## The Basics let声明常量，var声明变量 You can access the minimum and maximum values of each integer type with its min and max properties. 虽然有UInt，但能用Int的时候就用Int。 ~~~ // 各种进制的字面量表示 let decimalInteger = 17 let binaryInteger = 0b10001 // 17 in binary notation let octalInteger = 0o21 // 17 in octal notation let hexadecimalInteger = 0x11 // 17 in hexadecimal notation // 更易于阅读的写法 let paddedDouble = 000123.456 let oneMillion = 1_000_000 let justOverOneMillion = 1_000_000.000_000_1 ~~~ Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3. ~~~ // 定义类型别名 typealias typealias AudioSample = UInt16 // optional binding，只有当yyy是optional的时候才可以这样用。optional的yyy非空时为真，将yyy中的值取出赋给xxx，空时（nil）为假； if let xxx = yyy { // do something } else { // do other thing } // decompose一个tuple时，对于不想使用的元素用’_’接收 let http404Error = (404, "Not Found") let (justTheStatusCode, _) = http404Error println("The status code is \(justTheStatusCode)") // prints "The status code is 404 let possibleNumber = "123" let convertedNumber = possibleNumber.toInt() // convertedNumber is inferred to be of type "Int?", or "optional Int”，因为toInt()可能会失败（比如“123a”）导致返回nil ~~~ You can use an if statement to find out whether an optional contains a value. If an optional does have a value, it evaluates to true; if it has no value at all, it evaluates to false. Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation mark (!) to the end of the optional’s name. The exclamation mark effectively says, “I know that this optional definitely has a value; please use it.” This is known as forced unwrapping of the optional’s value。 If you define an optional constant or variable without providing a default value, the constant or variable is automatically set to nil for you.