Question

Quick Sort Beginner Java

Asked by: elliottbenzle

Hi all,
 I'm working on a Java program that will:

1) Take user input for a file name with numers
2) Read numbers into an array
3) Apply the quickSort/partition methods to the array
4) And print to the screen the sorted array

Much thanks to anyone that can help me get this program working right, Thanks

-Kyle
This is my current output:

Name of file with array: text.txt
Exception in thread "main" java.lang.StackOverflowError
      at ss.quickSort(ss.java:62)
      at ss.quickSort(ss.java:66)

import java.io.*;
import java.io.File;
import java.util.Scanner; 
public class ss {
	public static void main(String[] args) throws IOException {
		System.out.print("Name of file with array: ");
		Scanner readIn = new Scanner(System.in);
		String input = readIn.nextLine();
		testScan1(input);
		
	} 
	public static void testScan1(String filename) {
		File file = new File(filename);
		Scanner scan;
		int[] array;
		try {
			array = new int[5];
			scan = new Scanner(file);
		} catch (java.io.FileNotFoundException e) {
			System.out.println("couldn't open. file not found ");
			return;
		}
		int i =0;
		while (scan.hasNext()) {
			//for (i = 0; i <= file.length(); ++i) 
				array[i] = Integer.parseInt(scan.next());
				i++;}
				quickSort(array, 0, array.length-1);
			partition(array, 0, array.length-1);
			
				//System.out.println(java.util.Arrays.toString(array));
				for (int j = 0; j < array.length; ++j)
				System.out.println(array[i] + " ");
			}
		
	 
	static int partition(int[] arr, int left, int right) {
		int i = left;
		int j = right;
		int tmp;
		int pivot = arr.length-1;
		while (i <= j) {
			while (arr[i] < pivot)
				i++;
			while (arr[j] > pivot)
				j--;
			if (i <= j) {
				tmp = arr[i];
				arr[i] = arr[j];
				arr[j] = tmp;
				i++;
				j--;
			}
		}
		return i;
	} 
	static void quickSort(int[] arr, int left, int right) {
		int index = partition(arr, left, right);
		if (left < (index - 1)); {
			
		}
		quickSort(arr, left, index - 1);
		if (index < right) {
			quickSort(arr, index, right); 
		}
	}		
	//for (int j = 0; j < array.length; ++j)
	//	System.out.println(array[i] + " ");
}

                                  
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Asked On
2009-11-05 at 17:28:49ID24876759
Tags

quick sort

,

java

Topics

Java Programming Language

,

New to Java Programming

Participating Experts
3
Points
350
Comments
4

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Answers

 

by: jb1devPosted on 2009-11-05 at 17:51:38ID: 25756055


Your quicksort function looks wrong. It's just calling itself endlessly.

    static void quickSort(int[] arr, int left, int right) {
        int index = partition(arr, left, right);
        if (left < (index - 1)); {

        }
        quickSort(arr, left, index - 1); // <-- here
        if (index < right) {
            quickSort(arr, index, right);
        }
    }

 

by: elliottbenzlePosted on 2009-11-05 at 18:05:14ID: 25756096

Ok, I made the changes to quickSort, but there is no output now?

/*
 * Kyle Arthur Benzle
 * Lab 3
 * Oct. 30 9
 * Tagore
 */ 
import java.io.*;
import java.util.Scanner;
//Lab3 Class
public class Lab3 {
	public static void main(String[] args) throws IOException {
		//getting the user input from scanner, calling it "input"
		System.out.print("Name of file with array: ");
		Scanner readIn = new Scanner(System.in);
		String input = readIn.nextLine();
		//testScan1(input);
		
	}
	private String filename;
	//Calling this method testScan1 to read in the file info, taking in the filename as a parameter
	//public static void testScan1(String filename) {
		//Calling the file, file
		File file = new File(filename);
		Scanner scan;
		//making an int array called array of course.
		int[] array;{
		//using try/catch block to read from file, (Incase file is not found)
		try {
			//Lab assignment never said anything about what our input would be, I assume there will be 6 intergers like in the example, I think this is a fair assumption as it was not defined that we had to have the array length variable.
			array = new int[5];
			//scan to get file info
			scan = new Scanner(file);
			//if, the file name was not found, give this error
		} catch (java.io.FileNotFoundException e) {
			System.out.println("couldn't open. file not found ");
			
		}
		int i =0;
		//Now, finally, reading the file, while loop for as long as i is < file.length. 
		while (scan.hasNext()) {
			//the below comment is for printing the array, Just a test.
			//for (i = 0; i <= file.length(); ++i) 
			//setting each of the array element = to the next number in the file
				array[i] = Integer.parseInt(scan.next());
				//i++;
				i++;}
		//Calling the quicksort method with array, 0 for left and length-1 for right
				quickSort(array, 0, array.length-1);
				//calling partition method, with the same as the above
			partition(array, 0, array.length-1);
			//Just another test, trying to print array
				//System.out.println(java.util.Arrays.toString(array));
			//printing array
				//for (int j = 0; j < array.length; ++j)
				//System.out.println(array[i] + " ");
			}
		
	
//partition method
	static int partition(int[] arr, int left, int right) {
		int i = left;
		int j = right;
		int tmp;
		//making pivot = to the last element of array
		int pivot = arr.length-1;
		//3 while loops to move the left and right toward each other, until the can be swapped below.
		while (i <= j) {
			while (arr[i] < pivot)
				i++;
			while (arr[j] > pivot)
				j--;
			if (i <= j) {
				tmp = arr[i];
				arr[i] = arr[j];
				arr[j] = tmp;
				i++;
				j--;
			}
		}
		return i;
	}
//QuickSort Method, taking in arr, left and right
	static void quickSort(int arr[], int left, int right) {
		//Setting index = to the partition with parameters of array and left and right, when that method runs
		if (left>=1) return;
		int index = partition(arr, left, right); //ERROR this line is giving an exception when the program runs, a stack overflow. 
		//if (left < (index - 1)) ; 
		
		//Call quickSort recursively with the new right
		quickSort(arr, left, index); //ERROR, this line is giving the overflow error I think you were talking about in class, that a lot of people were having trouble with. 
		//if (index < right) {
			//or call quickSort recursively with the old #'s
		quickSort(arr, index+1, 1);
	}}
	
			//Now, trying to print out the array. 
	//for (int j = 0; j < arr.length; ++j)
	//System.out.println(arr[j] + " ");
	//	}
	
                                              
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by: objectsPosted on 2009-11-05 at 18:07:30ID: 25756101

try this:

    public static void quicksort(int[] a, int left, int right) {
        if (right <= left) return;
        int i = partition(a, left, right);
        quicksort(a, left, i-1);
        quicksort(a, i+1, right);
    }

    private static int partition(double[] a, int left, int right) {
        int i = left - 1;
        int j = right;
        while (true) {
            // find item on left to swap
            while (a[++i]< a[right]);
            // find item on right to swap
            while (a[right]< a[--j])      
                if (j == left) break;  
            if (i >= j) break;
            swap(a, i, j);  
        }
        swap(a, i, right);
        return i;
    }

    private static void swap(int[] a, int i, int j) {
        int swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }

 

by: dincaflorinPosted on 2009-11-11 at 03:39:20ID: 25794177

I think a better solution is to use TreeSet
TreeSet is an array object in Java that stores all the elements ordered. For example, when you use
TreeSet ts = new TreeSet();
ts = {1,3,6};
ts.add(5);
after that, ts would be {1,3,5,6};
it just adds the element in it's position;

import java.util.Collections;
import java.util.Set;
import java.util.TreeSet;

public class MainClass {
  public static void main(String args[]) throws Exception {
    String elements[] = { 3, 1, 6, 2, 4 };
    Set set = new TreeSet();
    for (int i = 0, n = elements.length; i < n; i++) {
      set.add(elements[i]);
    }
    System.out.println(set);
  }
}

the output will be 1, 2, 3, 4, 6

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