Trying to reverse array elements

Question

I am trying to sort the array elements in a reverse order, but it doesn't seem to give me correct output. i have tried many times to correct it but not able to do so. I am a complete beginner. any help?

CODE

import java.util.Arrays;
import java.util.Scanner;

public class main {

    public static void main(String[] args) {
        int[] arr = new int[5];
        Scanner input = new Scanner(System.in);
        System.out.println("Enter the array Elements");
        for (int i = 0; i < 5; i++) {
            arr[i] = input.nextInt();
        }
        swap(arr);
        input.close();
    }

    static void swap(int arr[]) {
        for (int start = 0; start < 5; start++) {
            for (int end = arr.length - 1; end >= 0; end--) {
                if (start >= end) {
                    System.out.println(Arrays.toString(arr));
                    System.exit(0);
                } else {
                    int temp = arr[start];
                    arr[start] = arr[end];
                    arr[end] = temp;
                }
            }

        }

    }
}

Hint: you don't need two loops, just one that cycles over the first half of the array. The rest is just some arithmetic for the array indexes. — Federico klez Culloca
– Federico klez Culloca, Commented Sep 22, 2021 at 9:27
@FedericoklezCulloca I Understnad, but why isn't my code working? — Abhinav K
– Abhinav K, Commented Sep 22, 2021 at 9:41
Note: you should essentially never invoke System.exit(), especially in a method that is just meant to reverse an array. If you want the method to stop, use return (or throw an exception, if you want to stop because of a problem). — Andy Turner
– Andy Turner, Commented Sep 22, 2021 at 9:44
@AbhinavK "but why isn't my code working" step through your code with a debugger; or, at the very least, print out arr regularly, so you can see how it's being changed. — Andy Turner
– Andy Turner, Commented Sep 22, 2021 at 9:45

La-comadreja · Accepted Answer · 2021-09-22 09:37:13Z

5

If you want to reverse the elements in the array, you need to write method swap() like this:

static void swap(int[] arr) {
    for (int start = 0; start < arr.length / 2; start++) {
        int tmp = arr[arr.length - 1 - start];
        arr[arr.length - 1 - start] = arr[start];
        arr[start] = tmp;
    }
}

answered Sep 22, 2021 at 9:37

La-comadreja

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Comments

sulox32 · Accepted Answer · 2021-09-22 10:03:58Z

0

You can use the ‘reverse’ method present in the collections framework. But for this, you first need to convert an array to a list as the ‘reverse’ method takes the list as an argument.

static void swap(Integer arr[]) { 
     Collections.reverse(Arrays.asList(arr)); 
     System.out.println(Arrays.toString(myArray));
  }

edited Sep 22, 2021 at 10:03

answered Sep 22, 2021 at 9:46

sulox32

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5 Comments

Andy Turner Over a year ago

Did you actually try this? If you didn't, give it a go.

sulox32 Over a year ago

yes i try this and it works!

Andy Turner Over a year ago

It certainly compiles and runs; it just doesn't reverse arr. Check the link in my previous comment. Arrays.asList(arr) is List<int[]> with 1 element, not a List<Integer> with n elements.

sulox32 Over a year ago

Yes i changed the parameter. look here

Andy Turner Over a year ago

Yes, that allows you to reverse an Integer[]; but OP has an int[], and so can't invoke this method.

James Mudd · Accepted Answer · 2021-09-22 11:01:23Z

0

You can use ArrayUtils#reverse https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/ArrayUtils.html#reverse-int:A- To avoid the creation of List and to reverse the array in-place.

answered Sep 22, 2021 at 11:01

James Mudd

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Comments

m4n0 · Accepted Answer · 2021-09-22 11:10:11Z

0

Personally, I would use the Collections method, transforming your array into a List, sorting it with the reverse() method and then transforming it back to an array, like so:

public void invert(int[] array) {
    List<int> list = Arrays.asList(array);
    Collections.reverse(list);
    array = list.toArray(new int[0]);
}

edited Sep 22, 2021 at 11:10

m4n0

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answered Sep 22, 2021 at 9:50

Antonio De Michele

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Comments

divyang4481 · Accepted Answer · 2021-09-22 09:44:09Z

import java.util.Arrays; 
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
         int[] arr = new int[5];
        Scanner input = new Scanner(System.in);
        System.out.println("Enter the array Elements");
        for (int i = 0; i < 5; i++) {
            arr[i] = input.nextInt();
        }
        swap(arr,arr.length  );
        input.close();
    }
    
     static void swap(int a[], int n) {
         int i, k, t;
        for (i = 0; i < n / 2; i++) {
            t = a[i];
            a[i] = a[n - i - 1];
            a[n - i - 1] = t;
        }
 
        /*printing the reversed array*/
        System.out.println("Reversed array is: \n");
        for (k = 0; k < n; k++) {
            System.out.println(a[k]);
        }

    }

}

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