Question

I have a variable List< Tuple< DateTime, double>> myList.
Given a datetime, hope it returns the Tuple that precedes
the datetime by using Linq.
For example, if "2013-Feb-08 21:34:00" is supplied, want to
see the last Tuple in the list whose datetime is before this timestamp.

How do I do this with Linq?

Edit:
myList.Where(t => t.Item1 < timestamp).Last();
solved my issue.
Which is better in terms of performance compared to
myList.TakeWhile(t => t.Item1 < timestamp).Last();

Answer 1

With MoreLinq MaxBy (available from NuGet):

myList.Where(t => t.Item1 < timestamp).MaxBy(t => t.Item1);

Or (if items are sorted):

myList.TakeWhile(t => t.Item1 < timestamp).Last();

---

UPDATE (with binary search) write comparer:

public class MyComparer : IComparer<Tuple<DateTime, double>>
{
    public int Compare(Tuple<DateTime, double> x, Tuple<DateTime, double> y)
    {
        return x.Item1.CompareTo(y.Item1);
    }
}

Then search

   int index = myList.BinarySearch(new Tuple<DateTime, double>(timestamp, 0), 
                                   new MyComparer());

   if (index == 0)
      // there is no items before timestamp

   if (index > 0)
      result = myList[index - 1]; // your item is previous

   if (index < 0) // no tuple with date equal to timestamp
      var nearestIndex = ~index;
      if (nearestIndex > 0)
          result = myList[nearestIndex - 1];