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c#arrayssortingbubble-sort
Sandro
Sandro

Reputation: 487

Reverse order of bubble sort of null elements

I'm trying to reverse my bubble sort so all null elements are pushed to the end of array instead of the beginning as they're getting sorted now. Any advice on how this can be achieved?

            for (int outerLoop = 0; outerLoop < students.Length-1; outerLoop++)
            {
                for (int innerLoop = 0; innerLoop < students.Length-1; innerLoop++)
                {
                    if (students[outerLoop+1] == null)
                    {
                        var tempObject = students[outerLoop+1];
                        students[outerLoop+1] = students[innerLoop];  
                        students[innerLoop] = tempObject;
                    }
                }
            }

Upvotes: 0

Views: 255

Answers (3)

Dmitrii Bychenko
Dmitrii Bychenko

Reputation: 186813

There's a faster, easier and stable approach with O(n) time complexity:

  int at = 0;

  for (int i = 0; i < students.Length; ++i)
    if (students[i] != null)
      students[at++] = students[i];

  for (int i = at; i < students.Length; ++i)
    students[i] = null;

However, if you insist on bubble sort: we can implement it in a bit different way

  for (bool wrongOrder = true; wrongOrder;) {
    wrongOrder = false;

    for (int i = 1; i < students.Length; i++) {
      if (students[i - 1] == null && students[i] != null) {
        // At least 2 items has wrong order, swap them
        var temp = students[i - 1];
        students[i - 1] = students[i];
        students[i] = temp;

        wrongOrder = true;
      }
    }
  }

Upvotes: 1

Tarik
Tarik

Reputation: 11209

Corrected code:

        for (int outerLoop = 0; outerLoop < students.Length-1; outerLoop++)
        {
            for (int innerLoop = 0; innerLoop < students.Length-1; innerLoop++)
            {
                if (students[innerLoop] == null)
                {
                    var tempObject = students[innerLoop+1];
                    students[innerLoop+1] = students[innerLoop];  
                    students[innerLoop] = tempObject;
                }
            }
        }

This will not sort your array but only drop the nulls at the bottom.

In fact you can do away with the temp variable:

        for (int outerLoop = 0; outerLoop < students.Length-1; outerLoop++)
        {
            for (int innerLoop = 0; innerLoop < students.Length-1; innerLoop++)
            {
                if (students[innerLoop] == null)
                {
                    students[innerLoop] = students[innerLoop+1];  
                    students[innerLoop+1] = null;
                }
            }
        }

Note: C# 7 introduced tuples which enables swapping two variables without a temporary one:

int a = 10;
int b = 2;
(a, b) = (b, a);

This assigns b to a and a to b.

Upvotes: 1

jdweng
jdweng

Reputation: 34421

Try following. You have to test for outer being null and inner not being null :

            for (int outerLoop = 0; outerLoop < students.Length - 1; outerLoop++)
            {
                for (int innerLoop = outerLoop + 1; innerLoop < students.Length; innerLoop++)
                {
                    if ((students[outerLoop] == null) && (students[innerLoop] != null))
                    {
                        var tempObject = students[outerLoop];
                        students[outerLoop] = students[innerLoop];
                        students[innerLoop] = tempObject;
                    }
                }
            }

Upvotes: 1

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