使用if 或者 swith进行条件判断，
使用for-in，while，和 repeat-while进行循环
if 后面的（）是可选的【例如下边的 if score > 50】，但是其后的{}必须有（非可选）
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：条件判断中，描述必须是一个明确的bool值，不能含蓄的跟0作比较。也就是必须是yes或者no，不能跟0还是非0做参考
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./
用国人的话来说就是：如果一个变量可能有值，也可能没有值，就在初始化的时候添加一个问号？。
另外，声明变量的时候不添加？，编译不通过
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")
}
?? 如果第一个为nil，就选择 ?? 后边的值
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./
支持各种数据类型，以及更多的变量对比（只要符合条件就执行）
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.注释default报错：Switch must be exhaustive（详尽）
2.case后边不再有break，只要满足了一个case，程序就不会再去判断其他case并执行
是否for-in遍历字典的时候，因为字典是无序的集合，因此取出的keys也是任意的顺序的
let interestingNumbers = [
"Prime": [2, 3, 5, 7, 11, 13],
"Fibonacci": [1, 1, 2, 3, 5, 8],
"Square": [1, 4, 9, 16, 25],
]
标记上面哪组拥有最大的数
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)会有警告，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 ..< 表示某个索引的范围
var total = 0
for i in 0..<4 {
total += i
}
print(total) 6
使用 ... 表示某个索引的范围【包含最后一个范围，即：（0..<=4）这个表达式写法是错的，这里就是表达这个意思而已】
var total2 = 0
for i in 0...4 {
total2 += i
}
print(total2) 10