Chapter 8
Sequential Files
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Outline and Objective
• Creating Sequential Files
• Adding items to Sequential Files
• Using Sequential Files
– Sorting Sequential Files
– Merging Sequential Files
– Control Break Processing
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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.
– Holds Data about a single individual
• A record is collection of fields.
– Individual item
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Creating a Sequential File
•
•
•
•
•
Choose a file name
Choose a reference number
Open the file for Output
Write to the file
Close the file
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Example:
Private Sub cmdCreateFile_Click()
Dim name1 As String, name2 As String
' Demonstrate use of Write # statement
Reference number
Open "PIONEER.TXT" For Output As #1
Write #1, "ENIAC"
File name
Write #1, 1946
Write #1, "ENIAC", 1946
Write to the file
name1 = "Eckert"
name2 = "Mauchly"
Write #1, 14 * 139, "J.P. " & name1, name2, "John"
Close #1
Close the file
End Sub
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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
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Example (Adding a player to a BASEBALL1.TXT file)
Private Sub Form_Load()
Open App.Path & "\BASEBALl.TXT" For Append As #1
End Sub
Opening the file in append mode
Private Sub cmdAddRec_Click()
Write #1, txtPlayer.Text, txtTimes.Text, txtHits.Text
txtPlayer.Text = “ “
txtTimes.Text =“ ”
txtHits.Text=“ ”
txtPlayer.SetFocus
End Sub
Private Sub cmdQuit_Click()
Close #1
End
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End Sub
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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.
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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”
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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.
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Error Numbers
Type of Error
Value of Error Number
Subscript out of range
9
Division by zero
11
File not found
53
File already open
55
File already exists
58
Disk full
61
Disk not ready
71
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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
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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
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The error-handling routine
ErrorHandler :
Line Label
‘ 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
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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.
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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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