1. Chapter 8 - Visual Basic Schneider
Sequential Files
Chapter 8 - Visual Basic Schneider

2. Chapter 8 - Visual Basic Schneider
Outline and Objective
Creating Sequential Files
Adding items to Sequential Files
Using Sequential Files
Sorting Sequential Files
Merging Sequential Files
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3. Chapter 8 - Visual Basic Schneider
What is a File?
A collection of related data kept in secondary storage
Two types of files: program files, text(data) files
Data files are usually made up of a collection of records.
A record is collection of fields.
Name First second
Ali
Access method: the way in which the computer transmits data between secondary storage and the primary storage.
Huda
record
Ahmed
field
Chapter 8 - Visual Basic Schneider

4. Creating a Sequential File
Choose a file name
Choose a reference number
Open the file for Output
Write to the file
Close the file
Open “FileName.txt” for Output as #1
Write#1, “Hello”, 5
Close #1
This means that if you want to access a particular data item, all the data proceeds the needed item must be accessed first.
Chapter 8 - Visual Basic Schneider

5. Chapter 8 - Visual Basic Schneider
Example:
Private Sub cmdCreateFile_Click()
Dim name1 As String, x as Integer
' Demonstrate use of Write # statement
Open “C:\user\temp\Marks.txt" For Output As #1
Write #1, “Ali"
Write #1, 22
Write #1, “Ahmed", 23
name1 = “Huda“
x= 20
Write #1, name1, x+2
Close #1
End Sub
Reference number
File name
Write to the file
Close the file
Chapter 8 - Visual Basic Schneider

6. Chapter 8 - Visual Basic Schneider
Example:
Write the string CS surrounded by quotation marks into the file
Write the number 116 without any leading or trailing space into the file
Private Sub cmdCreateFile_Click()
Dim name1 As String, name2 As String
Open “C:\user\Data.txt" For Output As #1
Write #1, “CS"
Write #1, 116
Write #1, “BASIC", 10, 20, ”One ”
name1 = “Student“
name2 = “Test"
Write #1, 2*3, “ID “ & name1, name2
Close #1
End Sub
“CS”
116
“BASIC",10,20,”One ”
6,“ID Student”,“Test” 6
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7. Chapter 8 - Visual Basic Schneider
Example:
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8. Chapter 8 - Visual Basic Schneider
Example:
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Example:
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10. Chapter 8 - Visual Basic Schneider
Caution
If an existing file is opened for output, the computer will erase the existing data and create a new empty file.
Chapter 8 - Visual Basic Schneider

11. Adding Items to a Sequential File
Choose a reference number for the file
Open the file for Append
Write to the end of the file
Close the file
1. Reference number should be from 1 through 511. It need not be the number that was used when the file was created.
2. It allows data to be output and recorded at the end of the specified file.
3. Write with the # write statements
4. Close with the same reference number
Note: Append is intended to add data to an existing file, but also can be used to create a new file.
Chapter 8 - Visual Basic Schneider

12. Chapter 8 - Visual Basic Schneider
Assume that the file “data.txt” contains the following entries:10,20,30,40
What is the content of the file “data.txt” after the user click on the command button
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13. Chapter 8 - Visual Basic Schneider
Sequential File
Different modes in which a file can be used:
Output
Input
Append
A file should not be open in two different modes at the same time.
1. The name of the file: follows the same rules for naming program files.
Use invest.bas and invest.dat
2. The mode in which the file is to be used , can be output, input, append .
Output: indicates data is written to the file from the program
input: the contents of the file are being read into the program
append :allows new records to be added to an existing file.
A butter is a reserved part of the primary storage unit used a s temporary storage area for the data that is being written or read from a file
Chapter 8 - Visual Basic Schneider

14. Chapter 8 - Visual Basic Schneider
ERROR: A file should not be open in two different modes at the same time.
Chapter 8 - Visual Basic Schneider

15. Chapter 8 - Visual Basic Schneider

16. Sorting Sequential Files
The records of a sequential file can be sorted on any field by first reading the data into parallel arrays and then sorting on a specific field, and then rewriting the data into a file.
Chapter 8 - Visual Basic Schneider

17. Merging Sequential Files
Steps to merge two sorted files:
1. Open the two sorted files For Input and a third file For Output
2. Get the two items of data from each file
3. Compare and repeat until the EOF.
If one item precedes the other, write it into the third file and
get another item from the file
If the two items are identical, write it into the third file and
advance to the next items in both files.
4. Write the remaining items to the third file
5. Close the three files.
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18. Chapter 8 - Visual Basic Schneider
Common Errors
Opening the file in wrong mode
Not using Write statement every time you need to add data to a file
Not closing the file
Chapter 8 - Visual Basic Schneider

19. Deleting or Changing a Record!
Create a new file
Read and change records
Create a new one
Erase the old one
Kill “FileSpec”
Rename the new file
Name “oldfilespec” As “newfilespec” 19
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20. Chapter 8 - Visual Basic Schneider
Example:
before
After
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21. Chapter 8 - Visual Basic Schneider
Example:
After
before
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22. Chapter 8 - Visual Basic Schneider
Error Trapping
Visual Basic has a device, called error-trapping, for preventing some types of errors.
If an error occurs while error-trapping is active, two things happen:
1. An identifying number is assigned to the Number property of an object called Err
2. The program jumps to error-handling routine.
Chapter 8 - Visual Basic Schneider

23. Setting up error-trapping in a procedure:
Make the first line of the procedure:
On Error GoTo ErrorHandler
Type the statements to carry out the procedure
End the procedure by typing
Exit Sub
ErrorHandler:
error-handling routine
Resume
Chapter 8 - Visual Basic Schneider

24. Example ( error-handling routine to handle a “division by zero” error)
Private Sub cmdDivide_Click()
On Error GoTo ErrorHandler
Dim a As Single, b As Single, c As Single
picResult.Cls
a = Val(InputBox("Enter the numerator."))
b = Val(InputBox("Enter the denominator."))
c = a / b
picResult.Print ”The result is = “; c
Exit sub
Chapter 8 - Visual Basic Schneider

25. The error-handling routine
errorHandler:
‘ Division by zero is error code 11
If Err = 11 Then
picResult.Print "You tried to divide by 0, which is illegal"
picResult.Print "Try again.”
b=Val(InputBox(“Enter the denominator”))
End If
Resume
End Sub
Line Label
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Example:
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27. Example ( error-handling routine to handle a “division by zero” error)27
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Example: 28
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Example:
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Example:
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Example:
Chapter 8 - Visual Basic Schneider

32. file1 contains: 1 4 7 file2 contains: 5 8 9
Question:
file1 contains: file2 contains: 5 8 9
Private Sub Command1_Click()
On Error GoTo fixMe
x = "file1.txt"
kill x
Open x For Input As #1
Input #1, x
Print x
Close #1
Exit Sub
fixMe:
Print "welcome to error handling"
x = "file2.txt"
Resume
End Sub
welcome to error handling 5
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33. Chapter 8 - Visual Basic Schneider
Question:
What is the output of the following code
private sub command1_click()
x = 12
y = 6
Print x / (y - 6);
On Error GoTo myRoutine
Print x / (y - 9);
Exit Sub
myRoutine:
Print "Error";
y = y + 3
Resume
End Sub
Run time error (Division by zero)
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34. Chapter 8 - Visual Basic Schneider
Example
Chapter 8 - Visual Basic Schneider