How to best implement a 2D sparse array of data?

There's a very simple one (not math-oriented) posted at http://www.blackbeltcoder.com/Articles/algorithms/creating-a-sparse-matrix-in-net.

Thank you very much, Alan!

That is a very good introduction for me to the use of dictionaries. I can see its usage from your example!

And I am sure I will definitely make good use of it in the future!

I thought I'd just clarify that a Dictionary is the generic equivalent of a Hashtable where both the key and value are strongly typed. In a Hashtable they are weakly typed because internally they are stored as type System.Object. This means that value types have to be boxed which is not good from a performance viewpoint.

You can use any type you like for the keys as long as the keys are unique and (in the case of reference type) don't change in a way which affects the hash code.

For future reference, here's a very simple example of how this works in practice:

using System;
using System.Collections.Generic;
using System.Drawing;

class Test
{
     static void Main()
     {
         Dictionary dict = new Dictionary();
         dict.Add(new Point(100,200), 300.3);
         dict.Add(new Point(150,250), 400.5);
         Point key = new Point(100,200);
         Console.WriteLine("The value for point {0} is {1}", key, dict[key]);
         key = new Point(150,250);       
         Console.WriteLine("The value for point {0} is {1}", key, dict[key]);
         Console.ReadKey();
     }
}

Thank you for the reply. I need to look these things up in help since I am not used to using them. But it sounds like you mean I should use a hashtable but where the key is a point structure? Or am I confused? I did not think that the key of a hashtable could be something other than a number or a string.

I have come up with a way of doing this which may be somewhat memory wasteful, but makes the coding rather easy to read. What I actually have to store in 2D sparse arrays are two pieces of information per coordinate location: a "chip type" and an inspection value. The chip type is only mildly sparse (looks like a round disk in a rectangular field) and the inspection value can be very sparse depending on how many inspections of the chips were make.

So I have defined a struct consisting of two int values for chip type and inspection value, and I store the data after retrieval from the database in a rectangular array of these structs.

The coding becomes rather easy because if my struct's members are called "chiptype" and "inspectvalue" and the 2D array is called "chipmap", then for a given x,y coordinate pair I access the values with

chipmap[x,y].chiptype

chipmap[x,y].inspectvalue

so I think I will go with this for now.

If you're using .NET 2.0 or later and the value is of type double, then I'd be inclined to use a Dictionary. As each  pair of coordinates is unique, then a Point is suitable as a key. 

Rather than define your own Point type, I'd just use the System.Drawing.Point struct. This is very lightweight and, as it's a value type, garbage collection is not a problem. Also it contains it's own implementation of the GetHashCode() method - which is used internally by the generic Dictionary class - and can be relied upon to be reasonably efficient, particularly for sparse data, even if it cannot be guaranteed that different Points won't produce the same hash. The Dictionary class contains code to deal with hash collisions in any case.