Question

简单的selectionSort无法正常工作

doSort - 不工作
doSortWorking方法正常运行

这里doSort方法不起作用只是因为操纵for循环内部的“最小”，有人能告诉我什么是根本原因，我无法理解根本原因

object SelectionSort extends App {
    val unSortedArray = ArrayBuffer(5, -1, 8, 4, 6, 21, 4, 5, -0, 0)

    def doSort (ary: ArrayBuffer[Int], startIndex: Int, endIndex: Int) = {
        var minimum = 0
        for {
            index <- startIndex to endIndex
            _ = minimum = ary(index)
            loopIndex <- index until endIndex
        } {
            if (minimum > ary(loopIndex + 1)) {
                minimum = ary(loopIndex + 1)
                val swap = ary(index)
                ary(index) = ary(loopIndex + 1)
                ary(loopIndex + 1) = swap
            }
        }
    }

    def doSortWorking (ary: ArrayBuffer[Int], startIndex: Int, endIndex: Int) = {
        var minimum = 0
        for {
            index <- startIndex to endIndex
            loopIndex <- index until endIndex
        } {
            minimum = ary(index)
            if (minimum > ary(loopIndex + 1)) {
                minimum = ary(loopIndex + 1)
                val swap = ary(index)
                ary(index) = ary(loopIndex + 1)
                ary(loopIndex + 1) = swap
            }
        }
    }

    doSort(unSortedArray, 0, unSortedArray.size - 1)
    println(unSortedArray)
    doSortWorking(unSortedArray, 0, unSortedArray.size - 1)
    println(unSortedArray)
}

Answer 1

只需在doSort（）中交换第二行和第三行进行理解：

def doSort (ary: ArrayBuffer[Int], startIndex: Int, endIndex: Int) = {
    var minimum = 0
    for {
        index <- startIndex to endIndex
        loopIndex <- index until endIndex
        _ = minimum = ary(index)
    } {
        if (minimum > ary(loopIndex + 1)) {
            minimum = ary(loopIndex + 1)
            val swap = ary(index)
            ary(index) = ary(loopIndex + 1)
            ary(loopIndex + 1) = swap
        }
    }
}

在doSortWorking函数内找到loopIndex后，您应该将新值重新指定为最小值。

编辑：为什么您的doSort版本不起作用？

让我们试试它的简化版本：

scala> val startIndex = 0
firstIndex: Int = 0

scala> val endIndex = 3
lastIndex: Int = 3

scala> var minimum = 0
minimum: Int = 0

scala> for {
     |   index <- startIndex to endIndex
     |   _ = minimum = index
     | } {
     |   println(minimum)
     | }
3
3
3
3

为什么不打印0 1 2 3？因为理解不像你在这里想象的那样有效。检查这个答案：https://stackoverflow.com/a/3754568/5053865这里有一些关于for-comprehension评估的规则。让我们应用它们：

scala> for ((p, _) <-
     |   for (index <- startIndex to endIndex)
     |   yield { val x0 = minimum = index; (index, x0) }
     | ) {
     |   println(minimum)
     | }
3
3
3
3

scala> startIndex to endIndex map {
     |   index => val x0 = minimum = index; (index, x0)
     | } foreach {
     |   _ => println(minimum)
     | }
3
3
3
3

现在你应该看看这里到底发生了什么。首先，将startIndex和endIndex中的值分配给变量minimum，然后每次都使用maximum = endIndex执行body！

scala for-loop问题分配问题

1 个答案: