I need help troubleshooting my code for MergeSort and Merge

Ben Tseng :

So I've been working on MergeSort for an Algorithm project, but I've ran into various problems when it comes to getting the code to sort the arrays. Whenever I generate a string and put it into MergeSort, it seems to just come out exactly the same. I want some help in finding where the mistake in my code is, why is it giving me this, and a solution with a simple, but good explanation.

Here's what I've tried in the past:

1. I've tried to use return arr[0] instead of arr, but it throws me an error with it being unable to convert from int to int[].
2. I've looked in my merge class and everything seems to be ok there.
3. I've discussed the issue with my teacher and he says that everything looks fine, but I know that there must be something wrong somewhere.
4. I've tried to remove return merge(arr1, arr2) but the system throws an error telling me I have to return something.
5. I've tried to print out the arrays individually, but it still shows no changes and is the exact same as the original string.

Merge method:

private static int[] merge(int[] a, int[] b)
{
    int[] c = new int[a.length + b.length];
    int counterA = 0;
    int counterB = 0;
    int counterC = 0;

    while (counterA != a.length && counterB != b.length)
    {
      if (a[counterA] < b[counterB])
      {
        c[counterC] = a[counterA];
        counterA++;
        counterC++;
      }
      else
      {
        c[counterC] = b[counterB];
        counterB++;
        counterC++;
      }
    }

    while (counterB == b.length && counterA != a.length)
    {
      c[counterC] = a[counterA];
      counterA++;
      counterC++;
    }

    while (counterA == a.length && counterB != b.length)
    {
      c[counterC] = b[counterB];
      counterB++;
      counterC++;
    }

    return c;
}

MergeSort method:

public static int[] mergeSort(int[] arr)
{ 
    if (arr.length == 1)
    {
      return arr[0];
    }

    int[] arr1 = new int[arr.length / 2];
    int[] arr2 = new int[arr.length - arr1.length];

    for (int i = 0; i < arr1.length; i++)
    {
      arr1[i] = arr[i];
    }

    for (int i = 0; i < arr2.length; i++)
    {
      arr2[i] = arr[i + arr1.length];
    }

    arr1 = mergeSort(arr1);
    arr2 = mergeSort(arr2);

    return merge(arr1, arr2);
}

Because the array is randomly generated, an example would be this:

9, 1, 7, 5, 7, 2, 2, 9, 8, 9

The intended result should be this:

1, 2, 2, 5, 7, 7, 8, 9, 9, 9

However, this is what the output is instead (The array comes out unchanged):

9, 1, 7, 5, 7, 2, 2, 9, 8, 9 

chqrlie :

There are 2 problems in your code:

• you return an array element arr[0] instead of the array itself arr
• you do not handle the case of arr.length == 0. It should return arr as well.

Note that the code can be simplified:

• you can use the ++ operator on index values when copying the values,
• you should remove the redundant tests on the second and third while loops.

Here is an improved version:

private static int[] merge(int[] a, int[] b)
{
    int[] c = new int[a.length + b.length];
    int counterA = 0;
    int counterB = 0;
    int counterC = 0;

    while (counterA != a.length && counterB != b.length)
    {
        if (a[counterA] <= b[counterB])
            c[counterC++] = a[counterA++];
        else
            c[counterC++] = b[counterB++];
    }

    while (counterA != a.length)
        c[counterC++] = a[counterA++];

    while (counterB != b.length)
        c[counterC++] = b[counterB++];

    return c;
}

public static int[] mergeSort(int[] arr)
{ 
    if (arr.length < 2)
        return arr;

    int[] arr1 = new int[arr.length / 2];
    int[] arr2 = new int[arr.length - arr1.length];

    for (int i = 0; i < arr1.length; i++)
        arr1[i] = arr[i];

    for (int i = 0; i < arr2.length; i++)
        arr2[i] = arr[i + arr1.length];

    arr1 = mergeSort(arr1);
    arr2 = mergeSort(arr2);

    return merge(arr1, arr2);
}