n Data and Object Structures in Visual Basic .NET

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ConsoleWriteLine(slItem("Alpha")) ConsoleWriteLine(slItem("Charlie")) ConsoleWriteLine( _ "Using GetByIndex() sorted access:") Dim index As Integer For index = 0 To slCount 1 ConsoleWriteLine(slGetKey(index) & ": " _ & slGetByIndex(index)) Next End Sub Listing 627: Using a SortedList
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Like a HashTable or any other form of Dictionary collection, it is normal to use a string as the Key and some other class of object as the Value of each entry In the above example, coding is fairly simple because we use strings for both parts and Visual Basic knows how to deal with strings (sorting them into order and displaying them) If we were to use an arbitrary class for the Value objects, we would not expect to be able to display them so easily, and it would be more usual to use the sorted list as a way of looking up Values in sort order to perform some operation on or with them We might also wish to use some other class, not a string, as the Key part of entries In this case, we would need to provide an ICompare Interface and a CompareTo() method as in Listing 69 to properly de ne the Key class We will look again at this issue in the next chapter
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6341 Binary Searches
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There are a number of reasons why you might want to keep all of the elements in a list sorted into some order (eg alphabetical order), but one of the most compelling is that it is possible to use a very fast algorithm for searching for an item in the list A binary search is a divide-and-conquer algorithm, which works ef ciently by repeatedly splitting the list into two equal halves and discarding the half that obviously does not contain the sought item Since the items in a sorted list are in strict order or key, it is easy to determine which half cannot contain the item you are looking for, and therefore ignore it for the remainder of the search In a way this is analogous to how you might look up a number in the phone book: open the book half way, check which half the name you are looking up is in by comparing it with the names at the part you have opened, and then repeat with the half that is left A binary search algorithm is built-in to the Array class and inherited by the SortedList class so that you can perform fast look-ups Since it is only possible to do a binary search on a sorted list of items, care is required when using it with a normal array (typically, use the Sort() method rst and then BinarySearch) No such care is necessary with a SortedList For a large list, a call to BinarySearch is not quite as fast as it would be if we were looking up an entry in a HashTable, but since a HashTable does not provide a way of retrieving the items in order, a BinarySearch on a SortedList is a good compromise, giving us a useful combination of ordered data and fast retrieval
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63 Other Data Structures
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635 Stacks and Queues
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Stacks and Queues are data structures that require that items are added to and
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removed from a speci c end Both are useful data structures for managing the way that objects are dealt with when they cannot be processed immediately for some reason
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A Queue data structure is sometimes referred to as a rst-in- rst-out structure, as this is descriptive of the way that items are added to and removed from it The most obvious analogy for a Queue structure is the real queue that you join to receive service at the bank or post of ce Every (well mannered) person who joins the line of customers attaches to the end of the queue, and customers leave the queue to get service only after all of the customers before them have left so that they are at the front In a bank or post of ce, if it is a quiet period there may be no queue, since if a customer arrives and there is a free service window they can be served immediately During busy periods, customers arrive at intervals more quickly than they can be served and in these circumstances a customer who arrives will join the queue, wait until all of the queue members in front of them have been served, and then leave the queue to get service We can see from this that a queue is a good way of providing fair access to a limited resource (eg a bank teller) Microsoft Windows uses Queue data structures in several ways most notably when dealing with user interactions Consider the action of a Windows PC that is running several application programs simultaneously The rst thing to be aware of is that a normal PC cannot do more than one thing at once (you can spend a lot on a multi-processor PC so that it can do more than one job at a time, but most people s needs would not justify this expense) When you see the computer apparently doing two jobs simultaneously, you are in fact watching it switching so rapidly between the two tasks that you cannot see the join Now think of what happens if you select some time-consuming task for your computer to do, such as saving a long text le, or redrawing a complex graphic While it is busy performing this lengthy task, it will probably appear to ignore your commands For example, if you click on the Start button to start another program up, the Start menu might not appear immediately You will most likely notice that after a delay (once it had completed the time-consuming task) the Start menu will pop up In fact, we are so used to this behaviour in Windows that we tend not to notice unless the delay between initiating some action and the action occurring becomes very long usually an indication that the computer is stressed When you clicked on the Start button, your command was not ignored, but simply placed into a queue until the PC could get round to it Any other commands that were in front of it in the queue would be processed before your mouse click, but you can be quite sure that, failing crashes, Windows will get around to displaying the Start menu eventually Typically you will see this behaviour whenever you try to access a printer, open a network drive or deal with some other slow media devices The Windows message queue is central to making a Windows computer usable It ensures that commands are processed fairly in the order that they were issued, and
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