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Listing 36 shows how the new type would be deployed in a program The Structure de nition that appears rst de nes the component parts of a Rectangle each Rectangle has a Length and Width part Within Main, the variable myRectangle is of the new type Once it has been declared, we can assign values to its component variables (myRectangleLength and myRectangleWidth) and later retrieve these values for use in calculations We could easily have dimensioned and used several rectangles in this program, since the structure Rectangle is de ned as a new type of variable Creating new types of variable like this brings several advantages
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Using structures like this helps us to keep together related items of data, so we can declare a whole Rectangle variable in a single statement instead of a number of individual ones We can declare as many of the type as we need in a program (in the same way that we can declare many variables of type Single) and assign composite values from one to the other in a single statement For example, given two Rectangle structure variables R1 and R2, we can copy the values from one to the other with the single statement R2 = R1 It can be clearer to anyone reading the program code what the intentions of the programmer are since the name given to a structure is often self-explanatory For example, one variable of type Rectangle has a more obvious purpose than two individuals of type Single While we can t often expect a general purpose programming language to provide us with exactly the range of data types we need to do what we want to in every program, we can easily build task-speci c data types, such as rectangles, into our programs
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There is one aspect where it is necessary to exercise care when creating new structured variable types: Visual Basic does not allow us to initialize the elements of a structure when it is declared, so for example, you could not do this:
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Dim rect As Rectangle = (3, 4) ' This is an error
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In this case, the attempt to initialize the Length and Width parts of a Rectangle simply causes the compiler to reject the program
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Exercises 37
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1 It is possible to create a structure that combines variables of different types (eg numbers and dates, strings, etc) Create structures that could be used to represent the following a) a Person, with Name, Address, DateOfBirth and NumberOfChildren; b) a Car, with EngineSize, NumberOfDoors, RegistrationDate and LicenceNumber;
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36 Enumerations
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c) d) e)
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a CD recording, with Title, Artist, DistributionLabel, IssueYear and NumberOfTracks; a MapReference, with gures for Longitude and Latitude; a window on a PC screen, with Top, Left, Width and Height settings
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2 For each of the above, write sample statements to declare and then assign values to a variable of the appropriate type
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Enumerations
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Structures are new types that we can de ne as needed in programs: they give us the power to create variables that can contain several pieces of related information Another way we can de ne our own variable types is to create variables that have a restricted range of values Enumerations are de ned as lists of names, each of which becomes a pseudonym for an integer value The keyword Enum is used to create one and Listing 37 is an example
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Enum MusicStyles Classical Swing Jazz Pop Rock Techno End Enum Listing 37: An enumeration (Enum)
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With this enumeration, we can now create variables that will be restricted to taking on one of these values and no other Once an enumeration is de ned, Visual Studio will assist you to assign a value to a variable of that type, as shown in Figure 33 Each entry in the MusicStyles list is represented by an integer number starting by default from 0 This makes enumerations very ef cient to work with, since integer numbers are the root of all data in computer programs In many enumerations, we do not really care what value is assigned to represent a particular entry in an enumeration list However, an uninitialized enumeration variable will always take on the rst value in the range (which will be the zero value) Enumerations can also be more explicitly de ned so that speci c entries in the list take on speci c integer values this will affect the default value assigned to an un-initialized variable We can use this to make the values assigned to members of an enumeration list relate to the names used, as shown, for example, in Listing 38
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