一、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）。
二、Swift语言概览
1.基本概念
注：这一节的代码源自The Swift Programming Language中的A Swift Tour。
1.1.Hello, world
类似于脚本语言，下面的代码即是一个完整的Swift程序。
println("Hello, world")
1.2.变量与常量
Swift使用var声明变量，let声明常量。
ar myVariable = 42
myVariable = 50
let myConstant = 42
1.3.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：
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）：
let label = "The width is "
let width = 94
let width = label + String(width)
1.4.字符串格式化
Swift使用(item)的形式进行字符串格式化：
let apples = 3
let oranges = 5
let appleSummary = "I have (apples) apples."
let appleSummary = "I have (apples + oranges) pieces of fruit."
1.5.数组和字典
Swift使用[]操作符声明数组（array）和字典（dictionary）：
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：
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。
2.控制流
2.1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：
