Why this reverse array method isnt working?

Question

I am trying to make a method to reverse an array, I don't know why it is not working?

When I said int[] arr = array, I want to do this so it will not be affected so I can use the elements for the second for loop and it should have elements {1,2,3,4,5,6} but when I use

for (int i=array.Length-1;i>array.Length/2;i--)
{
    array[i] = arr[array.Length - 1 - i];
}

In this case I have 6 elements so array.Length is 6 and since I started from array.Length-1 it must start from the last element and it must be array[5]=arr[6-1-5] which must be array[5]=arr[0] and arr[0] is 1 but I think it is getting it as 6, why?

Here is the complete code:

// ReverseArray method
static int [] ReverseArray(int [] array)
{
    int[] arr = array;
    for (int i=0; i<array.Length/2;i++)
    {
        array[i] = array[array.Length-1  - i];
    }
    for (int i=array.Length-1;i>array.Length/2;i--)
    {
        array[i] = arr[array.Length - 1 - i];
    }
    return array;
}

// Method for displaying elements of Array
static void DisplayArray(int [] array)
{
    int i;
    Console.Write("{");
    for (i = 0; i < array.Length-1; i++)
    {
        Console.Write(array[i] + ",");
    }
    Console.WriteLine(array[i] + "}");
}

static void Main(string[] args)
{
    int[] array = { 1, 2, 3, 4, 5 ,6};
    ReverseArray(array);
    DisplayArray(array);
    Console.ReadKey();
}

Might be an idea to tag the language? Looks like C# to me, but I'm not betting my job on it. — Bathsheba
– Bathsheba, Commented Jan 23, 2018 at 13:40
Try to run your code in a debugger and see what is happening on each line. — user202729
– user202729, Commented Jan 23, 2018 at 13:40
Hint: by the time you've made the first half of the array look like the second half in reverse order, how do you expect to get at the data that was in the first half? — Jon Skeet
– Jon Skeet, Commented Jan 23, 2018 at 13:42
I have to ask, are you aware there is a BCL function Array.Reverse()? — NightOwl888
– NightOwl888, Commented Jan 23, 2018 at 13:55
@NightOwl888 ah I know it exist but I just wanted to make my own — Endrit Shabani
– Endrit Shabani, Commented Jan 23, 2018 at 14:16

Sergey Kalinichenko · Accepted Answer · 2018-01-23 13:44:41Z

7

Your reversal approach is invalid: rather than swapping elements at indexes i and array.Length-1 - i, your code copies the tail portion of the array onto the initial one, effectively "folding" the array onto itself.

You need to remove the second loop, and replace the assignment with a swap: make a temporary variable, store array[i], copy array[array.Length-1 - i], then write the temporary in its place.

answered Jan 23, 2018 at 13:44

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ray Krungkaew · Accepted Answer · 2018-01-23 13:59:58Z

6

I couldn't help myself but start to write a piece of code trying to solve it.

    int[] arr = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    arr = Reverse(arr).ToArray();

Here is the shorter way to achieve that.

    public IEnumerable<int> Reverse(int[] arr)
    {
        for (int i = arr.Length-1; i >= 0; i--)
        {
            yield return arr[i];
        }
    }

answered Jan 23, 2018 at 13:59

Ray Krungkaew

7,0071 gold badge19 silver badges29 bronze badges

Collectives™ on Stack Overflow

Why this reverse array method isnt working?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related