10 Files

1
10 Files

• Random Access Files

2
Learning Objectives

• Explain what a file does and why a file is
  useful.
• Give the general code for dealing with random
  access files (remembering fixed length strings).

3
A File

• Stores data permanently
• In all programs so far, any stored data has been
  lost when the program closes, and has to be
  entered again when the program next runs.

4
Random (Direct) Access Files

• Store only records.
• You can read and write to any record position
  within the file.
• You can edit records within the file.
• Can be opened for read and write access at the
  same time.

5
Restrictions of Random Access Files

• You can only read or write whole records.
• You cannot read or write individual elements of
  records.
• Records must be of equal length so that VB knows
  where each record is.
• As you may already know (1.44 Files) Number
  variables already have fixed lengths.
• However, string variables use 1 byte per
  character (so are as long as the text they hold).
  
• String variables in a random access file record
  need to be of a fixed length (this will remove
  characters if the string is longer than the fixed
  length or add spaces to extend it to reach the
  fixed length).

6
Filename

• The full path and name of the file.
• It is best to use a variable to hold all this
• Dim FileName As String CurDir() "\....txt"

Name of file.txt

• Finds the path of the current program, this will
  store the file in the same folder.
• This will be in the program folder a folder
  with the same name bin Debug.
• It is best to do this otherwise every time you
  move the program you would have to change the
  path accordingly.

7
Decorating Business Example Project

• Open the Decorating Business Example Project.
• Open the code for the first form frmQuoteFinder
  (code view).

8
Globally in frmQuoteFinder

• Declare a public variable to hold the path and
  name of the customers file.
• Note it must be public as it needs to be
  accessible on other forms.
• Public CustomersFile CurDir()
• \Customers.txt
• Now write a similar line underneath for the
  FinishesFile

9
Declaring Fixed Length Strings

• To do this use
• ltVBFixedString(30)gt Dim As String
• before declaring the string.

Length in bytes (30 is suitable for most cases)
Variable name / identifier
10
Fixed Length Strings

• In the structure for customer records add the
  part in bold below
• Structure CustomerQuotesRecord
• ltVBFixedString(30)gt Dim CustomerName As String
• Dim QuotationDate As Date
• Dim Quote As Decimal
• End Structure
• Do the same for the Finishes record structure.

11
Open a random access file

• FileOpen(1, FileName, OpenMode.Random)

• Open using random access.

• Reference number of the file used by VB to
  identify it.
• You only need to use a higher number if you wish
  to open more than one file at a time.

12
Writing to a random access file

• FilePut(1, , )

Record to write
Record Number
13
Closing a random access file

• FileClose(1)

14
Storing records in the file

• As while as storing finishes and their prices in
  the array we also have to now store them in the
  file.

15
frmEnterAFinish - butEnterFinish code

• At the end of the procedure write
• FileOpen (1, frmQuoteFinder.FinishesFile,
  OpenMode.Random)
• FilePut (1, frmQuoteFinder. FinishesArray(frmQuote
  Finder.NumberOfFinishes), form1.NumberOfFinishes)
• FileClose(1)
• Now write similar lines to store customer details
  in the CustomersFile (in the frmCreateQuote
  butCreateQuote code).

16
Reading all records from a file and inserting
them into an array

• Do While Not EOF(1)
• NumberOfRecords NumberOfRecords 1
• FileGet(1, Array(NumberOfRecords))
• Trim (remove spaces) any fixed length string
  fields
• Loop

EOF End Of File
Makes rows in file match rows in array and counts
number of records on file.
17
Retrieving records from the file

• We now have to retrieve records from the file
  when the program is run.
• The best place to do this is in the
  frmQuoteFinders load event as this is the first
  form to appear and the load event is the first
  event which happens automatically.

18
Form1 load event

• FileOpen (1, FinishesFile, OpenMode.Random)
• Do While Not EOF(1)
• NumberOfFinishes NumberOfFinishes 1
• FileGet(1, FinishesArray(NumberOfFinishes))
• FinishesArray(NumberOfFinishes).Finish _
  Trim(FinishesArray(NumberOfFinishes). Finish)
• Loop
• FileClose(1)

19
Retrieving records from the CustomersFile

• Now write similar lines to retrieve customer
  details in the CustomersFile.

20
Plenary

• Explain what a file does and why a file is
  useful.
• Store data permanently.
• So data does need to be re-entered every time the
  program starts.

21
Plenary

• What is the general code for dealing with random
  access files (remembering fixed length strings)?
• ltVBFixedString(30)gt Dim As String
• FileOpen(1, FileName, OpenMode.Random)
• FilePut(1, , )
• FileGet(1, )
• FileClose(1)
• Use Do While EOF(1) Loop if you wish to read
  all records.

22
Homework

• Write a program to store student records in a
  random access file.
• Each record should hold the number of warnings,
  merits and a comment field.
• A user should be able to view the records on a
  separate form.
• Extension
• A user should be able to edit or delete a record.