Question

我正在通过Deitel＆amp; amp; Deitel＆＃34; Java - 如何编程＆＃34;而且我很难理解为什么我提出的通用选择排序方法的实现不起作用。我确定我必须遗漏一些小细节，但在研究了API和一些关于泛型的资源后，我感到很冷淡。虽然程序运行并执行某种排序，但它肯定不按数字顺序排序！我无法判断我是否误解了泛型，或者只是选择排序算法。任何帮助将不胜感激！

我在intArray上运行选择排序时收到的输出是： 0,1，-23,7,54

排序后floatArray的输出是： -1.1，-10.3,0.4,4.5

更新我只是尝试了这个而不使用负值而且排序很好，那是什么???

以下是执行选择排序的完整Sorter类：

import java.util.Arrays;
import java.util.ArrayList;

public class Sorter {
    public static void main(String[] args) {
        Integer[] intArray = {1, 7, -23, 54, 0};
        Float[] floatArray = {0.4f, -10.3f, 4.5f, -1.1f};

        ArrayList<Integer> intList = new ArrayList<>(Arrays.asList(intArray));
        ArrayList<Float> floatList = new ArrayList<>(Arrays.asList(floatArray));

        System.out.printf("Lists before selectionSort: %n%s%n%s%n%n",
            intList, floatList);

        selectionSort(intList);
        selectionSort(floatList);

        System.out.printf("Lists after selectionSort: %n%s%n%s%n%n",
            intList, floatList);
    }

    public static <T extends Comparable<T>> void selectionSort(ArrayList<T> list) {
        // helps determine whether or not a swap will occur
        boolean needsSorting = false;
        // keeps track of the index of the smallest value
        int smallest = 0;

        // outer for walks the portion of the list that will be swapped
        for (int i = 0; i < list.size() - 1; i++) {
            // inner for searches for a smaller value than the front of list
            for (int j = i + 1; j < list.size(); j++) {
                // if the inner value is less than the outer value
                if (list.get(j).compareTo(list.get(i)) < 0) {
                    // store the index of the smaller value
                    smallest = j;
                    // set the boolean flag to true so the sort will happen
                    needsSorting = true;
                }
            }

            // if the list needs sorting
            if (needsSorting) {
                // get the value of the outer loop, store in generic variable
                T temp = list.get(i);
                // replace value of outer loop with value at the smallest index
                list.set(i, list.get(smallest));
                // replace value at what was smallest index with the value that
                // was at the index of the outer loop
                list.set(smallest, temp);

                needsSorting = false;
            }
        }
    }
}

Answer 1

问题可能出在这一行，

if (list.get(j).compareTo(list.get(i)) < 0)

而不是list.get(i)，应该是list.get(smallest)

另外，当你应该在外部for循环的每次迭代中这样做时，你不会更新最小的。在for (int i = 0; i < list.size() - 1; i++)

之后添加此行代码

smallest = i;

Answer 2

DaneBrick让我朝着正确的方向前进。对于外循环的每次迭代，我需要将最小变量设置为等于外部循环的计数器i。以下是代码的更正部分：

    for (int i = 0; i < list.size() - 1; i++) {
        // inner for searches for a smaller value than the front of list
        //*** NEW PORTION, DIDN'T HAVE THIS BEFORE ***//
        smallest = i;
        for (int j = i + 1; j < list.size(); j++) {
            // if the inner value is less than the outer value
            if (list.get(j).compareTo(list.get(smallest)) < 0) {
                // store the index of the smaller value
                smallest = j;
                // set the boolean flag to true so the sort will happen
                needsSorting = true;
            }
        }

SelectionSort使用通用方法实现 - 错误的结果

2 个答案: