Why is my bubble sort giving an incorrect output when sorting data in ascending order?

Question

I have created an implementation of the Bubble sort algorithm in Java. The code runs fine and gives a valid output, however, for some reason, when I sort the data in ascending order, it is doing it properly, but I have a problem when I try to print out the statement. Below is my code, as well a slightly better description of the problem!

import java.util.Arrays;
import java.util.Scanner;
//import java.util.regex.Pattern;
//import java.util.stream.Stream;
public class BubbleSortNumeric {
    public static void main (String [] args) {
        Integer [] unsortedData = getDataInput();
        Integer [] sortedDataAscending;
        Integer [] sortedDataDescending;
        long start = System.nanoTime();
        sortedDataAscending = bubbleSortAscending(unsortedData);
        sortedDataDescending = bubbleSortDescending(unsortedData);
        long stop = System.nanoTime();
        System.out.println("Ascending: " + Arrays.toString(sortedDataAscending));
        System.out.println("Descening: " + Arrays.toString(sortedDataDescending));
        System.out.println("Execution time: " + ((stop - start) / 1e+6) + "ms.");
    }
    
    private static Integer [] getDataInput() {
        System.out.println("Enter a set of integers seperated by a space.");
        Integer [] userInput = {};
        String strInput;
        try(Scanner sc = new Scanner(System.in)) {
            strInput = sc.nextLine();
        }
        String [] inputData = strInput.split("\\s+");
        try {
            userInput = Arrays.asList(inputData).stream().map(Integer::valueOf).toArray(Integer[]::new); 
        }catch(NumberFormatException e) {
            System.out.println("ERROR. Invalid input.\n" + e.getMessage());
        }
       return userInput;
    }
    
    private static Integer [] bubbleSortAscending(Integer[] ascendingUnsorted) {
        int n = ascendingUnsorted.length;
        System.out.println(n);
        if(n == 1) {
            return ascendingUnsorted;
        }
        boolean swapped;
        int temp;
        do {
            swapped = false;
            for(int i = 1; i < n; i++) {
                if(ascendingUnsorted[i - 1] > ascendingUnsorted[i]) {
                    temp = ascendingUnsorted[i - 1];
                    ascendingUnsorted[i - 1] = ascendingUnsorted[i];
                    ascendingUnsorted[i] = temp;
                    swapped = true; 
                }
            }
            n--;
        }while(swapped == true);
        return ascendingUnsorted;
    }
    
    private static Integer [] bubbleSortDescending(Integer [] descendingUnsorted) {
        int n = descendingUnsorted.length;
        if(n == 1) {
            return descendingUnsorted;
        }
        boolean swapped;
        int temp;
        do {
            swapped = false;
            for(int i = 1; i < n; i++) {
                if(descendingUnsorted[i - 1] < descendingUnsorted[i]) {
                    temp = descendingUnsorted[i];
                    descendingUnsorted[i] = descendingUnsorted[i - 1];
                    descendingUnsorted[i - 1] = temp;
                    swapped = true;
                }
            }
            n--;
        }while(swapped == true);
        return descendingUnsorted;
    }
}

when I call bubbleSortAscending it works fine, and sorts the data in ascending order. Because I am timing the execution time of the program, I do not want to print out the results before also sorting the data in descending order.

My problem is, although both the methods work fine, I have a problem when printing out the results. An example of what happens is below:

Input

1 3 9 2 40 193

Output:

Ascending: [193, 40, 9, 3, 2, 1]

Descening: [193, 40, 9, 3, 2, 1]

Execution time: 0.527142ms.

If I were to move the print statement to just after the line sortedDataAscending = bubbleSortAscending(unsortedData); then it would give the correct output, however, as I've already stated I do not want that.

So my question, even though I am asigning the results to two distinct variables, why, when I print the answers of both the variables, is the output the same?

Answer 1

You must make copy of input array because in Java You passing values as reference not as copy:

public class BubbleSortNumeric {
    public static void main (String [] args) {
        Integer [] unsortedData1 = getDataInput();
        Integer [] unsortedData2 = new Integer[unsortedData1.length];
        System.arraycopy(unsortedData1, 0, unsortedData2, 0, unsortedData1.length);
        Integer [] sortedDataAscending;
        Integer [] sortedDataDescending;
        long start = System.nanoTime();
        sortedDataAscending = bubbleSortAscending(unsortedData1);
        sortedDataDescending = bubbleSortDescending(unsortedData2);
        // ...

Tested and Your algorythm works fine, only array mistake:

$ javac BubbleSortNumeric.java  && java BubbleSortNumeric 
Enter a set of integers seperated by a space.
1 3 9 2 40 193

6
Ascending: [1, 2, 3, 9, 40, 193]
Descening: [193, 40, 9, 3, 2, 1]
Execution time: 0.068779ms.

Answer 2

I'll give you a hint: swap these two rows, execute, and you will see the magic. From this

    sortedDataAscending = bubbleSortAscending(unsortedData);
    sortedDataDescending = bubbleSortDescending(unsortedData);

to this

    sortedDataDescending = bubbleSortDescending(unsortedData);
    sortedDataAscending = bubbleSortAscending(unsortedData);

What does this suggest you?

.

.

.

.

That you're modifying twice the same array! All of your references

unsortedData
sortedDataAscending
sortedDataDescending

are pointing to the same object: an array, that is unsorted at first, then sorted in ascending order and finally in descending order.

At this point you decide to print it out.

Twice.