• 常量关键字  let
• 变量关键字  var
• 数组和字典  ([])
• \ () 格式字符串标记

      在swift语法中使用let关键字声明常量，例如   

let myConstant = 42

     使用let声明的常量在编译时候不需要赋值，但是你必须在某个时候给它且只能赋值一次，意味着你可以声明一次，可以在许多地方使用多次。 

     使用var关键字声明变量，例如     

var myVariable = 42

     声明的let常量和var变量必须赋予相应的值，但是不需要每次都声明类型，因为编译器可以自动判断声明的变量类型。     

“let implicitInteger = 70
let implicitDouble = 70.0
let explicitDouble: Double = 70”

“let label = "The width is "
let width = 94
let widthLabel = label + String(width)”

   声明数组和字典使用 ([])，通过括号中的索引或者键来访问他们的元素。

“var shoppingList = ["catfish", "water", "tulips", "blue paint"]
shoppingList[1] = "bottle of water"
 
var occupations = [
    "Malcolm": "Captain",
    "Kaylee": "Mechanic",
]
occupations["Jayne"] = "Public Relations”

“let emptyArray = String[]()
let emptyDictionary = Dictionary<String, Float>()”

    如果数组或者字典类型能够被推断出来，数组可以写一个空数组[]，字典使用[:]

“shoppingList = []   // Went shopping and bought everything.”

“let apples = 3
let oranges = 5
let appleSummary = "I have \(apples) apples."
let fruitSummary = "I have \(apples + oranges) pieces of fruit.”