简单值：

1.let声明常量，var声明变量（声明的同时赋值，编译器会自动推断出类型）

var myVariable = 42

myVariable 50

let myConstant = 42

let implicitInteger = 70

let implicitDouble = 70.0

let explicitDouble:Double = 70

声明类型时在“量”后面加上“：”然后写上类型的名字。（值永远不会隐式转换，需要转换请显示转换“类型（量）”）

  更简单的办法将值转换成字符串：\（量）

let apples = 3

let oranges = 5

let appleSummary = "I have \(apples) apples."

let fruitSummary = "I have \(apples + oranges) pieces of fruit"

2.“[]”创建数组和字典，key或者下标取值

var shoppingList = ["catfish","water","tulips","blue paint"]

shoppingList[1] = "bottle of water"

var occupations = ["Malcolm":"Captain","KayLee":"Mechanic"]

occupations["Jayne"] = "Public Relations"

空数组和空字典声明

let emptyArray  = String[]()

let emptyDictionary = Dictionary<String,Float>()

如果类型信息可以别推断出来，你可以用[]和[:]来创建空数组和空字典，就想你声明变量或者给函数传递参数的时候一样

shoppingList = []

控制流

1.if,switch进行条件操作，for-in,for,while,do-while进行循环（包裹条件和循环变量括号可以省略，但是语句体的大括号是必须的）

let individualScores = [75,43,103,87,12]

var teamScore = 0

for score in individualScores{

        if score>50{

          teamScore += 3

     }

     else

     {

          teamScore += 1

     }

}

if语句的条件必须是布尔值，一个可选的值可能是一个具体的值或者是nil，表示值缺失，在类型后面加上一个问号来标记这个变量的值是可选的。

var optionalString : String? = "Hello"

optionalString == nil

var optionalName:String?="John Appleseed"

var greeting = "Hello!"

if let name = optionalName{

       greeting = "Hello, \(name)"

}

switch支持任意类型的数据以及各种比较操作

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."

}

使用for - in 来遍历字典，需呀两个变量来表示每个键值对

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

var n = 2

while n < 100{

     n = n*2

}

n

var m = 2

do{

     m = m*2

}while m < 100

m

var firstForLoop = 0

for i in 0..3{

    firstForLoop += i

}

firstForLoop

var secondForLoop = 0

for var i= 0;i < 3;++i{

     secondForLoop += 1

}

secondForLoop(..不包含上界，...包含上界)