Bubble sort a linked list with one pointer

Question

I'm trying to bubble sort a linked list with one pointer, p_previous . The pointer is supposed to look ahead one node and also look ahead two nodes, and if the first is greater than the second, they are to be switched using a temporary variable while p_previous stays out of the swap. p_previous should also check down two nodes to see if the list trailer is there, stopping the sort. I honestly have no clue what I am doing when it comes to bubble sorts, and the linked list implementation isn't helping.

Here is some code:

int sort_list(ID_NUMBER *p_list, int list_count)
{
    ID_NUMBER *p_previous = p_list; /* Previous node, use to swap next */
    ID_NUMBER *p_temp;              /* Temporary variable              */
    int count;                      /* Counts number of nodes passed   */

    for(count = 0; count < list_count; count++)
    {
        while(p_previous->p_next_student->p_next_student != NULL)
        {
        if(p_previous->p_next_student->student_id >
                p_previous->p_next_student->p_next_student->student_id)
        {
            p_temp = p_previous->p_next_student->p_next_student;
            p_previous->p_next_student = p_temp->p_next_student;
            p_temp = p_previous->p_next_student;
            p_previous->p_next_student = p_temp;
        }
        p_previous = p_previous->p_next_student;
        }

    }
    return 0;
}

Here is what I know.

If this is my list as entered.

H-->1-->3-->2-->4-->T

1 and 3 are already in order, move p_previous down.

3 and 2 are out of order, make the temp variable point to 2.

Make 3 point to the number 4.

Make 2 point to the number 3.

Make 1 point to the number 2.

I think thats how I'm supposed to do it, I just don't know how to put it into code. I am pretty sure a while loop inside a for loop inside is all that is necessary.

If someone could help, that would be great.

Also, if you need more information, just ask.

Answer 1

There's several fundamental problems here ( like you can't change the first item in the list), but the most basic one is your swap:

        p_temp = p_previous->p_next_student->p_next_student;
        p_previous->p_next_student = p_temp->p_next_student;
        p_temp = p_previous->p_next_student;
        p_previous->p_next_student = p_temp;

So lets say we have three items, we'll call them AB C. And we want AC B. Right now you start off doing:

p_previous = A
p_temp = A->next->next = C

This is going the wrong way. We're going to point A->next at C. The problem is, when we do that we'll lose B. p_temp should store B.

p_temp = A->next = B
p_previous->next= p_previous->next->next = C
p_temp->next= p_previous->next->next = {whatever came after}
p_previous->next->next= p_temp = B

Then you just have to deal with the other issues, like crashing when your list has only one item.

Also you may want to do this:

for(count = 0; count < list_count; count++)
{
  p_previous = p_list;

So that you go through the list bubbling values N times, instead of just once.