Question

我正在阅读Apple的Swift指南，我在do-while循环中遇到错误，我无法解决为什么Xcode会抛出错误。

本代码直接来自本书：

let finalSquare = 25

var board2 = [Int](count: finalSquare + 1, repeatedValue: 0) //26 values (0 included)

board2[03] = +08; board2[06] = +11; board2[09] = +09; board2[10] = +02 //the ladders
board2[14] = -10; board2[19] = -11; board2[22] = -02; board2[24] = -08 //the snakes
var square2 = 0
var diceRoll2 = 0

do {
    square2 += board2[square2]
    if ++diceRoll2 == 7 { diceRoll2 = 1 }
    square2 += diceRoll2    
} while square2 < finalSquare

但是我想在滚动后向上移动梯子或者直接向上移动蛇，所以我这样做了：

do {
    if ++diceRoll2 == 7 { diceRoll2 = 1 }
    square2 += diceRoll2
    square2 += board2[square2]
} while square2 < finalSquare

这会引发错误：

Execution was interrupted, reason:EXC_BAD_INSTRUCTION(code=EXC_1386_INVOP, subcode=0x0)

Answer 1

在原始代码中：

do {
    square2 += board2[square2]
    if ++diceRoll2 == 7 { diceRoll2 = 1 }
    square2 += diceRoll2    
} while square2 < finalSquare

square2递增，然后检查其值finalSquare，导致循环结束，如果它大于或等于。

代替您的代码：

do {
    if ++diceRoll2 == 7 { diceRoll2 = 1 }
    square2 += diceRoll2 // < incremented here
    square2 += board2[square2] // < exception here
} while square2 < finalSquare

你增加square2，然后你用它来访问相应的数组元素，但不检查它是否在数组边界后面（检查是在访问数组后完成的，所以在这种情况下没用）。

快速补丁是添加一个明确的检查：

do {
    if ++diceRoll2 == 7 { diceRoll2 = 1 }
    square2 += diceRoll2
    if square2 < finalSquare {
        square2 += board2[square2]
    }
} while square2 < finalSquare

但如果以这种形式书写，它可能更具可读性：

do {
    if ++diceRoll2 == 7 { diceRoll2 = 1 }
    square2 += diceRoll2
    if square2 < finalSquare {
        square2 += board2[square2]
    } else {
        break
    }
} while true

虽然我不是内心休息的无尽循环的忠实粉丝。

Swift - do-while循环意外错误

1 个答案: