快速排序Java

Question

Hi I'm currently working on getting my Quick Sort program working but cannot figure out where I'm going wrong, i have spent hours trying to find out why it's not working but no luck,when I run my code nothing happens. I have four other sorting algorithms working in a similar fashion which is what's confusing me the most. Below is my code for the Quick Sort program

import java.io.IOException;

public class QuickSort  {



public static int[] compute(int[] array, int lower, int higher )throws IOException{

    if(lower<higher){
        int pivot=split(array,lower,higher);
        if(pivot>1)
            compute(array, lower, pivot-1);

        if(pivot+1<higher)
            compute(array, pivot+1, higher);


    }

    return array;

}


public static int split(int[] array, int lower, int higher){

    while(true){
        int pivot=array[lower];

        while(array[lower]<pivot)
            lower++;

        while(array[higher]>pivot)
            higher--;


        if(lower<higher){

            int temp=array[higher];
            array[higher]=array[lower];
            array[lower]=temp;
        }
        else{
        return higher;
    }
    }
}

}

Here is my Test class that's running the code:

import java.io.IOException;
import java.util.Scanner;


public class Test extends ReadIn{ 

static BubbleSort bubble=new BubbleSort();


  public static void main(String[] args) throws IOException{
  System.out.println("Enter 1 for BubbleSort\nEnter 2 for Insertion Sort\nEnter 3      for Selection Sort\nEnter 4 for Merge Sort\nEnter 5 for QuickSort\nPlease input sorting algorithm to use for sorting:");
  Scanner input=new Scanner(System.in);
  int number=input.nextInt();
  final long startTime = System.currentTimeMillis();
  int[] Array=read();

  if(number==1){
      Array=BubbleSort.compute(Array);
        for(int i=0;i<BubbleSort.compute(Array).length;i++){
      System.out.println(Array[i]);

    }

  }
  if(number==2){
      Array=InsertionSort.compute(Array);
        for(int i=0;i<InsertionSort.compute(Array).length;i++){
      System.out.println(Array[i]);

    }

  }
  if(number==3){
      Array=SelectionSort.compute(Array);
        for(int i=0;i<SelectionSort.compute(Array).length;i++){
      System.out.println(Array[i]);

    }

  }
  if(number==4){
      Array=MergeSort.compute(Array);
        for(int i=0;i<MergeSort.compute(Array).length;i++){
      System.out.println(Array[i]);

    }

  }
  if(number==5){
  Array=QuickSort.compute(Array,0,Array.length-1);
        for(int i=0;i<QuickSort.compute(Array,0,Array.length-1).length;i++){
      System.out.print(Array[i]);

    }

  }


    final long endTime = System.currentTimeMillis();
    System.out.println("Total execution time: " + (endTime - startTime) );
}





}

When I press 5 nothing happens, the rest work perfectly. I cant figure out whatsoever what the problem is so any help or input would be appreciated.

Answer 1

Your Quicksort code loops forever if there's duplicate numbers in the input, since the numbers can just keep swapping with each other. As mentioned in the comment, you should manually try out your code with a sample array, or check the code running with a debugger . See the example array below.

You might want to switch the while(true) loop to a recursive call to make the code a little clearer. Also, you can practice going through the different steps of Quicksort on my Quicksort online tutorial .

Say the array = {3,1,2,3} and the pivot is 3. Nothing will change and the code will loop forever.

while(true){
    int pivot=array[lower];

    while(array[lower]<pivot)
        lower++;

    while(array[higher]>pivot)
        higher--;


    if(lower<higher){   //this will just swap the 3's with each other. 
        int temp=array[higher];
        array[higher]=array[lower];  
        array[lower]=temp;
    }
    else{
      return higher;
    }
}

Answer 2

While it looks like your program is doing nothing, it probably just loops infinitely inside this loop:

while(true){
    int pivot=array[lower];

    while(array[lower]<pivot)
        lower++;

    while(array[higher]>pivot)
        higher--;


    if(lower<higher){

        int temp=array[higher];
        array[higher]=array[lower];
        array[lower]=temp;

        // I recommend adding this line for you to see what's going on
        System.out.println("lower="+lower+", higher="+higher+", pivot="+pivot);
    }
    else{
        return higher;
    }
}

for example, if you have equal values in your Array, say value 65 is there twice, that loop will run forever...

int[] Array={1,41,2,90,32,65,12,43,78,65,46,67};

Also, in your loop above I added and extra printout of higher, lower, and pivot values - it will help you see what is happening inside that devious loop.

In general writing a while(true) loop is not a good practice, because it's so easy to hang your system on it.