Random Access Files

1
Chapter 9

• Random Access Files

2
Outline and Objectives

• Fixed-Length Strings
• Records
• Random Access Files

3
Fixed - Length String

• Each variable in the program is assigned to a
  data type.
• Fixed - Length String variable Stores character
  strings of fixed length
• To declare a string of length n use the following
  statement
• Dim var As String n
• Note n is a positive integer

4
Example

• Private Sub cmdGo_Click()
• Dim city As String 9
• Illustrate fixed-length strings
• picOutput.Cls
• picOutput.Print "123456789"
• city "San Francisco"
• picOutput.Print city
• city "Detroit"
• picOutput.Print city "MI"
• picOutput.Print Len(city)
• End Sub

5
Comparing Strings

• Care must be taken to compare two fixed-length
  strings of different length.
• Use the function RTrim to remove the right hand
  spaces from a variable - length or a fixed -
  length string.
• Example RTrim(city)

6
Records

• Record is a user defined data type
• A record is a collection of fields
• Each field in the record has a name, type and
  length which is the number of spaces allocated to
  it.

7
Syntax of a Record Definition

• In General Declarations
• Private Type recordType
• field-name1 As fieldType1
• field-name2 As fieldType2
• End Type

8
Record Declaration

• Dim record-variable As recordType

9
Example of Record Definition

• Private Type studentRecord
• name As String 30
• idNumber As String 11
• gpa As Single
• numOfCredit As Integer
• End Type

10
To create a record of a specific type

• Dim student1 As studentRecord

11
Storing Data in a Record

• student1.name John Smith
• student1.idNumber 456-78-9012
• student1.gpa 3.5
• student1.numOfCredits 124

12
Methods of accessing a File

• Sequential access
• Data is accessed in the same order in which it is
  physically stored in the file.
• Random access
• Records are accessed directly from the file in
  random order.

13
Random Access Files

• A Random Access file is usually made up of a
  collection of records.
• Records are accessed directly from the file in
  random order.
• Usually is done by using records number in the
  file.

14
Opening a Random Access File

• One statement is suffices for creating,
  appending, writing and reading a random access
  file.
• Open filespec For Random As n LenLen(recVar)

15
Example of entering a record into a file

• Private Sub cmdAddCollege_Click()
• Write a record into the file COLLEGES.TXT
• Dim college As collegeData
• college.nom txtCollege.Text
• college.state txtState.Text
• college.yrFounded Val(txtYear.Text)
• recordNum recordNum 1
• Put 1, recordNum, college
• txtCollege.Text ""
• txtState.Text ""
• txtYear.Text ""
• txtCollege.SetFocus
• End Sub

Entering a record into a file
16
Example of reading a record from a file

• Private Sub DisplayFile()
• Dim recordNum As Integer
• Access the random-access file COLLEGES.TXT
• Dim college As collegeData
• Open "COLLEGES.TXT" For Random As 1 Len
  Len(college)
• picOutput.Print "College", , "State", "Year
  founded"
• For recordNum 1 To 3
• Get 1, recordNum, college
• picOutput.Print college.nom, college.state,
  college.yrFounded
• Next recordNum
• Close 1
• End Sub

To Read a data from a file
17
Summary of Using Random Access Files

• Do not need to close between placing records into
  them or reading from them
• Do not need to input the records in any specific
  order
• To find out the most recent record number in a
  file n, then use the function Loc(n)
• Each record should have the same length, which is
  any number from 1 to 32767