Exercises 61 in VB.NET

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Exercises 61
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1 The standard telephone book may have more than a million entries in a big city or area a) What feature of it makes it possible for you to look up someone s number in a couple of minutes b) What makes it dif cult to add or delete an entry in a phone book 2 Insurance companies, banks and other large service organizations often identify you as a customer by number (eg policy number or account number) rather than by your name Can you think of two reasons why it is more ef cient for them to do this
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611 Multiple Data Items Individual Variables
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Up till now, every program we ve looked at has used individual variables to store data and provide operations For example, in the bank account program in 4, we used a variable, MyAccount, to allow us to create and work with one object of the BankAccount class If we had wanted two BankAccount objects, we could have declared a second reference variable Ten accounts would have
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required ten reference variables, and since each would have to have a distinct name, would have needed ten sets of similar statements to allow us to manipulate them Consider a simple program (Listing 61) to create and manipulate ten BankAccounts:
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Sub Main() Dim Account1 As BankAccount = New BankAccount() Dim Account2 As BankAccount = New BankAccount() ' Etc Dim Account10 As BankAccount = New BankAccount() Account1AccountName = "John Smith" Account2AccountName = "Mary Green" ' Etc Account10AccountName = "Billy Brown" Account1Deposit(100) Account2Deposit(200) ' Etc Account10Deposit(150) ' More operations End Sub Listing 61: Working with multiple individual variables
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Considering the power that a computer has to work tirelessly through hundreds, thousands or millions of program statements, this way of working with individual variables seems to be a very limiting constraint To unleash the full power of computer programs, it would be better if we could work with multiple variables, without having to deal with them on an instance-by-instance basis Visual Basic provides us with a number of ways of doing this The simplest of these is the array
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An array is a number of individual variables that all share the same identi er (variable name) We declare an array by indicating in brackets how many elements it will contain, and following this with their type name as if the array was a single variable For example:
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Dim Accounts(100) As BankAccount
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Elements (individual variables) of this array are accessed by combining their name with an index variable or value in brackets For example, Accounts(0) would be the rst element of the array of accounts, Accounts(1) would be the second element, etc If we think of a variable as a box for storing a piece of data, we might imagine an array to be a row of boxes, like a single row of pigeonholes, each holding its own piece of data, and each with a number to distinguish it from the others This is shown in Figure 61
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62 Arrays
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A Single integer variable and an array of integers
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The combination of an identi er and an index number gives us a simple and elegant way of de ning as many individual variables as we could use in a program (almost) However, there is a bit of an oddity to contend with The declaration of the Accounts array (Dim Accounts(100) As BankAccount) gives us 101 BankAccount reference variables, each of which could refer to an individual object of the BankAccount class Why 101 This is an awkward one to explain and retain any sense of dignity for the computer programming fraternity, but in the C programming language (which we must consider as being the archetype of everything in NET) arrays always start at element 0 In C, such a declaration would give us an array of integers with exactly 100 elements, numbered from 0 to 99 This in itself is awkward but excusable, because it does at least make it easier to implement arrays in the C language (this is an odd but consistent feature of C) In older versions of BASIC and Visual Basic, the same declaration would give us 101 elements because there would be elements 0 to 100 (101 inclusive) My understanding is that most people seeing a declaration of a 100 element array would expect 100 elements numbered 1 to 100, and the creators of BASIC decided that this was sensible Somewhere along the line, the C language went for the pragmatic element 0 approach, and the designers of Visual Basic (and subsequently VB NET) decided to hedge their bets and go for both approaches simultaneously An array declared like Dim a(<number>) gives us <number>+1 elements, starting from element 0 and going up to element <number> I m afraid you ll just have to get used to this Once we ve accepted the odd numbering scheme, working with lots of data elements opens up a whole new way of programming, in which our declarations do not need to impose a limit on the number of items we deal with in our programs Also, we get to use the arrays we declare very exibly For a start, where you would expect to place an index number to indicate a speci c element of an array, you can instead put a variable! Consider Listing 62
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