Lesson XVIII:         Sequential File Handling

 

Objectives

ü      To be able to open and close sequential files using Open and Close

ü      To be able to detect end-of-file condition

ü      To be able to write to files using Write

ü      To be able to fetch data from files using Input

 

 

 

 

Visual Basic offers file handling capabilities and database handling features. In database handling controls, the file manipulation is transparent to the programmer and user. However, in non-database applications the programmer has to handle virtually all aspects of reading or editing the data contained in a file. This is what this lesson will cover.

 

Simple text files (also called sequential files) are often used as the storage method for information. This is because of their universally standard format.  You can simple create text files using DOS Edit^TM program and the newer Windows NotePad^TM applications.

 

To start manipulating files, you must first open it.  You do this through the following:

 

Open <filename> for <mode>  As # <filenumber>

 

Close #<filenumber>

 

Open tells Visual Basic to locate a file specified by <filename>, open it for some purpose, and in some cases, create the file if it does not exist.  <filename> can be a string literal such as “D:\myfiles\names.txt” or a string variable containing the name of the file to be opened.  <mode> tells Visual Basic what to do with the opened file.  It can be any of the following:

 

Mode	Description
Input	Opens files for reading.  An error occurs if the specified file does not exist.
Output	Opens files for writing. If the specified file does not exist, VB creates the file for writing. Note: It overwrites the contents of the file specified.
Append	Opens files for appending.  It appends new data to the contents (if there are any) of a file. If the specified file does not exist, VB creates the file.
Random	Opens files for reading or writing.

 

The pound sign (#) is optional.  The <filenumber> value (also called file channel), which can be between 1 and 255 inclusive, represents the specified file.  If you need to manipulate a file, you refer to the file by the <filenumber> to which it is associated.  The number remains associated to the file until such the time when you close the file using Close statement.  Always remember that a <filenumber> can only be associated to one file only at one time.  If the file is closed, you may use the <filenumber> again for some other file.  If you have lost track of the available <filenumber>, you may use the FreeFile() function as in the following statement:

 

FileNum = FreeFile()

The Close statement on the other hand performs the opposite job from Open.  It closes a file and releases the <filenumber> for reuse by the Open statement.  There are 2 ways in which you can use this statement.  They are as follows:

 

Close #<filenumber>, …, #<filenumber>      e.g.       Close #1           /           Close #1, #2, #7

 

Close

 

The first Close format closes one or more files as specified by the <filenumber>.  The second format close all opened files, thereby releasing all <filenumber>s.

 

 

IN FOCUS: WRITING TO FILES

 

The Write command writes data of any data type or format to an opened file.  You need to open a file for Output, Append, or Random before you can use Write.  Below is the syntax of Write:

 

Write #<filenumber>, <expressionlist>

 

<filenumber> is the number of the opened file where we will write the data.  <expressionlist> contains the value or values that you will be writing to the file.  If you don’t specify <expressionlist>, then Write writes a carriage return (ASCII 13) and line feed (ASCII 10) character to the file.   If you have multiple values in <expressionlist>, separate these values by a comma.  VB automatically writes a carriage return (ASCII 13) and line feed (ASCII 10) character after these data.

 

Let’s take the following example:

 

StudeName(0) = “Antonio”

StudeName(1) = “Heidi”

StudeName(2) = “Joni”

StudeAge(0) = 24

StudeAge(1) = 25

StudeAge(2) = 29

 

Open “C:\studerec.txt” For Output As #1

For x = 1 To 2

    Write  #1, StudeName(x), StudeAge(x)

Next x

Close #1

 

When executed, the above code will produce the following file contents:

 

“Antonio”, 24

“Heidi”, 25

“Joni”, 29

 

 

IN FOCUS: READING FROM FILES

 

File handling does not only mean writing to files.  What are files for if we do not use the contents?  Retrieving files is easy.  You do this using the Input statement through the following syntax:

 

Input #<filenumber>, <expressionlist>

 

The Input statement reads the data stored in the file into a list of variables enumerated in <expressionlist>.  Note that the <expressionlist> should match the type and number of data stored in the file.  In our example above, we wrote the name followed by the age of the student.  If we fetch these data, a sample code might be:

 

Open “C:\studerec.txt” For Input As #1

Input  #1, StdName, StdAge

Close #1

 

Variable StdName will receive the name while StdAge will receive the age.  Thus, they must be of type String and Integer respectively.  More, the order of the variables in the <expressionlist> should match the order of data in the file. The first value will be stored in the first variable, the second value on the second variable, and so on.  Errors may occur if read more data than the file holds or when you are storing a data to a variable of incompatible type (e.g. a Boolean value into an Integer variable).

 

We are not done yet.  What the code above does is simply to fetch the values on the first line: store “Antonio” to StdName and 24 to StdAge.  From this, we say that Input just fetches one line only.  So do we fetch the remaining data?  Answer: We need to do a loop through the file until we reach the end of it.  How do we know that we have reached the end of the file?  Answer: VB provides you with the EOF() function.  It returns True if we have reached the End Of File.  Therefore, what we should do is to keep on executing Input while EOF() returns False.  The following code does all these routines:

 

Open “C:\studerec.txt” For Input As #1

Do Until (EOF(#1) = True)

    Input #1 StdName, StdAge

Loop

Close #1

 

This code works but with one flaw: Every time we iterate the Input statement, we overwrite the date previously stored in StdName and StdAge. This does not serve our purpose, after all!  What we should do then is to store the contents in an array.

 

Open “C:\studerec.txt” For Input As #1

x = 0

Do Until (EOF(#1) = True)

    Input #1 StdName(x), StdAge(x)

    x = x + 1

Loop

Close #1

 

After 3 iterations, we will have the following data:

 

StdName(0)             “Antonio”           StdName(1)       “Heidi”               StdName(2)       “Joni”

StdAge(0)                24                     StdAge(1)          25                     StdAge(2)          29

 

 

Let’s create a simple file handling application.  The Form below contains a TextBox, a ListBox and 2 CommandButtons.  The “Add Name” button adds the name entered in the TextBox to the ListBox.  “Save to File” button saves the contents of the ListBox in a text file for future use.

 

 

 

 

 

 

 

 

 

 

 

 

 

Enter the following code into the code window. 

 

‘This procedure adds the name in the TextBox in the ListBox

Private Sub cmdAddName_Click()

    lstNames.AddItem txtName.Text

End Sub

 

‘This procedure saves the contents of the ListBox to a text file named names.txt

Private Sub cmdSave_Click()

    Open App.Path & "\names.txt" For Output As #1

        For x = 0 To lstNames.ListCount - 1

            Write #1, lstNames.List(x)

        Next x

    Close #1

End Sub

 

 

‘When the form is loaded, this procedure retrieves the names saved in the file (if any)

Private Sub Form_Load()

    Open App.Path & "\names.txt" For Input As #1

        While Not EOF(1)

            Input #1, temp

            lstNames.AddItem temp

        Wend

    Close #1

End Sub

 

 

 

 

 

Instructions: Modify the sample program to include not just the name but also the age of the person.  The program should also be able to delete a person and edit a person’s data.

 

 

 

 

 

 

 

 

 

 

 

 

The < and > buttons are for browsing through the student records.  When > is pressed, the program displays the records of the next student (if any).  Disable this button when there are no more succeeding records. When < is pressed, the program displays the records of the previous student (if any).  Disable this button when there are no more preceding records.  The records of the first student (the one stored at index 0 of the array), if any, should be displayed during start-up.  The TextBoxes containing the student’s name and age should be locked to avoid being edited.  Unlock these TextBoxes only when the user adds a new student (when the New button is pressed) or when the Edit button is pressed.  Disable Edit and Delete buttons when there are no displayed records.  The Save button should be disabled until the user adds a new student record.  Use two arrays StudeName() and StudeAge() to store the students’ data and store these in StudeRec.txt.