Swift是什么？
Swift是苹果于WWDC 2014公布的编程语言。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。（预计也不会支持其他屌丝系统）
Swift吸取了C和Objective-C的长处，且更加强大易用。
Swift能够使用现有的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。以下的代码即是一个完整的Swift程序。
println("Hello, world") 变量与常量
Swift使用var声明变量，let声明常量。
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。假设须要指定类型：
let explicitDouble : Double = 70
Swift不支持隐式类型转换（Implicitly casting），所以以下的代码须要显式类型转换（Explicitly casting）：
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：
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）：
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>()
假设类型信息已知。则能够使用[]声明空数组。使用[:]声明空字典。
控制流
概览
Swift的条件语句包括if和switch，循环语句包括for-in、for、while和do-while，循环/推断条件不须要括号，但循环/推断体（body）必需括号：
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。能够方便的处理可空变量（nullable variable）。对于空值，须要在类型声明后加入?
显式标明该类型可空。
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支持各种各样的比較操作：
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."
}
其他循环
for-in除了遍历数组也能够用来遍历字典：
let interestingNumbers = [
"Prime": [2, 3, 5, 7, 11, 13],
"Fibonacci": [1, 1, 2, 3, 5, 8],
"Square": [1, 4, 9, 16, 25],
]
var largest = 0
for (kind, numbers) in interestingNumbers {
for number in numbers {
if number > largest {
largest = number
}
}
}
largest
while循环和do-while循环：
var n = 2
while n < 100 {
n = n * 2
}
n
var m = 2
do {
m = m * 2
} while m < 100
m
Swift支持传统的for循环。此外也能够通过结合..（生成一个区间）和for-in实现相同的逻辑。
var firstForLoop = 0
for i in 0..3 {
firstForLoop += i
}
firstForLoop
var secondForLoop = 0
for var i = 0; i < 3; ++i {
secondForLoop += 1
}
注意：Swift除了..还有...：..生成前闭后开的区间，而...生成前闭后闭的区间。