这句代码已经是一个完整的程序了
print("Hello World")
let 声明恒量 编译期不需要知道类型，但是必须正确的赋值
var 声明变量
自动类型推断这个恒量的类型，另外，也在冒号后面指定类型
let implicitFloat:Float = 4
print(implicitFloat)
不能含蓄的进行类型转换，需要明确的指定类型
let label = "the width is "
let width = 94
let labelWidth = label + String(width)
print(labelWidth)
拼接不同类型的变量
let oldFloat = 4.0
let Jack = "Jack"
let jackOld = "jack is \(oldFloat) years old"
print(jackOld)
""" 连续的三个双引号表示换行
let apples = 3
let oranges = 5
let quotation = """
I said "I have \(apples) apples."
And then I said "I have \(apples + oranges) pieces of fruit."
"""
print(quotation)
使用圆括号和方括号创建数组和字典([])，使用索引获取或者赋值，允许子啊末尾添加一个逗号
var shoppingList = ["catfish", "water", "tulips", "blue paint"]
print(shoppingList) ["catfish", "water", "tulips", "blue paint"]
shoppingList[1] = "bottle of water"
print(shoppingList) ["catfish", "bottle of water", "tulips", "blue paint"]
var occupations = [
"Malcolm": "Captain",
"Kaylee": "Mechanic",
]
occupations["Jayne"] = "Public Relations"
print(occupations)
初始化空的数组和字典，需要使用初始化格式
let emptyArrar = [String]()
let emptyDic = [String: Float]()
如果消息可以推断出来，就不用指定类型了, 比如，给新变量赋值，或者函数传参
shoppingList = []
occupations = [:]
print(shoppingList)
print(occupations)
