Random Access File IO

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Random Access File I/O
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Random-Access File Concepts

• Sequential files are not the most efficient means
  for getting disk file
• Information for instant-access applications.
• Instant-access application include the following
• Airline-reservation systems
• Banking systems
• Point-of-sale systems
• Automated-teller machine
• Instant-access applications are examples of
  transaction-processing systems.
• Such systems require rapid access to specific
  data.
• Random-access ( or direct-access) files are one
  way of achieving instant access.
• Sequential-access file search involves searching
  through large number of records. This is not the
  case with random-access files.
• This class illustrates the organization of a
  random-access file composed of fixed-length
  records of 100 bytes each.

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Random-Access File Concepts

• Advantages over Sequential-Access Files
• Data can be inserted into a random-access file
  without destroying
• data in the file
• Previously-stored data can be updated or deleted
  without having
• to re-write the entire file.

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Bank Library Example

• The examples that follow using random-access file
  I/O are
• based on a banking user interface form
  (BankUIForm). This form
• encapsulates a base-class GUI.
• BankUIForm Source
• In order to reuse BankUIForm, compile the GUI
  into a DLL by creating a project of type Windows
  Control Library. Name the DLL BankLibrary.

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Creating a Random-Access File

• This project solves the following problem for a
  credit-processing
• application
• Create a transaction-processing program capable
  of storing a maximum of 100
• fixed-length records for a company that can have
  a maximum of 100 customers.
• Each record consists of an account number (which
  is a record key), a last name, a
• first name, and a balance. The program can do
  the following
• Update an account
• Create an account
• Delete an account
• This program creates a random-access file
  composed of random records.
• Pay particular attention to what method
  SaveFile() is doing.
• Trace the for loop.
• This program should be created using a console
  application. Use BankLibrary.dll

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Writing Data Randomly to a Random-Access File

• A random access file has been created.
• Use class WriteRandomAccessFileForm to write data
  to that file.

• Note how the Open File and Enter buttons were
  added to the form
• Note the Enter button code.
• Run this program as a class exercise.
• Build the program as Windows Application.

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Writing Data Randomly to a Random-Access File
What to Look For

• Class WriteRandomAccessFileForm must determine
  the location
• in the FileStream at which to insert data
  from the TextBoxes.
• Use the Seek method of the FileStream object to
  locate an exact
• point in the file.
• Note where the file-position pointer is set
• (accountNumber 1) RandomAccessRecord.SIZE
• Account numbers are indexed starting at position
  0.
• The second argument to method Seek is a member of
  the enumeration SeekOrigin
• and it specifies the location at which the
  method should begin seeking.
• Use const SeekOrigin.Begin to seek in relation to
  the beginning of the file.
• After the file determines the location at which
  to place the record, lines 112-118 write the
  record to the file using the BinaryWriter.

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Reading Data Sequentially from a Random-Access
File

• The next task in our exploration of random-access
  file I/O is
• to develop a program that opens the file, reads
  records from it,
• and displays only the records that contain data
  (i.e., those records
• in which the account number is not zero.)
• Build this code using a Windows Form.

Trace the code. Understand how it works
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Reading Data Sequentially from a Random-Access
File

• The next task in our exploration of random-access
  file I/O is
• to develop a program that opens the file, reads
  records from it,
• and displays only the records that contain data
  (i.e., those records
• in which the account number is not zero.)
• Build this code using a Windows Form.

Trace the code. Understand how it works
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A Transaction Processing System

• This project maintains a banks account
  information.
• Users of this program can add new accounts,
  update existing
• accounts, and delete accounts.
• A class known as Transaction acts as a proxy to
  handle all transaction processing.
• Rather than providing transaction processing
  themselves, objects in this application use an
  instance of Transaction to provide the necessary
  functionality.
• By using a proxy, we can encapsulate
  transaction-processing behavior in only one
  class, enabling various other classes in our
  application to reuse this behavior.
• Methods of class Transaction
• OpenFile()
• GetRecord()
• AddRecord()
• The GUI for this program uses whats called a
  Multiple-Document Interface (MDI).
• Later in this course we will explore more details
  of what an MDI is.

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A Transaction Processing System

• The MDI is organized as follows
• Class TransactionProcessorForm is the parent
  window.
• Children windows include
• StartDialogForm
• NewDialogForm
• UpdateDialogForm
• DeleteDialogForm
• Transaction class
• Transaction Processor Form
• Start Dialog
• Update Dialog
• New Dialog
• Delete Dialog

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A Transaction Processing System
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Homework