swift是什么？
swift是苹果于wwdc 2014发布的编程语言，这里引用the swift programming language的原话：
swift is a new programming language for ios and os x apps that builds on the best of c and objective-c without the constraints of c compatibility.
swift adopts safe programming patterns and adds modern features to make programming easier more flexible and more fun.
swift’s clean slate backed by the mature and much-loved cocoa and cocoa touch frameworks is an opportunity to imagine how software development works.
swift is the first industrial-quality systems programming language that is as expressive and enjoyable as a scripting language.
简单的说：
swift用来写ios和os x程序。（估计也不会支持其它屌丝系统）
swift吸取了c和objective-c的优点，且更加强大易用。
swift可以使用现有的cocoa和cocoa touch框架。
swift兼具编译语言的高性能（performance）和脚本语言的交互性（interactive）。
swift语言概览
基本概念
注：这一节的代码源自the swift programming language中的a swift tour。
hello world
类似于脚本语言，下面的代码即是一个完整的swift程序。
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println(" hello world" )
变量与常量
swift使用var声明变量，let声明常量。
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var myvariable = 42
myvariable = 50
let myconstant = 42
类型推导
swift支持类型推导（type inference），所以上面的代码不需指定类型，如果需要指定类型：
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let explicitdouble : double = 70
swift不支持隐式类型转换（implicitly casting），所以下面的代码需要显式类型转换（explicitly casting）：
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let label = " the width is "
let width = 94
let width = label + string(width)
字符串格式化
swift使用\(item)的形式进行字符串格式化：
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let apples = 3
let oranges = 5
let applesummary = " i have \(apples) apples."
let applesummary = " i have \(apples + oranges) pieces of fruit."
数组和字典
swift使用[]操作符声明数组（array）和字典（dictionary）：
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var shoppinglist = [" catfish" " water" " tulips" " blue paint" ]
shoppinglist[1] = " bottle of water"
var occupations = [
" malcolm" : " captain"
" kaylee" : " mechanic"
]
occupations[" jayne" ] = " public relations"
一般使用初始化器（initializer）语法创建空数组和空字典：
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let emptyarray = string[]()
let emptydictionary = dictionary< string float> ()
如果类型信息已知，则可以使用[]声明空数组，使用[:]声明空字典。
控制流
概览
swift的条件语句包含if和switch，循环语句包含for-in、for、while和do-while，循环/判断条件不需要括号，但循环/判断体（body）必需括号：
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let individualscores = [75 43 103 87 12]
var teamscore = 0
for score in individualscores {
if score > 50 {
teamscore += 3
} else {
teamscore += 1
}
}
可空类型
结合if和let，可以方便的处理可空变量（nullable variable）。对于空值，需要在类型声明后添加?显式标明该类型可空。
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var optionalstring: string? = " hello"
optionalstring == nil
var optionalname: string? = " john appleseed"
var gretting = " hello!"
if let name = optionalname {
gretting = " hello \(name)"
}
灵活的switch
swift中的switch支持各种各样的比较操作：
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let vegetable = " red pepper"
switch vegetable {
case " celery" :
let vegetablecomment = " add some raisins and make ants on a log."
case " cucumber" " watercress" :
let vegetablecomment = " that would make a good tea sandwich."
case let x where x.hassuffix(" pepper" ):
let vegetablecomment = " is it a spicy \(x)?"
default:
let vegetablecomment = " everything tastes good in soup."
}
