Chapter 8 - Visual Basic Schneider1 Chapter 8 Sequential Files

Chapter 8 - Visual Basic Schneider2 Outline and Objective Creating Sequential Files Adding items to Sequential Files Using Sequential Files –Sorting Sequential Files –Merging Sequential Files

Chapter 8 - Visual Basic Schneider3 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. Ali Name First second Huda Ahmed record field

Chapter 8 - Visual Basic Schneider4 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 Schneider5 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 File name Reference number Write to the file Close the file

Chapter 8 - Visual Basic Schneider66 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 “CS” 116 “BASIC",10,20,”One ” 6,“ID Student”,”Test” Write the string CS surrounded by quotation marks into the file Write the number 116 without any leading or trailing space into the file Creates a new file for output

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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 Schneider11 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 Schneider12 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 Schneider13 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.

Chapter 8 - Visual Basic Schneider14 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 End Sub

Chapter 8 - Visual Basic Schneider15 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 Schneider16 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 Schneider17 ERROR: A file should not be open in two different modes at the same time.

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Chapter 8 - Visual Basic Schneider19 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 Schneider20 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 Schneider21 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 Schneider22 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” 22

Chapter 8 - Visual Basic Schneider23 before After

Chapter 8 - Visual Basic Schneider24 before After

Chapter 8 - Visual Basic Schneider25 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 Schneider26 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 Schneider27 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 Schneider28 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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Chapter 8 - Visual Basic Schneider30 Example ( error-handling routine to handle a “division by zero” error)

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Chapter 8 - Visual Basic Schneider35 file1 contains: file2 contains: 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 Question:

Chapter 8 - Visual Basic Schneider36 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 Schneider37 Example