使用if 或者 swith進行條件判斷，
使用for-in，while，和 repeat-while進行循環
if 后面的（）是可選的【例如下邊的 if score > 50】，但是其后的{}必須有（非可選）

let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0
for score in individualScores {
    if score > 50 {
        teamScore += 3
    } else {
        teamScore += 1
    }
}
print(teamScore)

注意：條件判斷中，描述必須是一個明確的bool值，不能含蓄的跟0作比較。也就是必須是yes或者no，不能跟0還是非0做參考

optionals
/You can use if and let together to work with values that might be missing. These values are represented as optionals. An optional value either contains a value or contains nil to indicate that a value is missing. Write a question mark (?) after the type of a value to mark the value as optional./
用國人的話來說就是：如果一個變量可能有值，也可能沒有值，就在初始化的時候添加一個問號？。
另外，聲明變量的時候不添加？，編譯不通過

var optionalString: String? = "Hello"
print(optionalString == nil)

var optionalName: String? = nil"John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
    greeting = "Hello, \(name)"
    print(greeting)
}else{
    print("your name is nil")
}

?? 如果第一個為nil，就選擇 ?? 后邊的值

let nickName: String? = nil
let fullName: String = "John Appleseed"
let informalGreeting = "Hi \(nickName ?? fullName)"
print(informalGreeting)

Switch
/Switches support any kind of data and a wide variety of comparison operations—they aren’t limited to integers and tests for equality./
支持各種數據類型，以及更多的變量對比（只要符合條件就執行）

let vegetable = "red pepper"
switch vegetable {
case "celery":
    print("Add some raisins and make ants on a log.")
case "cucumber", "watercress":
    print("That would make a good tea sandwich.")
case let x where x.hasSuffix("pepper"):
    print("Is it a spicy \(x)?") Is it a spicy red pepper?
default:
    print("Everything tastes good in soup.")
}

注意：
1.注釋default報錯：Switch must be exhaustive（詳盡）
2.case后邊不再有break，只要滿足了一個case，程序就不會再去判斷其他case并執行

是否for-in遍歷字典的時候，因為字典是無序的集合，因此取出的keys也是任意的順序的

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
 標記上面哪組擁有最大的數
var largest = 0
var lagestName:String? = nil
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
            lagestName = kind
        }
    }
}
print(lagestName)
print(largest)

/* 注意：print(lagestName)會有警告，fix的方案有三：
1.print(lagestName ?? <#default value#>)
2.print(lagestName!)
3.print(lagestName as Any)
*/

Use while to repeat a block of code until a condition changes. The condition of a loop can be at the end instead, ensuring that the loop is run at least once.

var n = 2
while n < 100 {
    n *= 2
}
print(n) 128

var m = 2
repeat {
    m *= 2
} while m < 100
print(m) 128

使用 ..< 表示某個索引的范圍

var total = 0
for i in 0..<4 {
    total += i
}
print(total)  6

 使用 ... 表示某個索引的范圍【包含最后一個范圍，即：（0..<=4）這個表達式寫法是錯的，這里就是表達這個意思而已】
var total2 = 0
for i in 0...4 {
    total2 += i
}
print(total2)  10