#字段 dictionary

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var animalGroupsDict = ["whales":"pod", "geese":"flock", "lions":"pride"]

var groupsDict = [String:String]()

##常见用法

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var lifeSpanDict = ["African Grey Parrot": 50...70, "Tiger Salamander": 12...15,
    "Bottlenose Dolphin": 20...30]

var averageLifeSpanDict = [String:Range<Int>]()

… 运算符代表某个区间 ，如 50… 70就是50到70之间的某个数 
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//Adding item to a dictionary

animalGrounpsDict["crows"] = "murder"
animalGrounpsDict["monkeys"] = "troop"
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//the count method is available to all collections, count方法所有的集合都有
animalGrounpsDict.count
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//Removing items from a dictionary
animalGroupsDict["monkeys"] = nil
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//Update a value 
animalGrounpsDict["monkeys"] = ["barrel"]
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var group = animalGrounpsDict.updateValue["gaggle", forkey:"geese"]
group.dynamicType （结果为 Optional<String>.Type） 
//首先更新gaggle的种类为geese，并返回 geese ,通过group.dynamicType获取group的类型
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//Retrieving the value for a particular key,获取key对应的值
let groupOfWhales = animalGroupsDict["whales"] 

if letgrounpOfWhales = animalGroupsDict[“whales”]{
	// 如果whales对应的value存在
}else{
	//如果whales对应的value不存在
}