# 18.循环的标签控制

## 滑梯游戏

![Game](/files/-MJaC1fawqTCPWf9Wixa)

游戏的规则如下：

* 游戏盘面包括 25 个方格，游戏目标是达到或者超过第 25 个方格；
* 每一轮，你通过掷一个六面体骰子来确定你移动方块的步数，移动的路线由上图中横向的虚线所示；
* 如果在某轮结束，你移动到了梯子的底部，可以顺着梯子爬上去；
* 如果在某轮结束，你移动到了蛇的头部，你会顺着蛇的身体滑下去。

```swift
let finalSquare = 25
var board = [Int](repeating: 0, count: finalSquare + 1)

// 一些转折点
board[03] = +08; 
board[06] = +11; 
board[09] = +09; 
board[10] = +02
board[14] = -10; 
board[19] = -11; 
board[22] = -02; 
board[24] = -08


var square = 0
var diceRoll = 0
while square < finalSquare {
    // 掷骰子
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    // 根据点数移动
    square += diceRoll
    if square < board.count {
        // 如果玩家还在棋盘上，顺着梯子爬上去或者顺着蛇滑下去
        square += board[square]
    }
}
print("Game over!")
```

## 滑梯变种-循环标签

> 增加一条规则：为了获胜，你必须刚好落在第 25 个方块中。

```swift
gameLoop: while square != finalSquare {
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    switch square + diceRoll {
    case finalSquare:
        // 骰子数刚好使玩家移动到最终的方格里，游戏结束。
        break gameLoop
    case let newSquare where newSquare > finalSquare:
        // 骰子数将会使玩家的移动超出最后的方格，那么这种移动是不合法的，玩家需要重新掷骰子
        continue gameLoop
    default:
        // 合法移动，做正常的处理
        square += diceRoll
        square += board[square]
    }
}
print("Game over!")
```

这种循环条件嵌套的场景下通过`标签`可以方便精确的进行控制

如果不使用标签的话，上面的`break` `continue` 只能控制`switch`

## 参考

> [SwiftGG](https://swiftgg.gitbook.io/swift/swift-jiao-cheng/05_control_flow)


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