Array Sorting Descending Order

Question

I'm a new programmer to java, been using this website a lot to learn the ways. Today I've stumbled into another problem towards a program I'm doing for practice.

I have an array:

    final int EXAMS = 5;
    int[] scores = new int [EXAMS];

The values for this array are then asked from the user through a Scanner object:

for (int index = 0; index < EXAMS; index++)
    {
        System.out.println("Enter the score for " + (index+1) +":");
        scores[index] = kb.nextInt();
        if (scores[index] < 0){
            System.out.println("The number you have entered is invalid.");
            scores[index] = kb.nextInt();
        }
    }

I managed to make an ascending order sorting of the values "scores[]":

       Arrays.sort(scores);
        System.out.println("The sorted int array is:");
        for (int number : scores) 
        {
            System.out.println("Number = "+ number);
        }

But I want the sorting to be in descending order. When I put the

Arrays.sort(scores, Collections.reverseOrder());

I get an error saying: "no suitable method found for sorting." Please help.

Sort the Array at the time of insertion..

gifpif
– gifpif

2015-08-10 16:41:48 +00:00
Commented Aug 10, 2015 at 16:41 — gifpif
– gifpif, Commented Aug 10, 2015 at 16:41

Danny Varela · Accepted Answer · 2015-08-09 06:41:35Z

3

Collections.reverseOrder()

Won't work because the array is of primitive type. You can change from int to Integer or try something different like:

Arrays.sort(scores);
ArrayUtils.reverse(scores);

Your final option would be to make your own code.

answered Aug 9, 2015 at 6:41

Danny Varela

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Comments

Nir Alfasi · Accepted Answer · 2015-08-09 06:42:05Z

3

The following works (using the Integer Objects instead of primitive int):

Integer[] test = {1, 2, 4, 3, 5};
Arrays.sort(test, Collections.reverseOrder());
System.out.println(Arrays.toString(test)); // [5, 4, 3, 2, 1]

edited Aug 9, 2015 at 6:42

answered Aug 9, 2015 at 6:36

Nir Alfasi

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Comments

Eran · Accepted Answer · 2015-08-09 06:47:02Z

There's no sort method in Arrays class that accepts an int[] and a Comparator. The sort methods in the Arrays class that accept a Comparator require an array of reference type (while an int[] is an array of a primitive type).

If you change the type of your scores array from int[] to Integer[], your code will work, since the method public static <T> void sort(T[] a, Comparator<? super T> c) of the Arrays class will match your Arrays.sort(scores, Collections.reverseOrder()); call.

final int EXAMS = 5;
Integer[] scores = new Integer [EXAMS]; // the only required change
for (int index = 0; index < EXAMS; index++) {
    System.out.println("Enter the score for " + (index+1) +":");
    scores[index] = kb.nextInt();
    if (scores[index] < 0){
        System.out.println("The number you have entered is invalid.");
        scores[index] = kb.nextInt();
    }
}
Arrays.sort(scores, Collections.reverseOrder());
System.out.println("The sorted int array is:");
for (int number : scores)  {
    System.out.println("Number = "+ number);
}

Nitesh Virani · Accepted Answer · 2015-08-09 06:51:38Z

1

You just need to change your array type from Primitive type to Primitive wrapper class i.e. int[] scores = new int [EXAMS] to Integer[] scores = new Integer[EXAMS]

Arrays.sort accept wrapper array in the first argument not primitive.

answered Aug 9, 2015 at 6:51

Nitesh Virani

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Comments

Parag Kadam · Accepted Answer · 2015-08-09 06:39:34Z

0

Arrays.sort(a, Collections.reverseOrder());

does not work with primitives. Try this,

Integer[] a = new Integer[]{1,2,3,5,4};
            Arrays.sort(a,Collections.reverseOrder());
            for(int i:a)
            {
                System.out.println(i);
            }

answered Aug 9, 2015 at 6:39

Parag Kadam

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Comments

pramod kumar · Accepted Answer · 2015-08-09 06:40:01Z

0

There is no sort method for arrays in java.But ArrayList have a sort method.You can the below code to sort the list in reverse order.

List l=new ArrayList();
l.add(5);
l.add(1);
l.add(2);
l.add(3);
Collections.sort(l,Collections.reverseOrder());
System.out.println(l);

answered Aug 9, 2015 at 6:40

pramod kumar

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Comments

Sayyid Iskandar Khan · Accepted Answer · 2023-04-07 10:00:57Z

0

i would initialise the values first.

// we do 1 to 6 because, 6th iteration is where we going to stop
// so that it will count up to 5.
int[] arr = IntStream.range(1, 6).toArray();

then the value of it u pass in the array below.

if we need to sort with primitives int, this would be the approach i would take.

sort the list in ascending order
reverse all the values

NOTE: this approach would be an N log N time complexity. of course it will be able to sort faster at N time complexity but this code is most readable and easy to understand for me.

// this func sorts in n log n time complexity
    public void sortReverse(int[] arr) {
        // 1. now sort all values in descending order
        for (int i = 0, j = arr.length - 1; i < arr.length / 2;i++) {
            int tmp = arr[i];
            arr[i] = arr[j];
            arr[j] = tmp;
            j--;
        }
    }

edited Apr 7, 2023 at 10:00

answered Apr 7, 2023 at 9:43

Sayyid Iskandar Khan

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1 Comment

Sayyid Iskandar Khan Over a year ago

i have updated the function to be in camelcase according to java naming convention ;)

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