Using LINQ to take the top 100 and bottom 100?

Question

I would like to do something like this (below) but not sure if there is a formal/optimized syntax to do so?

.Orderby(i => i.Value1) .Take("Bottom 100 & Top 100") .Orderby(i => i.Value2); 

basically, I want to sort by one variable, then take the top 100 and bottom 100, and then sort those results by another variable.

Any suggestions?

Answer 1

var sorted = list.OrderBy(i => i.Value); var top100 = sorted.Take(100); var last100 = sorted.Reverse().Take(100); var result = top100.Concat(last100).OrderBy(i => i.Value2); 

I don't know if you want Concat or Union at the end. Concat will combine all entries of both lists even if there are similar entries which would be the case if your original list contains less than 200 entries. Union would only add stuff from last100 that is not already in top100.

Some things that are not clear but that should be considered:

• If list is an IQueryable to a db, it probably is advisable to use ToArray() or ToList(), e.g.

  var sorted = list.OrderBy(i => i.Value).ToArray(); 

  at the beginning. This way only one query to the database is done while the rest is done in memory.

• The Reverse method is not optimized the way I hoped for, but it shouldn't be a problem, since ordering the list is the real deal here. For the record though, the skip method explained in other answers here is probably a little bit faster but needs to know the number of elements in list.

• If list would be a LinkedList or another class implementing IList, the Reverse method could be done in an optimized way.

Answer 2

You can use an extension method like this:

public static IEnumerable<T> TakeFirstAndLast<T>(this IEnumerable<T> source, int count) {     var first = new List<T>();     var last = new LinkedList<T>();     foreach (var item in source)     {         if (first.Count < count)             first.Add(item);         if (last.Count >= count)             last.RemoveFirst();         last.AddLast(item);     }      return first.Concat(last); } 

(I'm using a LinkedList<T> for last because it can remove items in O(1))

You can use it like this:

.Orderby(i => i.Value1) .TakeFirstAndLast(100) .Orderby(i => i.Value2); 

Note that it doesn't handle the case where there are less then 200 items: if it's the case, you will get duplicates. You can remove them using Distinct if necessary.