Using LINQ to Get Sum/ Average of a List with custom objects

Question

I have a class:

public class PointD
{
    public double X
    { get; set; }
    public double Y
    { get; set; }

    public PointD(double x, double y)
    {
        X = x;
        Y=y;
    }
   //operator for +,-, * and / are overridden
 }

Given a list<PointD>, how to get the average of it using LINQ? A for loop equivalent will be something like this:

  double xx, yy;
  for ( int i=0; i< ptlist.Count; i++)
     {
         xx+=ptlist[i].X;
         yy+=ptlist[i].Y; 
     }
  return new PointD(){X=xx, Y=yy};

You can use any built-in LINQ function only. You can't define an extension that takes care of this function.

Any idea?

Edit: Of course, you can use two separate Sum extension method to Sum for X and Y component before merging them. But this is not what I want. What I want is a single query/ method that does the job

Answer 1

The Aggregate function would come in handy here.

var sum = list.Aggregate((acc, cur) => acc + cur);
var average = list.Aggregate((acc, cur) => acc + cur) / list.Count;

Just insure that you have the / operator defined for types PointD and int, and this should do the job.

Note: I'm not quite sure whether you want the sum or average here (your question is somewhat ambiguous about this), but I've included examples for both.

Answer 2

You'll need to use the Aggregate method instead so you can provide your own aggregation function (Sum is just a convenient specialized case of this method). Something like:

points.Aggregate(new PointD(0, 0), (a, p) => a + p);

I know you say you don't want any additional methods defined, but if this is a common operation I'd be inclined to write an extension method for this, i.e.

public static PointD Sum(this IEnumerable<PointD> source)
{
    return source.Aggregate(new PointD(0, 0), (a, p) => a + p);
} 

Because it's much more readable to be able to write:

points.Sum();