Two Keys Are Better Than One

Question

I have a widget class:

public class Widget
{
    ...
    public string UniqueID = "856D9PWW";
    public int Price = 325;
    public byte[] Data;
    ...
}

I would like a data structure to store my widgets in. Here's the catch - sometimes I need to find a group of widgets based in their price, and sometimes I need to find a specific widget based on it's ID.

I do not want to use two data structures and use each of them when necessary and remove it from the other when I remove it from the first (this includes creating my own data structure that does this 'under the covers'). I would like on data structure, that allows me to use multiple keys, that only needs to remove an item from it using one of the keys, and has the capibility to store multiple items under one of the keys.

Answer 1

You can create your own data container that maintains the necessary data structures to allow indexing with two keys. If you implement one of the standard container interfaces, you can even use this custom container as any other framework container (of the same kind).

Since you control Add/Remove methods, you can easily make sure the different structures are updates as needed. You can also make sure lookups for both keys are as fast as possible. The fact that the container uses more than one structure to implement the different lookups is an implementation detail.

Answer 2

To store your data in a relational database you need a single table to store all of your Widget data in (assuming there is no common repeatable data e.g. Widget Type that should be stored in a lookup table - do a search on the web for Database Normalization).

Then once your data table has been created you can retrieve widgets by any combination of fields by using a Database Query.

If you mean a data structure in code just create a list class e.g.

 public class Widgets : List<Widget>{


 } 

and implement different Find methods depending on your search criteria.

Answer 3

Would a List of Widgets suffice?

List<Widget> widgetList = new List<Widget>();

widgetList.Add(new Widget(UniqueID, price, data));

//To query by UniqueID
Widget uniqueWidget =  widgetList.Single(x => x.uniqueID.Equals("123"));

//To query by price
List<Widget> widgetsByPrice = widgetList.Where(x => x.price.Equals(100.00));

//To remove
widgetList.Remove(uniqueWidget);

Answer 4

There is no magic container that does this. The way database systems handle this is the same way you should: Have a separate index on each access method you need. Yes, it seems ugly, but it's the only way to index on multiple fields. As already mentioned, you can encapsulate this in a class that handles adding and removing elements from each 'index' for you.