ISM 3230 Introduction to Programming

 Chapter 10: Data Files

Data Organization Hierarchy:

Data Files

• Data files consist of records and fields
• Record key determines a record’s position in a file
• Two common file organizations are sequential and random
• Process a file by

Opening Files

• When you open a sequential file, the first few records are read automatically and placed in memory to speed processing.
• These records are stored in a buffer for later processing.
• The goal is to always have records available in high speed memory rather than having to read, in real time, from relatively slow I/O devices.

Open statement’s form:

• Open "filename" For {Input|Output|Append|Random} as #Filenumber [Len= No.]

Examples:

Open "A:\DataFile.Dat" For Input As #1

Open stFileName For Input As #intFileNumber

• Four File Modes:
  • Output Writes at front of file, overwriting any extant data
  • Input Reads data from disk
  • Append Writes new data at end of file
  • Random Read/Write data to specific location & in any order

Close statement form: Close [#filenumber]

• Close #1 or Close #1, #2
• Close issued automatically when you end
• Issue Close statement before leaving prog.
• Close empties the output buffer, writes an EOF (end-of-file) mark, releases buffer, and releases the file number (so it can be reused, if necessary)

FreeFile Statements

• FreeFile function returns the next unused file number
• intFileNumber = FreeFile
• In large projects, file numbers are hard to track and allocate

Locating a File Number

• To locate an available file number for a large project, always use the FreeFile function to locate an unused file number.
• You don’t care what number is chosen—only that it is available.
• Store the retrieved file number in a module or global variable so other forms can manipulate the files, if necessary.

Obtain an available file number this way:

Dim intFileNum As Integer

intFileNum = FreeFile ‘Obtain next available file number

Open "File.dat" For Output As #intFileNum

• Whenever you reference the preceding file, you do so with the intFileNum variable rather than a literal (number).

Sequential File Organization

• Sequential file records are stored one after another
• Use Write to output data
• Use Input to read data
• File must be opened prior to first use
• Form:

  Write #fn, item₁, item₂,…, item_n

• EOF function signals end of file

Reading a Sequential File

• Form: Input #fn, item₁,…,item_n
• Separate fields items with commas
• Input statement reads sequential data

  Input #1, lblName.Caption, lblStreet.Caption

Input #2, stAccount, stDescription, cPrice

• File number must be that of an open file
• When you read the last record, end-of-file signals
• You detect end of file with the EOF function: EOF(FileNumber)

Example: reading sequential data file and populating two list boxes:

Do Until EOF(intFileNum)

Input #intFileNum, stName, stPhone

lstName.AddItem stName ‘add name to a list box

lstPhone.AddItem stPhone ‘add phone to a list box

Loop

Close #intFileNum ‘close the input file

End of File (EOF)

• When reading a sequential file, the EOF condition is raised when you attempt to read the n+1st record (n records total).
• The EOF function is used to determine when EOF occurs
• EOF form: EOF(filenumber)
• EOF returns false until end of file is reached.

• Typical read loop:

  Do Until EOF(1)

Input #1, strCofFlavor, strRoastName, strRoastCity

Loop

• If the file contains five records, then EOF is raised on the fifth input execution, not on the sixth Input.

Writing to a Sequential File

• Write statement places data in output
• Form: Write #file, o₁, o₂, …, o_n
• You can separate fields with commas or semicolons
• Output fields are separated with commas, and strings are quoted.
• <Cr>/<Lf> ends each record

  Write #1, txtLname.Text, 54.89

• Output fields are placed one after another and end with <CR>
• If you create a form to add records to a file, the Open statement is commonly placed in the form Load event procedure
• Whereas, the write statement is placed in the click procedure of an Add button.

Trapping Program Errors

• Errors may be trapped asynchronously
• Visual Basic generates an error number whenever an error occurs
• To handle errors, you must
• Turn on error handling feature: On Error...
• Create error handling code
• Determine what is to be done after processing the error (continue, end program,…)

Examples of Handling Errors

• Place On Error in any procedure that is to traps and processes errors
• Example:

Private Sub Form_Load()

On Error GoTo HandleErrors 'Turn on error trapping

Open "A:\Testdata.dat" For Input As #1

… (more code here)

Exit Sub 'exit to avoid executing error routine

HandleErrors:

‘some code to handle error

Resume

End Sub

• A line label is a name followed by a colon on a line by itself
• HandleErrors is a line label
• Forms of On Error statement:
• On Error GoTo linelabel transfer control to label in module
• On Error Resume Next skip error line & continue with next
• On Error GoTo 0 turns off error handling

The Err Object

• The Err object holds information about error that just occurred
• Err.Source holds the source of the error
• Err.Number holds the error number
• Err.Description contains error description
• You can raise an error condition—turn on an error—with:

  Err.Raise Number:=xx

Error Numbers

Err.Number	Err.Description
7	Out of memory
9	Subscript out of range
11	Division by zero
13	Type mismatch
52	Bad filename or number
53	File not found (fnf)
54	Bad file mode
58	File already exists
61	Disk full
67	Too many files
68	Device unavailable
70	Permission denied
71	Disk not ready
75	Path/file access error
76	Path not found
482	Printer error

Example: Raise error 76 in program code:

Err.Raise Number:=76

Coding Error-Handling Routines

• On Error statement designates error handler
• Code to handle errors follows line label
• Line label appears on line by itself and ends with a colon
• A Case statement works well to sort out errors with a "switchboard"
• Continue execution with Resume statement

Using Case to handle errors

• Using Case to handle errors with a "Switchboard":

  Select Case Err.Number

  Case 53 'Handle error # 53 here

  …

  Case 71 'Handle error # 71 here

  ...

  Case Else 'Handle all other errors here

  …

  End Select

Resume statement forms:

• Resume continues with line causing error (careful!)
• Resume Next continues with line following one that caused error
• Resume linelabel continues at line label, which must be in same proc

Exit and Exit Sub Statements

• Exit statement jumps out of structure--a function, subroutine, or loop
  • Exit should be placed before line label of error handling routine so normal program flow doesn't drop through the error code.
• Exit Sub immediately exits current sub procedure
• Exit Function immediately exits current function
• Exit is used to prematurely halt execution of sub procedure or function when extraordinary conditions occur
• Place Exit Sub above error handling label

Saving Changes to a File

• When data changes, ask users if they want to save the changes before program ends
• Changed data is known as "dirty" data
• Keep track of data changes with a global boolean flag
• Any procedure that allows data to change should set the flag to True—indicating "dirty" data file
• Check file just before ending program

Unload and End

• You should Unload all forms before executing an End statement.
• This triggers each form’s Unload event (and each Class’s Terminate event), giving you control at that point to check for "dirty" data files.

QueryUnload

• The best way to ask the user if they want to save changes is in the QueryUnload event procedure.
• The QueryUnload event procedure obtains control when the (what else!) QueryUnload event occurs.
• QueryUnload occurs when the user:
  • clicks an Exit button or menu command to exit
  • clicks on a window’s Close button
  • or exits Windows altogether

   

QueryUnload procedure example:

Private Sub Form_QueryUnload(Candel As Integer, UnloadMode As Integer)

Dim intResponse As Integer

If mblnIsDirty Then

IntResponse = MsgBox("Data has changed. Save it?", _

vbYesNo + vbQuestion, "Coffee List Changed Warning")

If intResponse = vbYes Then

mnuFileSave_Click

End If

End If

End Sub

Sequential File Prog. Example

• You can load a combo box by reading from a data file and executing the AddItem method
• Reading file & list filling halts when EOF occurs on input file
• (See Hands On Programming example)

Random data files

• You can read/write data in any order
• Open "filename" For Random As #1 LEN=x
• Get #filenumber, [record#], RecordName
• RecordName is a user-defined data type:

• Get and Put statements read an entire record
• You refer to fields in user-defined structure by recordname.fieldname

lstName.AddItem mRec.LastName

• end of file is calculated via record lengths
• Output: Put

Put #1, iRecordNo, pCustomerRecord

LOF - length of file

• LOF function used to locate end of file for a random file

LOF(filenumber)

• To find out how many records a file has, divide the file length by the record length

iNumberRecords = LOF(1) / Len(mMemberRecord)

• mMemberRecord is a user defined record type
• Len function returns the length of a single record

 

• The number of records in a random file can be used to drive a For/Next loop
• Remember, a random file DOES NOT have an end of file mark in the traditional sense
• Next record number (at end of file) is also calculated:

iRecordNumber = LOF(1) / Len(mMemberRecord) + 1

• The Seek function returns the current location of the file pointer—which record is the current record.
• Form: Seek(filenumber)

InputBox

• An alternative to a textbox to get input from user
• InputBox is a function that prompts the user for data and then returns the value
• Similar to MsgBox, but returns data which can then be assigned to a variable

Form:

VariableName = InputBox("Prompt", [... optional parameters] )

strName = InputBox("Enter your name.")

Chapter Summary:

OBJECTS		PROPERTIES		METHODS		STATEMENTS		FUNCTIONS
Err		Err.Number		Raise		Close		EOF
		Locked				Exit Sub		FreeFile
		Number				Exit Function		InputBox
		Source				Get		LOF
						Input		LTrim
						On Error		RTrim
						Open		Seek
						Put		Trim
						Resume
						Write