swift 学习（3）Optionals & Colle

1，Optionals

Swift introduces a whole new type, optional, that handles the possibility a value could be nil. If you’re handling a non-optional type, then you’re guaranteed to have a value and don’t need to worry about the existence of a valid value. Similarly, if you are using an optional type then you know you must handle the nil case. It removes the ambiguity introduced by using sentinel values.

Optionals are Swift’s solution to the problem of representing both a value and the absence of a value. An optional type is allowed to reference either a value or nil.

Optionals 相当于一个容器，里面可以存放一个确切存在的值或者nil。

var errorCode: Int?
errorCode = 100
print(errorCode)//Optional(100)
errorCode = nil
print(errorCode)//nil

可见当errorCode 有值时，打印的是Optional(100) 并不是我们想要的 100这个值，如何获得呢。

!

可以使用!强制获得。但当值为nil的时间会报错。

var errorCode: Int?
errorCode = 100
print(errorCode!)// 100
errorCode = nil
print(errorCode!)//报错

安全的做法
??

使用？？给一个默认值。当只为nil时使用默认值。

var errorCode: Int?
errorCode = 100
print(errorCode!)//Optional(100)
errorCode = nil
print(errorCode ?? 0)//0

Optional binding

Swift includes a feature known as optional binding, which lets you safely access the value inside an optional. You use it like so:

var authorName: String? = "Matt Galloway"
if let unwrappedAuthorName = authorName {
 print("Author is \(unwrappedAuthorName)")
} else {
 print("No author.")
}

2，数组

相同类型的数据的有序集合。

An array is an ordered collection of values of the same type. The elements in the array are zero-indexed, which means the index of the first element is 0, the index of the second element is 1 and so on. Knowing this, you can work out that the last element’s index is the number of values in the array minus 1.

创建数组

let evenNumbers = [2, 4, 6, 8]
var subscribers: [String] = []
let allZeros = [Int](repeating: 0, count: 5)
let allY = [String](repeating: "Y", count: 5)
let allZerosInferred = Array(repeating: 0, count: 5)

判断数组是否为空

var players = ["Alice", "Bob", "Cindy", "Dan"]

if !players.isEmpty {
    print(players)
}

if players.count > 0 {
    print(players)
}

推荐使用isEmpty 方法，isEmpty 应该是只要判断数组中有元素，就直接返回值了，而count会把数组中所有的元素都统计一遍，然后返回。如果数组的元素非常非常多，就很耗时间了。

读取操作

var currentPlayer = players.first
print(currentPlayer)
// > Optional("Alice")

print(players.last)
// > Optional("Dan")


currentPlayer = players.min()
print(currentPlayer)
// > Optional("Alice")

使用下表读取

var players = ["Alice", "Bob", "Cindy", "Dan"]
var firstPlayer = players[0]
var firstPlayer01 = players.first
print(firstPlayer)//Alice
print(firstPlayer01)//Optional("Alice")

使用rang读取

var players = ["Alice", "Bob", "Cindy", "Dan"]
var firstPlayer = players[0]
var firstPlayer01 = players.first
print(firstPlayer)//Alice
print(firstPlayer01)//Optional("Alice")

let upcomingPlayers = players[1...2]
print(upcomingPlayers)
// > ["Bob", "Cindy"]

操作数组

players.append("Eli")
players += ["Gina"]
players.insert("Frank", at: 5)
var removedPlayer = players.removeLast()
removedPlayer = players.remove(at: 2)
players[0] = "100"

print(players)//["100", "Bob", "Dan", "Eli", "Frank"]
players[0...1] = ["Donna", "Craig", "Brian", "Anna"]
print(players)//["Donna", "Craig", "Brian", "Anna", "Dan", "Eli", "Frank"]

迭代

for player in players {
    print(player)
//    Donna
//    Craig
//    Brian
//    Anna
//    Dan
//    Eli
//    Frank
}


for (index, player) in players.enumerated() {
    print("\(index + 1). \(player)")
//    1. Donna
//    2. Craig
//    3. Brian
//    4. Anna
//    5. Dan
//    6. Eli
//    7. Frank
}

3，字典

A dictionary is an unordered collection of pairs, where each pair is comprised of a key and a value.

字典包含键、值，键是唯一的，不同的键指向的值可以相同。键必须是同一类型，值也是同一类型。

创建字典

var namesAndScores = ["Anna": 2, "Brian": 2, "Craig": 8, "Donna": 6]
print(namesAndScores)
// > ["Craig": 8, "Anna": 2, "Donna": 6, "Brian": 2]
var pairs: [String: Int] = [:]

判断是否为空

isEmpty

count

推荐使用 isEmpty 理由通array

读取

var pairs: [String: Int] = [:]
var namesAndScores = ["Anna": 2, "Brian": 2, "Craig": 8, "Donna": 6]
print(namesAndScores["Anna"] ?? "测试")
print(namesAndScores["不存在Key"] ?? "测试")

Modifying dictionaries

bobData.updateValue("CA", forKey: "state")
bobData["city"] = "San Francisco"

上面两个方法的区别是updateValue 会把原有的key的值返回回来，如果之前没有这个key对应的值，返回nil。

bobData.removeValue(forKey: "city")
bobData["city"] = nil

上面两个方法的区别是removeValue 会把原有的key的值返回回来，如果之前没有这个key对应的值，返回nil。

迭代

for (player, score) in namesAndScores {
    print("\(player) - \(score)")
}