Question

Aim

To improve the algorithm provided in Snippet 2.3 by reducing the number of passes.

public void sortImprovement1(int[] numbers) {
    for (int i = 1; i < numbers.length; i++) {
        for (int j = 0; j < numbers.length - i; j++) {
            if (numbers[j] > numbers[j + 1]) {
                swap(numbers, j, j + 1);
            }
        }
    }
}

^{Snippet 2.3: Bubble sort improvement 1}

Change the bubble sort method so that it stops sorting if the array is untouched after an inner loop pass.

Consider changing the outer for loop into a while loop.

Steps for Completion

The solution can be developed easily if the outer for loop is changed into a while loop and a flag is kept to indicate if any elements have been swapped while going through the array. This is shown in the following code block:

public  void  sortImprovement2(int[]  numbers)  {
    int  i  =  0;
    boolean  swapOccured  =  true;
    while  (swapOccured)  {
        swapOccured  =  false;
        i++;
        for  (int  j  =  0;  j  < numbers.length -  i;  j++)  {
            if  (numbers[j]  >  numbers[j  +  1])  {
                swap(numbers,  j,  j  +  1);
                swapOccured  =  true;
            }
        }
    }
}

^{Snippet 2.4: Bubble sort improvement 2}

Implement the swap method to swap array elements:

private void swap(int[] numbers, int j, int k) {
    int temp = numbers[j];
    numbers[j] = numbers[k];
    numbers[k] = temp;
}

Code provided:

import java.util.Arrays;

public class BubbleSort {

public void sort(int[] numbers) {
for (int i = 1; i < numbers.length; i++) {
for (int j = 0; j < numbers.length - 1; j++) {
if (numbers[j] > numbers[j + 1]) {
int temp = numbers[j];
numbers[j] = numbers[j + 1];
numbers[j + 1] = temp;
}
}
}
}

// Write your code here

public static void main(String[] args) {
BubbleSort bubbleSort = new BubbleSort();
int[] numbers = new int[]{2, 5, 7, 2, 4, 2, 8, 1, 0};
int[] numbers2 = new int[]{2, 5, 7, 2, 4, 2, 8, 1, 0};
bubbleSort.sort(numbers);
bubbleSort.sortImprovement2(numbers2);
System.out.println(Arrays.toString(numbers));
System.out.println(Arrays.toString(numbers2));
}
}

Answer 1

Program Code to Copy:

import java.util.Arrays;

public class BubbleSort {

    public void sort(int[] numbers) {
        for (int i = 1; i < numbers.length; i++) {
            for (int j = 0; j < numbers.length - 1; j++) {
                if (numbers[j] > numbers[j + 1]) {
                    int temp = numbers[j];
                    numbers[j] = numbers[j + 1];
                    numbers[j + 1] = temp;
                }
            }
        }
    }

    //SortImprovement2 method placed here
    public void sortImprovement2(int[] numbers) {
        int i = 0;
        boolean swapOccured = true;
        //If swap becomes false while loop will terminate
        while (swapOccured) {
            swapOccured = false;
            i++;
            for (int j = 0; j < numbers.length - i; j++) {
                if (numbers[j] > numbers[j + 1]) {
                    swap(numbers, j, j + 1);
                    //Making swapOccurred true if a swap has occurred
                    swapOccured = true;
                }
            }
        }
    }

    //Swap method placed here
    private void swap(int[] numbers, int j, int k) {
        int temp = numbers[j];
        numbers[j] = numbers[k];
        numbers[k] = temp;
    }


    public static void main(String[] args) {
        BubbleSort bubbleSort = new BubbleSort();
        int[] numbers = new int[]{2, 5, 7, 2, 4, 2, 8, 1, 0};
        int[] numbers2 = new int[]{2, 5, 7, 2, 4, 2, 8, 1, 0};
        bubbleSort.sort(numbers);
        bubbleSort.sortImprovement2(numbers2);
        //Printing sorted numbers
        System.out.println(Arrays.toString(numbers));
        //Printing sorted numbers using Improvement2 method
        System.out.println(Arrays.toString(numbers2));
    }
}

Output: