Java：具有并行数组的Quicksort算法

Question

I am trying to implement the quicksort algorithm in Java:

// Sort parallel arrays with quick sort

import java.util.Scanner;
import java.text.DecimalFormat;

class FunRunQuiSort {
    static int iSize = 5; // Set value
    static double[] daFinishTime = new double[iSize];
    static String[] saRunnerName = new String[iSize];

static void getInput() {
    System.out.println("\n***** 5K RUNNERS *****");
    for(int i = 0; i < saRunnerName.length; i++) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter runner name: ");
        saRunnerName[i] = input.nextLine();

        System.out.print("Enter finish time: ");
        daFinishTime[i] = input.nextDouble();

        System.out.print("\r\n");
    }
}

static void doQuickSort(double[] daFinishTime) {
    int i = daFinishTime[0], j = daFinishTime.length - 1;
    double dTemp;
    String sTemp;
    // Pivot
    double dPivot = daFinishTime[(daFinishTime[0] + daFinishTime.length - 1) / 2];
    // Partition
    do {    
        while(daFinishTime[i] < dPivot) {
            i++;
        } 
        while(daFinishTime[j] > dPivot) {
            j--;
        }
        if(i <= j) {
            dTemp = daFinishTime[i];
            daFinishTime[i] = daFinishTime[j];
            daFinishTime[j] = dTemp;

            sTemp = saRunnerName[i];
            saRunnerName[i] = saRunnerName[j];
            saRunnerName[j] = sTemp;

            i++;
            j--;
        }
    } while(i <= j);
    // Recursion
    if(daFinishTime[0] < j) {
        doQuickSort(daFinishTime, daFinishTime[0], j);
    }
    if(i < daFinishTime.length - 1) {
        doQuickSort(daFinishTime, i, daFinishTime.length - 1);
    }
}

static void printOutput() {
    DecimalFormat df = new DecimalFormat("##.00");

    System.out.println("***** SORTED RUNTIMES *****");
    for(int i = 0; i < daFinishTime.length; i++) {
        System.out.println("[" + (i + 1) + "] " +
            df.format(daFinishTime[i]) + " mins. by " +
            saRunnerName[i]);
    }

    System.out.println("\n***** TOP 3 RUNTIMES *****");
    for(int i = 0; i < 3; i++) {
        System.out.println("#" + (i + 1) + " Place: " +
            df.format(daFinishTime[i]) + " mins. by " +
            saRunnerName[i]);
    }
}

public static void main(String[] args) {
    getInput();
    doQuickSort(daFinishTime);
    printOutput();
   }
}

It is returning loss of precision errors, but when I change the data type of the said line, it returns more loss of precision errors.

Can someone fix the code? I just need to see the quicksort in action.

Answer 1

double dPivot = daFinishTime[(daFinishTime[0] + daFinishTime.length - 1) / 2];

How you are selecting pivot element is confusing. if you want mid element as pivot, it should be

double dPivot = daFinishTime[(i+j)/2];

Update this method in @Tobias solution

 static void doQuickSort(double[] daFinishTime, int i, int j) {
        double dTemp;
        String sTemp;

        int start = i;
        int end = j;
        // Pivot
        double dPivot = daFinishTime[(i + j) / 2];

        // Partition
        while (start <= end) {
            while (daFinishTime[start] < dPivot) {
                start++;
            }
            while (daFinishTime[end] > dPivot) {
                end--;
            }
            if (start <= end) {
                dTemp = daFinishTime[start];
                daFinishTime[start] = daFinishTime[end];
                daFinishTime[end] = dTemp;

                sTemp = saRunnerName[start];
                saRunnerName[start] = saRunnerName[end];
                saRunnerName[end] = sTemp;

                start++;
                end--;
            }
        }

        // Recursion
        if (start < j) {
            doQuickSort(daFinishTime, start, j);
        }
        if (i < end) {
            doQuickSort(daFinishTime, i, end);
        }
    }

Answer 2

You had answers of how to change your quick-sort so I thought I'd introduce a few other suggestions and a slightly different approach. I'm not sure what your requirements for having two separate arrays are but it seems to me like bad practice maintaining the two - using their positions as identifiers. In my opinion, you need a Runner object, which perhaps contains some of their details (if needed). For my example, I've just created a 'RunnerTime' object which contains the two fields Name(String) and Time(Double).

I'v then populated an Array or RunnerTime objects to send into my quickSort method. Here is how it works.

I've tested this out locally adding 4 runners:

RunnerTime rt1 = new RunnerTime("Bob", 10.13);
RunnerTime rt3 = new RunnerTime("Craig", 11.65);
RunnerTime rt2 = new RunnerTime("Dave", 11.45);
RunnerTime rt4 = new RunnerTime("Marley", 5.62);

I've then added them to an array and sent them to the following method:

private static RunnerTime[] doSort(RunnerTime[] runnerTimes) {
        RunnerTime currentRunnerTime;
        RunnerTime nextRunnerTime;
        boolean swapped;

        do {
            swapped = false;
            for (int x = 0; x < runnerTimes.length - 1; x++) {
                currentRunnerTime = runnerTimes[x];
                nextRunnerTime = runnerTimes[x + 1];
                if (currentRunnerTime.time > nextRunnerTime.time) {
                    runnerTimes[x] = nextRunnerTime;
                    runnerTimes[x + 1] = currentRunnerTime;
                    swopped = true;
                }
            }
        } while (swapped);
        return runnerTimes;
    }

After doing this they sorted to:

Runner Name: Marley  Runner Time: 5.62
Runner Name: Bob     Runner Time: 10.13
Runner Name: Dave    Runner Time: 11.45
Runner Name: Craig   Runner Time: 11.65

The advantages of doing it this way are as follows:

• Your name and time are united, so there's no chance of a time being mistakenly assigned so another runner.
• Your sort method can be much shorter as you only need to swap one array.
• The 'swapped' boolean means that your array is only looped through as many times as it needs to. Once there's has been a cycle that has not made any swaps (meaning that everyone is in order) - the loop will exit.

Answer 3

If Possible Refer to Implementing quicksort algorithm . Moreover why are you complicating with ints and doubles in the program.As the comments on your code state correctly your program doesnt compile because of that.

Answer 4

I've made some changes in the doQuickSort-method. Now its compiling, and it seems to work.

// Sort parallel arrays with quick sort

import java.text.DecimalFormat;
import java.util.Scanner;

class FunRunQuiSort {
static int iSize = 5; // Set value
static double[] daFinishTime = new double[iSize];
static String[] saRunnerName = new String[iSize];

static void getInput() {
    System.out.println("\n***** 5K RUNNERS *****");
    for(int i = 0; i < saRunnerName.length; i++) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter runner name: ");
        saRunnerName[i] = input.nextLine();

        System.out.print("Enter finish time: ");
        daFinishTime[i] = input.nextDouble();

        System.out.print("\r\n");
    }
}

static void doQuickSort(double[] daFinishTime, int i, int j) {
    double dTemp;
    String sTemp;

    // Pivot
    double dPivot = daFinishTime[(i + j) / 2];

    // Partition
    while(i <= j) {
        while(daFinishTime[i] < dPivot) {
            i++;
        } 
        while(daFinishTime[j] > dPivot) {
            j--;
        }
        if(i <= j) {
            dTemp = daFinishTime[i];
            daFinishTime[i] = daFinishTime[j];
            daFinishTime[j] = dTemp;

            sTemp = saRunnerName[i];
            saRunnerName[i] = saRunnerName[j];
            saRunnerName[j] = sTemp;

            i++;
            j--;
        }
    }

    // Recursion
    if(i < i - 1) {
        doQuickSort(daFinishTime, i, i - 1);
    }
    if(i < j) {
        doQuickSort(daFinishTime, i, j);
    }
}

static void printOutput() {
    DecimalFormat df = new DecimalFormat("##.00");

    System.out.println("***** SORTED RUNTIMES *****");
    for(int i = 0; i < daFinishTime.length; i++) {
        System.out.println("[" + (i + 1) + "] " +
                df.format(daFinishTime[i]) + " mins. by " +
                saRunnerName[i]);
    }

    System.out.println("\n***** TOP 3 RUNTIMES *****");
    for(int i = 0; i < 3; i++) {
        System.out.println("#" + (i + 1) + " Place: " +
                df.format(daFinishTime[i]) + " mins. by " +
                saRunnerName[i]);
    }
}

public static void main(String[] args) {
        getInput();
        doQuickSort(daFinishTime, 0, daFinishTime.length - 1);
        printOutput();
    }
}