Using the Hashtable Class in VS .NET

Create UPC-A in VS .NET Using the Hashtable Class

Now look at the CustomCompare class This is really just a class that includes a single function, Compare The function simply checks the types of the objects coming in If the object on the left is an instance of Sortable Class, the function calls the SortableClass object s CompareTo function Otherwise, if the right object is an instance SortableClass, I call that object s CompareTo function But notice what I had to do in that case: I had to subtract the result form 0 Why did I do that Because I m comparing the objects backwards from what the Compare function is supposed to do; I m calling the CompareTo object on the right-hand object thus the less-than and greater-than result is reversed from what it should be To remedy the situation, I flip the sign of the result by subtracting the result from 0 Finally, if both objects are integers, I call the CompareTo function on the first integer, passing the second Result: I can compare integers, SortableClass instances, or any combination thereof When you run the code, you see this output:

4 5 (5,3) (5,4) (5,6) 7 (10,5) (15,4)

Notice that the 5 comes before the three SortableClass instances that have 5 in the first element That happens because I made sure that if an integer had the same value as the X value of a SortableClass instance, the integer would always have a less-than value thus the integer would appear first in the list

The Hashtable class is perhaps one of the most useful data structures in the NET Framework, even if the name is quite unusual (If you ve worked in other languages, you might know a type called map The Hashtable in NET is the same as a map in other languages) A Hashtable instance lets you associate data For example, if you want to store a list of names and associate a salary with each name, you can use a Hashtable Or, if you want to store a list of vehicle ID numbers and associate each with the color of the car you can use a Hashtable

Following is some VB code that demonstrates putting a list of car IDs into a Hashtable and associating a color with each:

Dim cars As New Hashtable cars( 6789Q345254 ) = green cars( 35T893W2A21 ) = red cars( T9U3W8OQQ53 ) = blue cars( T5WQY374833 ) = blue ResponseWrite( The car 6789Q345254 is ) ResponseWrite(cars( 6789Q345254 ))

The first line of this code creates a new Hashtable instance The next four lines put some data into the Hashtable instance You can see the general format for inserting the data: You type the name of the Hashtable, followed by a key surrounded by parentheses In this example, the keys are strings (and are supposed to be vehicle ID numbers, even though I made them shorter than real vehicle ID numbers) but you can use any type you want for the keys Next comes the equal sign, followed by the value you want to associate with the key Again, in this case, I m using strings, but you re free to use any type You can see then that I m associating the value green with the ID 6789Q345254 using this line:

Do you notice anything familiar with the Hashtable The Hashtable works very much like an array except instead of having to use only integers for the index, you can use any type you want (Oh, yes instead of the term index, which is associated with a Hashtable, the term is key) To access the values in the Hashtable, you again put your key inside parentheses, just like you would with an array Here s an example line from the preceding code:

ResponseWrite(cars( 6789Q345254 ))

This code writes out the value for the key 6789Q345254 (which you can see is green ) Each element in a Hashtable instance has a unique key No two elements can share the same key However, two elements can share the same value Think of the car example: Two cars won t share the same vehicle ID number, but two cars can have the same color Knowing that keys are unique, you can probably realize how to change the value in a key: Just reassign it Suppose you have the following code (notice that the keys are the same in these two lines):