Tutorial 10 Random Access Files

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Tutorial 10Random Access Files
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Random Access FilesLesson A Objectives

• After completing this lesson, you will be able
  to
• Define the terminology used with random access
  files
• Create a record structure
• Open and close a random access file
• Write data to and read data from a random access
  file
• Initialize a random access file
• Test for the end of a random access file

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Random Access Files Versus Sequential Access Files

• A random access file is composed of fields and
  records
• A field is a single item of information about a
  person, place, or thing
• A record is a group of related fields that
  contain all of the necessary data about a
  specific person, place, or thing
• Each record in a random access file also has a
  unique number, called a record number, that
  indicates its position in the file

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Records and Fields

• You can directly access a record in a random
  access file by its record number
• For this reason, random access files also are
  referred to as direct access files

A set of student records
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Creating a Record Structure

• Each record in a random access file must contain
  the same number of fields
• You ensure that each record contains the exact
  same fields by creating a record structure
• The record structure itself actually becomes a
  data type, called a user-defined data type, which
  is separate and distinct from the standard data
  types available in Visual Basic .NET
• You use the Structure statement to create a
  record structure in Visual Basic .NET
• An attribute is simply an instruction that
  provides additional information for the Visual
  Basic .NET compiler

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Creating a Record Structure

• The Structure statement begins with the keyword
  Structure followed by the
  name of the record structure the
  statement ends with the keywords EndStructure

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Declaring a Record Variable

• A record variable will itself contain one or more
  variables called field variables
• You separate the record variables name from the
  field variables name with a period
• Declare the variable
• Dim udtEmploy As EmployStruc
• Referring to the field variables
• udtEmploy.intEmployNumber
• udtEmploy.strFirstName
• udtEmploy.strLastName
• udtEmploy.sngSalary

A User-Defined Data Type
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Opening a Random Access File

• In Visual Basic .NET, you use the FileOpen
  function to open either a new random access file
  or an existing random access file

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Writing Records to a Random Access File

• You use the FilePut function to write a record to
  a random access file

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Initializing a Random Access File
Declare a variable
Initialize field variables
Open file
Write records
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Reading Records from a Random Access File

• You use the FileGet function to read a record
  from a random access file

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Testing for the End of a Random Access File

• You can use the EOF function, which stands for
  end of file, to determine whether the file
  pointer is at the end of a random access file

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Closing a Random Access File

• You use the FileClose function to close a random
  access file

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Using a Random Access File in an
ApplicationLesson B Objectives

• After completing this lesson, you will be able
  to
• Use a record structure in an application
• Initialize a random access file in an application
• Add records to a random access file
• Display a specific record contained in a random
  access file
• Verify that the record number is valid

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Coding the InitializeButton Click Event Procedure

• Figure 10-4 shows the pseudocode for the
  Initialize File buttons Click event procedure

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Coding the AddButtonClick Event Procedure

• The AddButton Click event procedure is
  responsible for adding a participant record to
  the random access file
• Figure 10-18 shows the pseudocode for the
  AddButton Click event procedure

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Coding the DisplayButton Click Event Procedure

• The DisplayButton Click event procedure is
  responsible for displaying the record associated
  with a specific seat number
• Figure 10-23 shows the pseudocode for the
  DisplayButton Click event procedure

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Completing the Seminar ApplicationLesson C
Objectives

• After completing this lesson, you will be able
  to
• Delete the information from a record stored in a
  random access file
• Print the contents of a random access file

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Coding the Remaining Procedures in the Seminar
Application

• You need to code the Click event procedures for
  the RemoveButton and PrintButton controls, and
  the PrintPage event procedure for the
  SeminarPrintDocument control

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Coding the RemoveButtonClick Event Procedure

• The RemoveButton Click event procedure is
  responsible for removing a record from the
  participants.data file
• Figure 10-27 shows the pseudocode for the
  RemoveButton Click event procedure

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Coding the PrintButton Click Event Procedure

• The PrintButton Click event procedure is
  responsible for using the SeminarPrintDocument
  control to print the contents of the random
  access file

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Coding the SeminarPrintDocument PrintPage Event
Procedure

• The SeminarPrintDocument controls PrintPage
  event procedure is responsible for printing the
  Seminar Participant Report
• The report should contain three columns of
  information
• The first column should list 20 seat numbers, and
  the second and third columns should list the
  corresponding participant and company names,
  which are stored in the participants.data file