How to reverse SelectionSort to display in descending order?

Question

I am trying to create a selectionSort method to arrange values in descending order. This code arranges them in ascending order. How can I swap this code to arrange it the other way.

public void selectionSort()
{
    int n = data.length;

    for(int index = 0; index < n-1; index++)
    {
        int min_idx = index;
        for(int j = index+1; j < n; j++)
            if(data[j] < data[min_idx])
                min_idx = j;

        int temp = data[min_idx];
        data[min_idx] = data[index];
        data[index] = temp;
    }
}

Possible duplicate of [Reverse Array Order](https://stackoverflow.com/questions/9995432/reverse-array-order) — Ori Marko, Nov 04 '18 at 08:14

score 3 · Accepted Answer · answered Nov 04 '18 at 08:18

3

As of now I can think of two methods. I haven't tried them but worth giving a try.

Multiply all the numbers by -1 and apply the original selection sort to sort for ascending order. After sorting is complete multiply all the numbers by -1 to get back originwl numbers but now they are sorted in descending order.
Try changing the comparison condition if(data[j] < data[min_idx])
to if(data[j] >= data[min_idx])

Let me know if there is some issue with these methods.

answered Nov 04 '18 at 08:18

Akhilesh Pandey

868
8
18

The second method worked out great! Thanks for the assistance! – Christian De La Rosa Nov 04 '18 at 08:21
You are welcome @Christian.I am glad it helped you. Could you please mark this answer as accepted? – Akhilesh Pandey Nov 04 '18 at 08:29

score 1 · Answer 2 · answered Nov 04 '18 at 08:19

Instead of selecting the minimal item in each iteration, you should instead select the maximum one. All you really need to do is change the < condition to >, but I've also changed the names accordingly to make the code easier to understand:

public void selectionSort()
{
    int n = data.length;

    for(int index = 0; index < n-1; index++)
    {
        int max_idx = index; // max_idx instead of min_idx
        for(int j = index+1; j < n; j++)
            if(data[j] > data[min_idx]) // > instead of <
                max_idx = j;

        int temp = data[max_idx];
        data[max_idx] = data[index];
        data[index] = temp;
    }
}

score 1 · Answer 3 · answered Nov 04 '18 at 08:33

To solve this problem, I would reccomend you to refactor your code first. Move swap and findMin into separate method:

private static int getMin(int[] arr, int from, int to) {
    int min = from;

    for (int j = from; j < to; j++)
        min = arr[j] < arr[min] ? j : min;

    return min;
}

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

Here, you probably can see, that implementation of ASC and DESC sorting are trivial:

public static void selectionSortAsc(int[] arr) {
    for (int i = 0; i < arr.length - 1; i++)
        swap(arr, getMin(arr, i + 1, arr.length), i);
}

public static void selectionSortDesc(int[] arr) {
    for (int i = arr.length - 1; i > 0; i--)
        swap(arr, getMin(arr, 0, i + 1), i);
}

Giorgio F. · Answer 4 · 2020-01-01T01:42:04.520

public static class SelectionSort
{
    static int min;
    public static void Sort(int[] data)
    {
        for (int i = 0; i < data.Length; i++)
        {
            for (int j = 0; j < data.Length; j++)
            {
                min = j;
                if (data[i] < data[j])
                    Swap(x: ref data[i], y: ref data[min]);
            }
        }
    }

    private static void Swap(ref int x, ref int y)
    {
        int temp = x;
        x = y;
        y = temp;
    }
}

For Ascending this line   
if (data[i] < data[j])

For Descending this line  
if (data[i] > data[j])

How to reverse SelectionSort to display in descending order?

4 Answers4