Question

Is there anyway to make a quicksort sort by multiple conditions? For example, I have a set of edges. Each edge has a source, destination, and length. I want to put the edge with a smaller length in my array first. But if the lengths are the same, I want to sort by that with a smaller source vertex. If these source vertexes are the same, I want to sort by the smaller of the two destination vertices.

For example:

4 (source) 2 (destination) 3 (length)

1 (source) 5 (destination) 3 (length)

Since they both have the same length, we look at the source vertex. Since the second edge is smaller than the first edge, we swap them because we compare by source vertex.

Below is my quicksort and I’m honestly not sure why it’s not sorting correctly.If there’s a way to make quicksort less efficient but more stable, I would gladly take suggestions!

void quickSort(edge *e, int left, int right)
{
  int i = left, j = right;
  int temp, temp1, temp2;
  int pivot = (left + right)/2;
  while(i <= j)
  {
    while(e[i] < e[pivot])
      i++;
    while(e[pivot] < e[j])
      j--;
    if(i <= j)
    {
      temp = e[i].getLength();
      temp1 = e[i].getEdgeSrc();
      temp2 = e[i].getEdgeDes();
      e[i].setLength(e[j].getLength());
      e[i].setEdgeSrc(e[j].getEdgeSrc());
      e[i].setEdgeDes(e[j].getEdgeDes());
      e[j].setLength(temp);
      e[j].setEdgeSrc(temp1);
      e[j].setEdgeDes(temp2);
      i++;
      j--;
    } //if statement
  }///while loop
  if(left < j)
    quickSort(e, left, j);
  if(i < right)
    quickSort(e, i, right);
}

My sorting of conditions:

bool edge::operator<(const edge &other) const 
{
    if (length < other.length)
        return true;
     else if ((length == other.length) && (source < other.source))
        return true;
     else if((length == other.length) && (source == other.source) && (destination < other.destination))
        return true;
     return false;
}

Again, if anyone knows a way to make this quicksort correctly by reducing the time complexity of it but making it stable, I would gladly take any suggestions! Thank you! Any help?

Edit: This is how I invoked my quicksort. I invoked it based on the number of edges read.

    quickSort(e, 0, edges-1); //-1 because if you put in edges, it'd go past the bounds of the array

EDIT: when I try to put in something like this in my algorithm:

0 1 1

0 3 1

1 3 1

2 5 1

4 10 1

4 8 1

10 8 1

11 6 2

11 7 2

6 7 1

9 6 1

9 7 1

This is the output:

0 1 1

0 3 1

1 3 1

2 5 1

4 8 1

4 10 1

6 7 1

6 9 1

8 10 1 <- should be below 7 9 1

7 9 1 <- should be above 8 10 1

6 11 2

7 11 2

Answer 1

It is cleaner to write it this way

if (length != other.length)
   return length<other.length;

if ( source != other.source)
   return source < other.source;

return destination < other.destination;

You should also be able to do temp = e[i] and so on since the members are all ints.

This (and the code you submitted) should do the task you want I think.

If you are having stability issues, thats because quicksort isnt stable. You could get around it by adding more conditions so that lhs==rhs doesnt happen. Alternatively you can try Mergesort

I dont have much experience with Quick sort frankly, but your impl does look markedly different from Wikipedias In Place Algorithm. For instance, your pivot is not moved at all. Could you check if that is the problem?

---

Edit

After looking at your link

It looks like the algorithm linked also uses pivot as a value instead of as an index (as you do). It looks syntactically identical to yours until you consider that your pivot value might move, after which your pivot index would point to something else

int pivot = arr[(left + right) / 2];

Does this help?