Swift 流程控制

一.for循环

for－in

实例1：

for index in 1...5{
print("\(index)times 5 is \(index * 5)")
}

结果：

1times 5 is 5
2times 5 is 10
3times 5 is 15
4times 5 is 20
5times 5 is 25

1...5表示：［1，5］大于等于1且小于等于5

如果括号内不使用index，可以用_代替：

var a = 5
for _ in 1...5{
a += 1
}
print("\(a)")

结果：

10

遍历数组：

let nameArr = ["Lili","Marry","xiaoming"]
for name in nameArr {
print("Hello,\(name)!")
}

结果：

Hello,Lili!
Hello,Marry!
Hello,xiaoming!

遍历字典：

let ageDict = ["Lili":9,"Marry":10,"xiaoming":12]
for (name,age) in ageDict {
print("\(name) is \(age)!")
}

结果：

Marry is 10!
Lili is 9!
xiaoming is 12!

由于字典是无序的，所以结果不一定按顺序输出。

二。while循环

语法：

while condition {

statements

}

举例：

//唉，举例无能
var a = 10
while a>0 {
a = a-2
}
print(a)

结果：0

原来的do...while循环在此处替换成了repeat...while循环。（不举例了）

三。switch

实例：

let someCharacter: Character = "z"
switch someCharacter {
case "a":
print("The first letter of the alphabet")
case "z":
print("The last letter of the alphabet")
default:
print("Some other character")
}

结果：

The first letter of the alphabet

＊ 注意：与C和Objective_C不同，3.0的swift中switch语句的case中不再需要使用break跳出switch语句，匹配条件后即跳出Switch语句，同时，switch中的default是必须的。

虽然break在switch中不再是必须的，但是由于switch中的case／default中语句不允许有空语句，此时，我们可以使用break跳出语句。

例如：

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a": // Invalid, the case has an empty body
case "A":
print("The letter A")
default:
print("Not the letter A")
}
if let integerValue = possibleIntegerValue {
print("The integer value of \(numberSymbol) is \(integerValue).")
} else {
print("An integer value could not be found for \(numberSymbol).")
}

case "a":是无效的，这将导致程序报错： 'case' label in a 'switch' should have at least one executable statement。

再看下面这个例子

let numberSymbol: Character = "三"  // Chinese symbol for the number 3
var possibleIntegerValue: Int?
switch numberSymbol {
case "1", "١", "一", "๑":
possibleIntegerValue = 1
case "2", "٢", "二", "๒":
possibleIntegerValue = 2
case "3", "٣", "三", "๓":
possibleIntegerValue = 3
case "4", "٤", "四", "๔":
possibleIntegerValue = 4
default:
break
}
if let integerValue = possibleIntegerValue {
print("The integer value of \(numberSymbol) is \(integerValue).")
} else {
print("An integer value could not be found for \(numberSymbol).")
}
// Prints "The integer value of 三 is 3."

此时，default中的可能性很多，我们可能不再需要设置一个possibleIntegerValue的值，此时就可以使用break跳出switch语句。

间隔匹配：

例如：

let grade = 67
switch grade {
case 0...59:
print("不及格")
case 60...79:
print("及格")
case 80...89:
print("良好")
case 90...100:
print("优秀")
default:
break
}
//输出：及格

元祖（Tuples）匹配：

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

下划线（_）也称为通配符，可以匹配任何可能的值。

值绑定：

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"

where：Switch中还可以使用where来设置附加条件

例如：

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"

控制转移语句：

1.continue；

2.break（不再重复说明）;

3.fallthrough;

4.return.

5.throw。

continue:跳出当前循环，执行下一次循环。

例如：

let puzzleInput = "great minds think alike"
var puzzleOutput = ""
for character in puzzleInput.characters {
switch character {
case "a", "e", "i", "o", "u", " ":
continue
default:
puzzleOutput.append(character)
}
}
print(puzzleOutput)
// Prints "grtmndsthnklk"

fallthrough:条件匹配成功后，继续执行后面的语句。

例如：

let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe {
case 2, 3, 5, 7, 11, 13, 17, 19:
description += " a prime number, and also"
fallthrough
default:
description += " an integer."
}
print(description)
// Prints "The number 5 is a prime number, and also an integer."

return：跳出整个循环与语句。