1. Chapter 9
Random Access Files

2. Outline and Objectives
Fixed-Length Strings
Records
Random Access Files
Chapter 9 - Visual Basic Schneider

3. Chapter 9 - Visual Basic Schneider
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
Chapter 9 - Visual Basic Schneider

4. Chapter 9 - Visual Basic Schneider
Example
Private Sub cmdGo_Click()
Dim city As String * 9
‘ Illustrate fixed-length strings
picOutput.Cls
picOutput.Print " "
city = "San Francisco"
picOutput.Print city
city = "Detroit"
picOutput.Print city; "MI"
picOutput.Print Len(city)
End Sub
Chapter 9 - Visual Basic Schneider

5. Chapter 9 - Visual Basic Schneider
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)
Chapter 9 - Visual Basic Schneider

6. Chapter 9 - Visual Basic Schneider
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.
Chapter 9 - Visual Basic Schneider

7. Syntax of a Record Definition
‘In General Declarations
Private Type recordType
field-name1 As fieldType1
field-name2 As fieldType2
………
End Type
Chapter 9 - Visual Basic Schneider

8. Chapter 9 - Visual Basic Schneider
Record Declaration
Dim record-variable As recordType
Chapter 9 - Visual Basic Schneider

9. Example of Record Definition
Private Type studentRecord
name As String *30
idNumber As String *11
gpa As Single
numOfCredit As Integer
End Type
Chapter 9 - Visual Basic Schneider

10. To create a record of a specific type
Dim student1 As studentRecord
Chapter 9 - Visual Basic Schneider

11. Storing Data in a Record
student1.name = “John Smith”
student1.idNumber = “ ”
student1.gpa = 3.5
student1.numOfCredits = 124
Chapter 9 - Visual Basic Schneider

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.
Qbasic allows you to create two different types of files: sequential and random accesss.
Chapter 9 - Visual Basic Schneider

13. Chapter 9 - Visual Basic Schneider
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 record’s number in the file.
Access method: the way in which the computer transmits data between secondary storage and the primary storage.
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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 Len=Len(recVar)
Chapter 9 - Visual Basic Schneider

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
Chapter 9 - Visual Basic Schneider

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
Chapter 9 - Visual Basic Schneider

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
Chapter 9 - Visual Basic Schneider