What .NET collection provides the fastest search

Question

I have 60k items that need to be checked against a 20k lookup list. Is there a collection object (like List, HashTable) that provides an exceptionly fast .Contains() method? Or will I have to write my own? In otherwords, is the default .Contains() method just scan each item or does it use a better search algorithm.

foreach Record item in LargeCollection
{
  if(LookupCollection.Contains(item.Key))
  {
     // Do something
  }
  else
  {
     // Don't
  }
}

Note. The lookup list is already sorted.

Answer 1

System.Collections.Generic.HashSet

Answer 2

If you don't need ordering, try HashSet<Record> (new to .Net 3.5)

If you do, use a List<Record> and use check BinarySearch.

Answer 3

Have you considered List.BinarySearch(item)?

You said that your large collection is already sorted so this seems like the perfect opportunity? A hash would definitely be the fastest, but this brings about its own problems and requires a lot more overhead for storage.

Answer 4

If you aren't worried about squeaking every single last bit of performance the suggestion to use a HashSet or binary search is solid. Your datasets just aren't large enough that this is going to be a problem 99% of the time.

But if this just one of thousands of times you are going to do this and performance is critical (and proven to be unacceptable using HashSet/binary search), you could certainly write your own algorithm that walked the sorted lists doing comparisons as you went. Each list would be walked at most once and in the pathological cases wouldn't be bad (once you went this route you'd probably find that the comparison, assuming it's a string or other non-integral value, would be the real expense and that optimizing that would be the next step).

Answer 5

If it's possible to sort your items then there is a much faster way to do this then doing key lookups into a hashtable or b-tree. Though if you're items aren't sortable you can't really put them into a b-tree anyway.

Anyway, if sortable sort both lists then it's just a matter of walking the lookup list in order.

Walk lookup list
   While items in check list <= lookup list item
     if check list item = lookup list item do something
   Move to next lookup list item

Answer 6

If you're using .Net 3.5, you can make cleaner code using:

foreach (Record item in LookupCollection.Intersect(LargeCollection))
{
  //dostuff
}

I don't have .Net 3.5 here and so this is untested. It relies on an extension method. Not that LookupCollection.Intersect(LargeCollection) is probably not the same as LargeCollection.Intersect(LookupCollection) ... the latter is probably much slower.

This assumes LookupCollection is a HashSet

Answer 7

Keep both lists x and y in sorted order.

If x = y, do your action, if x < y, advance x, if y < x, advance y until either list is empty.

The run time of this intersection is proportional to min (size (x), size (y))

Don't run a .Contains () loop, this is proportional to x * y which is much worse.