ifturnipsAreDelicious {
println("Mmm, tasty turnips!")
 }else {
println("Eww, turnips are horrible.")
 }
// prints "Eww, turnips are horrible."

 leti = 1
 ifi == 1 {
 // this example will compile successfully
}

元组

 lethttp404Error = (404,"Not Found")
 // http404Error is of type (Int, String), and equals (404, "Not Found")

let(statusCode,statusMessage) =http404Error
println("The status code is\(statusCode)")
// prints "The status code is 404"
println("The status message is\(statusMessage)")
// prints "The status message is Not Found"

 let(justTheStatusCode,_) =http404Error
 println("The status code is\(justTheStatusCode)")
 // prints "The status code is 404"

 println("The status code is \(http404Error.0)")
 // prints "The status code is 404"
 println("The status message is\(http404Error.1)")
 // prints "The status message is Not Found"

lethttp200Status = (statusCode:200,description:"OK")
 println("The status code is\(http200Status.statusCode)")
 // prints "The status code is 200"
 println("The status message is\(http200Status.description)")
 // prints "The status message is OK"

可选类型

 letpossibleNumber = "123"
 letconvertedNumber = possibleNumber.toInt()
 // convertedNumber is inferred to be of type "Int?", or "optional Int"

 ifconvertedNumber {
println("\(possibleNumber)has an integer value of \(convertedNumber!)")
 }else {
println("\(possibleNumber)could not be converted to an integer")
 }
 // prints "123 has an integer value of 123"

 if let actualNumber = possibleNumber.toInt() {
println("\(possibleNumber)has an integer value of \(actualNumber)")
 }else {
println("\(possibleNumber)could not be converted to an integer")
 }
// prints "123 has an integer value of 123"

空值(nil)

 varserverResponseCode:Int? =404
 // serverResponseCode contains an actual Int value of 404
 serverResponseCode= nil
// serverResponseCode now contains no value

 varsurveyAnswer:String?
 // surveyAnswer is automatically set to nil

隐式可选类型

 letpossibleString:String? ="An optional string."
 println(possibleString!)// requires an exclamation mark to access its value
 // prints "An optional string."

let assumedString: String! = "An implicitly unwrapped optional string."
 println(assumedString)// no exclamation mark is needed to access its value
 // prints "An implicitly unwrapped optional string."

ifassumedString {
println(assumedString)
 }
 // prints "An implicitly unwrapped optional string."

断言

 letage = -3
 assert(age>= 0, "A person's age cannot be less than zero")
 // this causes the assertion to trigger, because age is not >= 0