.NET 2.0+

A Generic Searchable Range Collection

A common programming task is to match a value to a group of ranges in order to find a value associated with that range. The .NET framework does not provide a collection class to support this functionality so a new generic collection must be created.

Tabular Lookup

There are many programming situations where a one value is linked to another and, given the first value, you need to determine the second. In some cases the link is a simple calculation within the code. For example, the relationship between a value and its square can be determined with a simple multiplication.

In more complicated situations, or where the link between two values is arbitrary, a tabular lookup may be used. Here you may decide to store the information in a dictionary collection such as a SortedList or Hashtable. The collection's key element can hold the lookup value and the value element can contain the related item. This is ideal for relationships where calculations would be inefficient or impossible such as those in the table of customer codes below:

As the table uses discrete values for the customer codes, finding a related name is easy. When stored using a Hashtable, the items can be added as key / value pairs and found using the unique key as an indexer.

Hashtable customers = new Hashtable();

customers.Add("ABC001", "ABC Technology Limited");
customers.Add("ABC002", "ABC Construction Plc");
customers.Add("JOH001", "John Smith");

Console.WriteLine(customers["ABC002"]);     // Outputs "ABC Construction Plc"

The tabular lookup method becomes problematic when the key values are divided into ranges. As an example, the following table shows the available rankings for the 1980's computer game "Elite" and the number of enemies that must be destroyed to achieve each rating.

This information could be held in a simple dictionary but to permit a lookup using the standard indexer, each item would need to be repeated multiple times. As an example, the "Harmless" option would require four entries so that a lookup could be performed for zero, one, two or three battles won. To support all of the possible searches the collection would include 6,001 items and a rule would be added to the code to check for the special case of over six thousands kills.

The problem of duplicating entries becomes even more serious when the lookup values are not integers. If strings or floating-point values are included in the key column of the table, the table size is limited only by the length of string or accuracy of the fractional values. In either case, a simple lookup is unusable except for very small tables.

The resolution to this problem is to have a collection class that represents value ranges. Unfortunately no such class exists in the .NET framework. The remainder of this article will describe the process and the code required for creating one.