1.变量申明var greeting:String = "Hello"如果编译能够推断出变量的类型也可以去掉类型标注var greeting = "Hello"很明显”Hello”是一个String类型Swift会在编译时检查类型是否匹配2.字符连接var greeting = "Hello" greeting = greeting +" World!"Swift 的String类型是可变的3.如果

可选（optionals) var str:String? = "Hello Swift"str 可能为nil解包（unwrap)一个可选变量if let unwrappedStr = str{ println("unwrapped!\(unwrappedStr.uppercaseString)") }else{ println("was nil") }2.强制解包（Forced u