Microsoft Visual Basic 2005: Reloaded Second Edition Chapter 9 Structures and Sequential Access...
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Transcript of Microsoft Visual Basic 2005: Reloaded Second Edition Chapter 9 Structures and Sequential Access...
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Microsoft Visual Basic 2005: Reloaded
Second Edition
Chapter 9Structures and Sequential Access Files
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Microsoft Visual Basic 2005: Reloaded, Second Edition 2
Structures
• Structure statement: creates your own data type• User-defined data type (or structure): data types
created by using the Structure statement• Member variables: variables defined within a
structure• Structure can include member variables of:
– Any of the standard data types– Other structure types
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Microsoft Visual Basic 2005: Reloaded, Second Edition 3
Structures (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 4
Using a Structure to Declare a Variable
• Structure variables: variables declared using a structure
• Refer to an entire structure in code using its name
• Refer to a member variable using the structure name with the dot operator and the member variable name
• Member variables can be used like any other variables
• Structures are used to group related items into one unit
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Microsoft Visual Basic 2005: Reloaded, Second Edition 5
Using a Structure to Declare a Variable (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 6
Using a Structure to Declare a Variable (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 7
Passing a Structure Variable to a Procedure
Open the Pool program and Flow Chart
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Microsoft Visual Basic 2005: Reloaded, Second Edition 8
Passing a Structure Variable to a Procedure (continued)
• Sample application using a structure:– Less code to pass a structure variable to a
procedure– Stores all of the data in a single unit
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Microsoft Visual Basic 2005: Reloaded, Second Edition 9
Creating an Array of Structure Variables
• Use a one-dimensional array of structure variables instead of two-dimensional or parallel one-dimensional arrays
• Refer to a member variable in an array element by:
arrayname(subscript).memberVariableName
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Microsoft Visual Basic 2005: Reloaded, Second Edition 10
Creating an Array of Structure Variables (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 11
Creating an Array of Structure Variables (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 12
Creating an Array of Structure Variables (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 13
Creating an Array of Structure Variables (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 14
File Types
• Reading a file: getting information from a file
• Writing to a file: sending information to a file
• Output files: files to which information is written
• Input files: files that are read by the computer
• Three types of files in Visual Basic:– Sequential – Random– Binary
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Microsoft Visual Basic 2005: Reloaded, Second Edition 15
File Types (continued)
• Sequential file: data is accessed sequentially, in consecutive order from the beginning to the end
• Random file: data can be accessed in consecutive or random order
• Binary file: data is accessed by its byte location in the file
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Microsoft Visual Basic 2005: Reloaded, Second Edition 16
Sequential Access Files
• Sequential access file: often called a text file because it is composed of lines of text
• Data must be read sequentially from beginning to end
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Microsoft Visual Basic 2005: Reloaded, Second Edition 17
Sequential Access Files (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 18
Writing Information to a Sequential Access File
• WriteAllText method: – Writes information to a sequential access file– File argument: contains the name of the file to be
used– Text argument: contains the data to be written– Append argument: if True, data is added to end of
existing file; otherwise all existing data is erased
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Microsoft Visual Basic 2005: Reloaded, Second Edition 19
Writing Information to a Sequential Access File (continued)
• My feature: exposes a set of commonly used objects to the programmer
• My.Computer object: allows access to objects and methods to manipulate files
• Computer uses a file pointer to trace where the next character will be read from or written to a file
• Strings.Space method: writes a specific number of spaces to a file
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Microsoft Visual Basic 2005: Reloaded, Second Edition 20
Writing Information to a Sequential Access File (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 21
Aligning Columns of Information in a Sequential Access File
• PadLeft, PadRight methods: – Pads a string with a specified number of characters
based on current length of the string being padded– Aligns columns of information written to a sequential
access file
• Aligns columns of numeric information by the decimal point
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Microsoft Visual Basic 2005: Reloaded, Second Edition 22
Aligning Columns of Information in a Sequential Access File (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 23
Aligning Columns of Information in a Sequential Access File (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 24
Reading Information from a Sequential Access File
• ReadAllText method: reads information stored in a sequential access file– File argument: contains the name of the sequential
access file
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Microsoft Visual Basic 2005: Reloaded, Second Edition 25
Reading Information from a Sequential Access File (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 26
Determining Whether a File Exists
• An attempt to read a file that does not exist will cause an error
• FileExists method: determines if the file exists before attempting to read it– File argument: contains the name of the file
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Microsoft Visual Basic 2005: Reloaded, Second Edition 27
Determining Whether a File Exists (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 28
The FormClosing Event
• FormClosing event: – Occurs when a form is about to be closed by the
program code or by the user– Allows you to trap the closing action and take any
necessary actions such as saving data– Can be used to cancel the close action– Set e.Cancel = True to cancel the closing action
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Microsoft Visual Basic 2005: Reloaded, Second Edition 29
The FormClosing Event (continued)
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Microsoft Visual Basic 2005: Reloaded, Second Edition 30
The Friends Application