LINQ to Get Closest Value?

Question

I have a List, MyStuff has a property of Type Float.

There are objects with property values of 10,20,22,30.

I need to write a query that finds the objects closest to 21, in this case it would find the 20 and 22 object. Then I need to write one that finds the object closes to 21 without going over, and it would return the object with a value of 20.

I have no idea where/how to begin with this one. Help?

Thanks.

Update - wow there are so many awesome responses here. Thanks! I don't know which one to follow so I will try them all. One thing that might make this more (or less) interesting is that the same query will have to apply to LINQ-to-SQL entities, so possibly the answer harvested from the MS Linq forums will work the best? Don't know.

Answer 1

Here's a solution that satisfies the second query in linear time:

var pivot = 21f;
var closestBelow = pivot - numbers.Where(n => n <= pivot)
                                  .Min(n => pivot - n);

(Edited from 'above' to 'below' after clarification)

As for the first query, it would be easiest to use MoreLinq's MinBy extension:

var closest = numbers.MinBy(n => Math.Abs(pivot - n));

It's also possible to do it in standard LINQ in linear time, but with 2 passes of the source:

var minDistance = numbers.Min(n => Math.Abs(pivot - n));
var closest = numbers.First(n => Math.Abs(pivot - n) == minDistance);

If efficiency is not an issue, you could sort the sequence and pick the first value in O(n * log n) as others have posted.