控制转移 #

一、控制转移概述 #

Swift提供以下控制转移语句：

continue：跳过当前迭代，继续下一次
break：终止整个循环
fallthrough：switch穿透执行
return：返回函数
throw：抛出错误

二、continue语句 #

2.1 基本用法 #

swift

for i in 1...10 {
    if i % 2 == 0 {
        continue
    }
    print(i)
}

2.2 过滤特定值 #

swift

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

for number in numbers {
    if number == 5 {
        continue
    }
    print(number)
}

2.3 处理可选值 #

swift

let items: [String?] = ["A", nil, "B", nil, "C"]

for item in items {
    guard let item = item else {
        continue
    }
    print(item)
}

2.4 复杂条件 #

swift

let students = [
    ("张三", 85),
    ("李四", 55),
    ("王五", 90),
    ("赵六", 45)
]

for (name, score) in students {
    guard score >= 60 else {
        continue
    }
    print("\(name): \(score)分 - 及格")
}

三、break语句 #

3.1 基本用法 #

swift

for i in 1...10 {
    if i == 5 {
        break
    }
    print(i)
}

3.2 查找元素 #

swift

let numbers = [3, 7, 2, 9, 5, 1]
let target = 9
var found = false
var index = -1

for (i, number) in numbers.enumerated() {
    if number == target {
        found = true
        index = i
        break
    }
}

print(found ? "找到在索引\(index)" : "未找到")

3.3 条件终止 #

swift

var sum = 0

for i in 1...100 {
    sum += i
    if sum > 1000 {
        print("当i=\(i)时，sum=\(sum)超过1000")
        break
    }
}

3.4 while循环中使用 #

swift

var count = 0

while true {
    count += 1
    if count > 5 {
        break
    }
    print(count)
}

四、带标签的语句 #

4.1 标签语法 #

swift

labelName: for i in 1...3 {
    for j in 1...3 {
        print("(\(i), \(j))")
    }
}

4.2 带标签的break #

swift

outerLoop: for i in 1...3 {
    for j in 1...3 {
        if i == 2 && j == 2 {
            break outerLoop
        }
        print("(\(i), \(j))")
    }
}

4.3 带标签的continue #

swift

outerLoop: for i in 1...3 {
    for j in 1...3 {
        if j == 2 {
            continue outerLoop
        }
        print("(\(i), \(j))")
    }
}

4.4 实际应用 #

swift

let matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
let target = 5

search: for (i, row) in matrix.enumerated() {
    for (j, value) in row.enumerated() {
        if value == target {
            print("找到\(target)在位置(\(i), \(j))")
            break search
        }
    }
}

五、fallthrough语句 #

5.1 基本用法 #

swift

let number = 2

switch number {
case 1:
    print("一")
    fallthrough
case 2:
    print("二")
    fallthrough
case 3:
    print("三")
default:
    print("其他")
}

5.2 模拟C风格switch #

swift

let char: Character = "a"

switch char {
case "a":
    print("是a")
    fallthrough
case "A":
    print("是A或a")
default:
    break
}

5.3 注意事项 #

swift

let number = 2

switch number {
case 1:
    print("一")
case 2:
    print("二")
    fallthrough
case 3:
    print("三")
default:
    print("其他")
}

六、return语句 #

6.1 基本用法 #

swift

func greet(name: String) -> String {
    return "Hello, \(name)!"
}

print(greet(name: "Swift"))

6.2 提前返回 #

swift

func divide(_ a: Int, by b: Int) -> Int? {
    if b == 0 {
        return nil
    }
    return a / b
}

if let result = divide(10, by: 2) {
    print(result)
}

6.3 隐式返回 #

swift

func add(_ a: Int, _ b: Int) -> Int {
    a + b
}

print(add(1, 2))

七、throw语句 #

7.1 基本用法 #

swift

enum ValidationError: Error {
    case emptyInput
    case invalidLength
}

func validate(_ input: String) throws {
    guard !input.isEmpty else {
        throw ValidationError.emptyInput
    }
    
    guard input.count >= 3 else {
        throw ValidationError.invalidLength
    }
}

do {
    try validate("ab")
} catch {
    print(error)
}

7.2 实际应用 #

swift

enum NetworkError: Error {
    case noConnection
    case timeout
    case serverError(Int)
}

func fetchData() throws -> String {
    let connected = false
    
    guard connected else {
        throw NetworkError.noConnection
    }
    
    return "Data"
}

do {
    let data = try fetchData()
    print(data)
} catch NetworkError.noConnection {
    print("无网络连接")
} catch {
    print("其他错误: \(error)")
}

八、实际应用 #

8.1 表单验证 #

swift

func validateForm(name: String?, email: String?, age: Int?) -> Bool {
    guard let name = name, !name.isEmpty else {
        print("名字不能为空")
        return false
    }
    
    guard let email = email, email.contains("@") else {
        print("邮箱格式不正确")
        return false
    }
    
    guard let age = age, age >= 18 else {
        print("年龄必须大于等于18岁")
        return false
    }
    
    return true
}

8.2 搜索算法 #

swift

func binarySearch(_ array: [Int], target: Int) -> Int? {
    var left = 0
    var right = array.count - 1
    
    while left <= right {
        let mid = (left + right) / 2
        
        if array[mid] == target {
            return mid
        } else if array[mid] < target {
            left = mid + 1
        } else {
            right = mid - 1
        }
    }
    
    return nil
}

let sortedArray = [1, 3, 5, 7, 9, 11, 13]
if let index = binarySearch(sortedArray, target: 7) {
    print("找到在索引\(index)")
}

8.3 状态机 #

swift

enum State {
    case idle
    case connecting
    case connected
    case disconnecting
}

var state = State.idle

func processState() {
    switch state {
    case .idle:
        print("空闲状态")
        state = .connecting
    case .connecting:
        print("连接中...")
        state = .connected
    case .connected:
        print("已连接")
        state = .disconnecting
    case .disconnecting:
        print("断开连接")
        state = .idle
    }
}

for _ in 1...5 {
    processState()
}

九、最佳实践 #

9.1 使用guard提前退出 #

swift

func process(data: [Int]?) {
    guard let data = data, !data.isEmpty else {
        return
    }
    
    for item in data {
        print(item)
    }
}

9.2 避免深层嵌套 #

swift

func validate(user: User?) -> Bool {
    guard let user = user else { return false }
    guard user.isActive else { return false }
    guard user.hasPermission else { return false }
    return true
}

9.3 合理使用标签 #

swift

search: for i in 0..<matrix.count {
    for j in 0..<matrix[i].count {
        if matrix[i][j] == target {
            return (i, j)
        }
    }
}

十、总结 #

本章学习了Swift的控制转移语句：

continue：跳过当前迭代
break：终止循环
fallthrough：switch穿透
return：函数返回
throw：抛出错误

最佳实践：

使用guard提前退出
避免深层嵌套
合理使用标签
保持代码可读性

下一章，我们将学习guard语句！