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Understanding Sequential Files in Visual Basic

Learn how to create, add items, sort, merge, and handle errors in sequential files in Visual Basic with detailed examples and best practices. Understand different file operations and common errors to avoid.

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Understanding Sequential Files in Visual Basic

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

  2. Outline and Objective • Creating Sequential Files • Adding items to Sequential Files • Using Sequential Files • Sorting Sequential Files • Merging Sequential Files Chapter 8 - Visual Basic Schneider

  3. 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 20 22 Huda 20 22 record Ahmed 20 22 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 Chapter 8 - Visual Basic Schneider

  5. Example: Private Sub cmdCreateFile_Click() Dim name1 As String, x as Integer ' Demonstrate use of Write # statement Open “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. Example: Private Sub cmdCreateFile_Click() Dim name1 As String, name2 As String Open “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 Creates a new file for output Write the string CS surrounded by quotation marks into the file Write the number 116 without any leading or trailing space into the file “CS” 116 “BASIC",10,20,”One ” 6,“ID Student”,”Test” Chapter 8 - Visual Basic Schneider 6

  7. Chapter 8 - Visual Basic Schneider

  8. Chapter 8 - Visual Basic Schneider

  9. Chapter 8 - Visual Basic Schneider

  10. 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 Chapter 8 - Visual Basic Schneider

  12. 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 Chapter 8 - Visual Basic Schneider

  13. Sequential File • Different modesin which a file can be used: • Output • Input • Append • A file should not be open in two different modes at the same time. Chapter 8 - Visual Basic Schneider

  14. Incorrect Code Private Sub Command1_Click() Open "file.txt" For Output As #1 Open "file.txt" For Append As #1 Write #1, "col1", "col2" Close #1 Close #1 End Sub Chapter 8 - Visual Basic Schneider

  15. Incorrect Code Open "file.txt" For Output As #1 Write #1, "Number", "Name" Open "file.txt" For Append As #2 Write #1, 123, "Ali" Close #1 Close #2 Chapter 8 - Visual Basic Schneider

  16. Correct Code Private Sub Command1_Click() Open "file.txt" For Output As #1 Write #1, "Number", "Name" Close #1 Open "file.txt" For Append As #1 Write #1, 123, "Ali" Close #1 End Sub Chapter 8 - Visual Basic Schneider

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

  18. Chapter 8 - Visual Basic Schneider

  19. 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

  20. 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. Chapter 8 - Visual Basic Schneider

  21. 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

  22. 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” Chapter 8 - Visual Basic Schneider 22

  23. before After Chapter 8 - Visual Basic Schneider

  24. After before Chapter 8 - Visual Basic Schneider

  25. 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

  26. 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

  27. 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 Chapter 8 - Visual Basic Schneider

  28. 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 Chapter 8 - Visual Basic Schneider

  29. Chapter 8 - Visual Basic Schneider

  30. Example( error-handling routine to handle a “division by zero” error) Chapter 8 - Visual Basic Schneider 30

  31. Chapter 8 - Visual Basic Schneider 31

  32. Chapter 8 - Visual Basic Schneider

  33. Chapter 8 - Visual Basic Schneider

  34. Chapter 8 - Visual Basic Schneider

  35. Question: file1 contains: 1 4 7file2 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 Chapter 8 - Visual Basic Schneider

  36. 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) Chapter 8 - Visual Basic Schneider

  37. Example Chapter 8 - Visual Basic Schneider

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