Swapping two elements in an array

Question

I'm doing mooc.fi, my code works but it won't submit.

errors I'm getting

the method swap does not work correctly with parameter 4, 7, 8, 6 index1=0 index2=3

the result was 4, 7, 8, 6 but it should have been 6, 7, 8, 4

and

the method sort does not work correctly with parameter 10, 20, 6, -1, 13, 11

the result was 10, 20, 6, -1, 13, 11 but it should have been -1, 6, 10, 11, 13, 20

I know the errors are connected but I'm not too sure what to do to fix this, any help is appreciated! thank you!

My Code:

import java.util.Arrays;

public class Main {

    public static int smallest(int[] array) {
        int start = array[0];
        for (int i = 0; i < array.length; i++) {
            if (array[i] < start) {
                start = array[i];
            }
        }
        return start;
    }

    public static int indexOfTheSmallest(int[] array) {
        for (int i = 0; i < array.length; i++) {
            if (array[i] == smallest(array)) {
                return i;
            }
        }
        return 0;
    }

    public static int indexOfTheSmallestStartingFrom(int[] array, int index) {
        int minIndex = index;
        for (int i = index; i < array.length; i++) {
            if (array[i] < array[minIndex]) {
                minIndex = i;
            }
        }
        return minIndex;
    }

    public static void swap(int[] array, int index1, int index2) {
        int hold = 0;
        for (int i = 0; i < array.length; i++) {
            hold = array[index1];
            array[index1] = array[index2];
            array[index2] = hold;
        }
    }

    public static void sort(int[] array) {
        System.out.println(Arrays.toString(array));
        for (int i = 0; i < array.length; i++) {
            swap(array, i, indexOfTheSmallestStartingFrom(array, i));
            System.out.println(Arrays.toString(array));
        }
    }

    public static void main(String[] args) {
        int[] values = {8, 5, 3, 7, 9, 6, 1, 2, 4};
        sort(values);
    }
}

This sounds like something you should ask on the MOOC forums or chat: mooc.fi/courses/2013/programming-part-1/… — Ilmari Karonen
– Ilmari Karonen, Commented Sep 7, 2015 at 16:41

Jeff Watkins · Accepted Answer · 2015-09-07 16:21:48Z

1

This algorithm is designed very badly. both smallest and indexOfSmallest use a loop, this will degrade geometrically.

The swap should not be scanning the entire array as you only want to swap two elements, not all of them!

You're effectively doing a whack version of a bubble-sort, but in a really convoluted way with too many loops. This will start to perform horribly with any decent sized list.

A much simpler version can be found here

answered Sep 7, 2015 at 16:21

Jeff Watkins

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2 Comments

syntax Over a year ago

I can see how this way can be inefficient but to pass the exercise I think I had to do it this way.

Ilmari Karonen Over a year ago

@syntax: Your swap() method has a useless (and potentially harmful) for loop around the code. Remove it, and at least your first error should go away. (Bonus exercise: Figure out in which cases your original code will work despite the needless loop, and in which cases it will fail.)

Cr0w3 · Accepted Answer · 2015-09-07 16:22:16Z

0

You do not need a loop for the swap method. As you only want to change the two element once and not n-times (which results in not swapping them at all if n is an even number).

answered Sep 7, 2015 at 16:22

Cr0w3

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Comments

Abhishek Singh · Accepted Answer · 2017-01-11 11:50:19Z

0

public int[] swapArrayElement(int arr[], int a, int b) {
        int index1;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == a || arr[i] == b) {
                index1 = i;
                if (arr[i] == a) {
                    for (int k = index1; k < arr.length; k++) {
                        if (arr[k] == b) {
                            arr[k] = arr[index1];
                            arr[index1] = b;
                        }
                    }
                    return arr;
                } else if (arr[i] == b) {
                    {
                        for (int k = index1; k < arr.length; k++) {
                            if (arr[k] == a) {
                                arr[k] = arr[index1];
                                arr[index1] = a;
                            }
                        }
                    }
                    return arr;
                }

            }

        }
        return arr;
    }

answered Jan 11, 2017 at 11:50

Abhishek Singh

171 bronze badge

1 Comment

Hermann Döppes Over a year ago

This does not answer the question. It is just a dump of code and does not help the OP to understand their mistakes or how to do it better. Instead of dumping code, you should always explain why and how stuff works.

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