Control Flow

for in 循环

for index in 1...5 {print("\(index) times 5 is \(index * 5)")
}
// index是一个常数无需声明
let base = 3
let power = 10
var answer = 1
for _ in 1...power {answer *= base
}
// 如果不需要索引值，怎可以使用 下划线_ 代替

let minutes = 60
for tickMark in 0..<minutes {// render the tick mark each minute (60 times)
}let minuteInterval = 5
for tickMark in stride(from: 0, to: minutes, by: minuteInterval) {// render the tick mark every 5 minutes (0, 5, 10, 15 ... 45, 50, 55)
}let hours = 12
let hourInterval = 3
for tickMark in stride(from: 3, through: hours, by: hourInterval) {// render the tick mark every 3 hours (3, 6, 9, 12)
}

While 循环

• while 每次循环执行时都会评估其条件。
• repeat- while在每次循环结束时评估其条件。在考虑循环条件之前，会先执行一次循环循环。然后，它将继续重复循环，直到条件为false。类似于 do - while

while condition {statements
}repeat {statements
} while condition

条件语句

if

多个判断条件可以用逗号连接

switch

break在Swift中不是必需的，但是可以使用break语句来匹配和忽略特定的情况，或者在该情况完成执行之前中断匹配的情况。
每个case都必须包含一个可执行语句，一个case两种value用逗号隔开

switch some value to consider {
case value 1:respond to value 1
case value 2,value 3:respond to value 2 or 3
default:otherwise, do something else
}//间隔匹配
let approximateCount = 62
let countedThings = "moons orbiting Saturn"
let naturalCount: String
switch approximateCount {
case 0:naturalCount = "no"
case 1..<5:naturalCount = "a few"
case 5..<12:naturalCount = "several"
case 12..<100:naturalCount = "dozens of"
case 100..<1000:naturalCount = "hundreds of"
default:naturalCount = "many"
}
print("There are \(naturalCount) \(countedThings).")
// Prints "There are dozens of moons orbiting Saturn."

let somePoint = (1, 1)
switch somePoint {
case (0, 0):print("\(somePoint) is at the origin")
case (_, 0):print("\(somePoint) is on the x-axis")
case (0, _):print("\(somePoint) is on the y-axis")
case (-2...2, -2...2):print("\(somePoint) is inside the box")
default:print("\(somePoint) is outside of the box")
}
// Prints "(1, 1) is inside the box"

(0，0）可以匹配所有四种情况。但是，如果可能有多个匹配项，则始终使用第一个匹配情况，所有其他匹配情况都将被忽略。

Value Bindings

let anotherPoint = (2, 0)
switch anotherPoint {
case (let x, 0):print("on the x-axis with an x value of \(x)")
case (0, let y):print("on the y-axis with a y value of \(y)")
case let (x, y):print("somewhere else at (\(x), \(y))")
}
// Prints "on the x-axis with an x value of 2"// 复核case
let stillAnotherPoint = (9, 0)
switch stillAnotherPoint {
case (let distance, 0), (0, let distance):print("On an axis, \(distance) from the origin")
default:print("Not on an axis")
}
// Prints "On an axis, 9 from the origin"

该switch陈述没有任何default理由。但最后一个case，声明了两个可以匹配任何值的占位符常量的元组。因为始终是两个值的元组，所以此情况与所有可能的剩余值匹配，并且不需要使情况穷举的情况。

where

let yetAnotherPoint = (1, -1)
switch yetAnotherPoint {
case let (x, y) where x == y:print("(\(x), \(y)) is on the line x == y")
case let (x, y) where x == -y:print("(\(x), \(y)) is on the line x == -y")
case let (x, y):print("(\(x), \(y)) is just some arbitrary point")
}
// Prints "(1, -1) is on the line x == -y"

Control Transfer Statements

• continue
• break
• fallthrough
• return
• throw

Labeled Statements（待研究）

label name: while condition {statements
}gameLoop: while square != finalSquare {diceRoll += 1if diceRoll == 7 { diceRoll = 1 }switch square + diceRoll {case finalSquare:// diceRoll will move us to the final square, so the game is overbreak gameLoopcase let newSquare where newSquare > finalSquare:// diceRoll will move us beyond the final square, so roll againcontinue gameLoopdefault:// this is a valid move, so find out its effectsquare += diceRollsquare += board[square]}
}
print("Game over!")

如果break上面的语句不使用gameLoop标签，它将脱离该switch语句，而不是该while语句。使用gameLoop标签可以清楚地表明哪个控制语句应该终止。

Early Exit：guard（待研究）

func greet(person: [String: String]) {guard let name = person["name"] else {return}print("Hello \(name)!")guard let location = person["location"] else {print("I hope the weather is nice near you.")return}print("I hope the weather is nice in \(location).")
}greet(person: ["name": "John"])
// Prints "Hello John!"
// Prints "I hope the weather is nice near you."
greet(person: ["name": "Jane", "location": "Cupertino"])
// Prints "Hello Jane!"
// Prints "I hope the weather is nice in Cupertino."

如果满足guard语句的条件，则执行在该guard语句的右括号后的代码。在guard语句出现的条件绑定，在其余代码部分依然有效。

如果不满足该条件，则执行else分支内的代码。该分支必须转移控制权以退出该guard语句所在的代码块。它可以控制转移语句做到这一点，如return，break，continue，throw，也可以调用一个函数或方法不返回，如fatalError(_:file:line:)。

检查API可用性（待研究）

if #available(platform name version, ..., *) {statements to execute if the APIs are available
} else {fallback statements to execute if the APIs are unavailable
}if #available(iOS 10, macOS 10.12, *) {// Use iOS 10 APIs on iOS, and use macOS 10.12 APIs on macOS
} else {// Fall back to earlier iOS and macOS APIs
}

上面的可用性条件指定在iOS中，if语句主体仅在iOS 10及更高版本中执行；在macOS中，仅在macOS 10.12及更高版本中。最后一个参数*是必需的，它指定在任何其他平台上，if执行程序的主体均在目标所指定的最小部署目标上执行。