Microsoft Visual Basic 2005: Reloaded

Second Edition

Chapter 9Structures and Sequential Access Files

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Objectives

After studying this chapter, you should be able to:

• Create a structure

• Declare and manipulate a structure variable

• Create an array of structure variables

• Write information to a sequential access file

• Align the text written to a sequential access file

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Objectives (continued)

• Read information from a sequential access file

• Determine whether a file exists

• Code the FormClosing event

• Prevent a form from closing

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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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Structures (continued)

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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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Using a Structure to Declare a Variable (continued)

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Using a Structure to Declare a Variable (continued)

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Passing a Structure Variable to a Procedure

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Passing a Structure Variable to a Procedure (continued)

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Passing a Structure Variable to a Procedure (continued)

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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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Passing a Structure Variable to a Procedure (continued)

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Passing a Structure Variable to a Procedure (continued)

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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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Creating an Array of Structure Variables (continued)

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Creating an Array of Structure Variables (continued)

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Creating an Array of Structure Variables (continued)

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Creating an Array of Structure Variables (continued)

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Creating an Array of Structure Variables (continued)

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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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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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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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Sequential Access Files (continued)

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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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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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Writing Information to a Sequential Access File (continued)

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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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Aligning Columns of Information in a Sequential Access File (continued)

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Aligning Columns of Information in a Sequential Access File (continued)

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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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Reading Information from a Sequential Access File (continued)

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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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Determining Whether a File Exists (continued)

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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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The FormClosing Event (continued)

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The Friends Application

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The Friends Application (continued)

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The Friends Application (continued)

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The Friends Application (continued)

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The Friends Application (continued)

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Programming Tutorial

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Programming Example

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Summary

• Structure statement: defines a user-defined data type or structure

• Structure variable contains one or more member variables

• Structure variable can be passed to procedures

• Create a one-dimensional array of structure variables– Use the dot member access operator to access the

structure member variables

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Summary (continued)

• Application can read from and write to a file

• Sequential access files are always accessed in consecutive (sequential) order from beginning to end

• ReadAllText method: reads text from a sequential access file

• WriteAllText method: writes text to a sequential access file

• Use Strings.Space, PadLeft, and PadRight methods to align data in a sequential access file

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Summary (continued)

• FileExists method: determines if a file exists before trying to read from it

• FormClosing event: occurs when the form is about to be closed

• Use the FormClosing event to save data or to prevent a form from being closed