What is the most efficient pattern/algorithm to compare two lists and find the delta between those two lists?

Question

We have two lists, let\'s say students and their scores. I want to compare these two lists and find the delta between the new list and the old list, then find the least intrusi

Answer 1

If you're looking for a general, language-agnostic solution, then you're looking for some kind of data synchronization of ordered lists. The basic algorithm would be:

i1 = 0
i2 = 0
while (i1 < list1.size && i2 < list2.size)
{
  if (list1[i1] < list2[i2])
  {
    //list1[i1] is not in list2
    i1++
  }
  else if (list1[i1] > list2[i2])
  {
    //list2[i2] is not in list1
    i2++
  }
  else
  {
    //item is in both lists
    i1++
    i2++
  }
}
if (i1 < list1.size)
   //all remaining list1 items are not in list2
if (i2 < list2.size)
   //all remaining list2 items are not in list1

Answer 2

Can you use Linq:

List<ListItem> list1 = GetYourList1();
List<ListItem> list2 = GetYourList2();
var diff = list1.Except(list2);

Your example specific:

var diff = newList.Except(existingList);

Not sure if it is the most efficient but it's concise :)

Answer 3

This should be able to do the trick if you don't have the same name twice in your list. In your example, you have 2x Steve, but you need a way to distinct between them.

public static List<ListItem> CompareLists(List<ListItem> existingList, List<ListItem> newList)
{
    List<ListItem> mergedList = new List<ListItem>();
    mergedList.AddRange(newList);
    mergedList.AddRange(existingList.Except(newList, new ListItemComparer()));
    return mergedList.OrderByDescending(x => x.Score).Take(10).ToList();
}

public class ListItemComparer : IEqualityComparer<ListItem>
{
    public bool Equals(ListItem x, ListItem y)
    {
        return x.Name == y.Name;
    }

    public int GetHashCode(ListItem obj)
    {
        return obj.Name.GetHashCode();
    }
}

You can call it like this:

newList = CompareLists(existingList, newList);