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Data Files PowerPoint Presentation

Data Files

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Data Files

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  1. Data Files • In all your projects so far, the user has entered information through text-boxes • This input method is unsatisfactory for large quantities of data • Many computer applications require data to be saved from one run to the next e.g. inventory records, customer files etc.

  2. File Organisation • The manner in which data are organised,stored and retrieved. • 2 Common file organisations: • Sequential • Random

  3. Opening and Closing Data Files • Open the file. Before any data may be placed on the disk or read from the disk, the file must be opened. Generally done in Form_Load procedure. • Read or Write the data records. • Close the file.

  4. Open Statement Open “Filename” For {Input/Output/Append/Random} As #FileNumber • The Open statement elements shown in the chain brackets are the file mode, indicating the way that you will access the file. This is a required entry, so one of the modes must be chosen. • The first three choices are used for sequential files.

  5. Open Statement • The FileNumber may be from 1 to 511 • Record Length may be up to 32,767 characters Open “A:\DataFile.dat” For Output As #1 Open “C:\VB6\Names.txt” For Input As #2

  6. FreeFile and Close Statements • FreeFile function returns the next unused file number • intFileNumber = FreeFile • In large projects, file numbers are hard to track and allocate • Close issued automatically when you end • Issue Close statement before leaving prog. • Close #1 or Close #1, #2

  7. 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.

  8. Obtain an available file number this way: • Dim intFileNumb As Integer • intFileNumb = FreeFile ‘Obtain next available file number • Open “File.dat” For Output As #intFileNumb • Whenever you reference the preceding file, you do so with the intFileNumb variable rather than a literal (number).

  9. Example: Private Sub cmdCreateFile_Click() Dim name1 As String, name2 As String ' Demonstrate use of Write # statement Open "PIONEER.TXT" For Output As #1 Write #1, "ENIAC" Write #1, 1946 Write #1, "ENIAC", 1946 name1 = "Eckert" name2 = "Mauchly" Write #1, 14 * 139, "J.P. " & name1, name2, "John" Close #1 End Sub Reference number File name Write to the file Close the file

  10. 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,…, itemn Input #fn, item1, item2,…, itemn • EOF function signals end of file

  11. 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. • Input statement reads sequential data • Input #1, lblName.Caption, lblStreet.Caption • Input #2, stAccount, stDescription, cPrice • When reading a sequential file, the EOF (End Of File) condition is raised when you attempt to read the n+1st record (n records total).

  12. The EOF function is used to determine when EOF occurs • EOF form: EOF(filenumber) • Example reading sequential data file and populating two list boxes: Do Until EOF(intFileNumb) Input #intFileNumb, stName, stPhone lstName.AddItem stName ‘add name to a list box lstPhone.AddItem stPhone ‘add phone to a list box Loop Close #intFileNumb ‘close the input file

  13. Writing to a Sequential File • Write statement places data in output • Form: Write #file, o1, o2, …, on • 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

  14. Example (Adding a player to a BASEBALL1.TXT file) Private Sub Form_Load() Open App.Path & "\BASEBALl.TXT" For Append As #1 End Sub 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 End Sub Opening the file in append mode

  15. Reading Data in a Sequential File • Form: Input #fn, item1,…,itemn • Separate fields items with commas • 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)

  16. 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.

  17. 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,…)

  18. 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

  19. 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 GoTolinelabel transfer control to label in module • On Error Resume Next skip error line & continue with next • On Error GoTo 0 turns off error handling

  20. 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

  21. 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 • Raise error 76 in program code: Err.Raise Number:=76

  22. 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

  23. 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

  24. 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 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.

  25. Exit and Exit Sub Statements • 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

  26. 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

  27. 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. • 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

  28. QueryUnload procedure example: Private Sub Form_QueryUnload(Cancel 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

  29. 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

  30. Random Access Files • A Random Access file is like an array of records stored on a disk. • The records are numbered 1,2,3, and so on, and can be referred to by their numbers. • Therefore, a Random Access File resembles a box of index cards, each having a numbered tab. Any card can be selected from the box without first reading every index card preceding it; similarly any record of a random access file can be read without having to read every record preceding it.

  31. Random File Organization • Records are fixed length and have a record number (data type=long integer) for reference • Fields are fixed length and position • Less data then length will be padded with spaces • More data than length will be truncated

  32. Random File Example • Read or written in any order • Think of the structure being "like a table"

  33. Defining the Record Structure • Must be done before reading or writing • Use Type/End Type statements • Code in General Declarations • Use fixed length strings • Specify length in Dim statement • Ex: Dim strFName as String * 20 • Number variables do not require explicit length

  34. Type/End Type Private Type Person intEmpNum AsInteger strFName AsString* 20 strLName AsString*30 strPhone AsString*12 curRate AsCurrency End Type Dim mudtPersonRecord As Person Open App.Path & "\Names.dat"For Random as #1 Len=Len (mudtPersonRecord) Note: mudt prefix for User Defined Type

  35. Open Statement • Once the random file is opened it can be used for both input and output unlike sequential files! • If you open a file that does not exist, VB will create it as an empty file • Once opened, data are available for read/write operations one record at a time

  36. Reading a Random File • Open an existing file for Random • Use Get Statement to read the records

  37. Get Statement • Usually coded in Form_Load • If populating a listbox or combo use a Do Until reach last record to read data Get #FileNumber, [RecordNumber],RecordName Get #2, 4, mudtPersonRecord Get #2,intRecordNumber, mudtPersonRecord [ ] indicates optional, if RecordNumber is omitted, the next record is read

  38. Put Statement • Use to place/save data in a random file • File must already be opened "For Random" • Place code in mnuFileSave, mnuFileClose or Form_QueryUnload • Follow with a Close statement

  39. Put Statement Example Put #FileNumber, [RecordNumber],RecordName Put #2,4, mudtPersonRecord Put #2,intRecordNumber, mudtPersonRecord [ ] indicates optional, if RecordNumber is omitted, the next record is read

  40. Accessing the Fields • Get and Put operate on an entire record • Reference the individual fields using dot notation for the User Defined Type

  41. Accessing the Fields (cont.) • Read using Get then update textbox Get #2, 1, mudtPersonRecord txtLName.Text = mudtPersonRecord.strLName • Update field from textbox the Write using Put: mudtPersonRecord.strPhone = txtPhone.Text Put #2, 1, mudtPersonRecord

  42. LOF Function • Length of File function returns the size of the file in bytes • Use instead of EOF used for sequential files • To determine the highest record number in the file divide LOF by the size of one record LOF(FileNumber) LOF(3) intNumRecords=LOF(3)/Len(mudtPersonRecord)

  43. Seek Function • Returns the current location of the pointer = the next record in the file to be processed Seek(FileNumber) intNextRecord=Seek(3)

  44. Trim Functions • Remove extra blank spaces in a string • Trim ==> removes spaces from both ends • LTrim==> removes spaces at left end • RTrim ==> removes spaces at right end Trim(String) LTrim(String) RTrim(String) txtLName.Text=RTrim(mudtPersonRecord.strLName)

  45. Reading/Retrieving Records – Sample Code Sub GetRecord(lngRecNum as Long) Get #1, lngRecNum, mudtPersonRecord With mudtPhoneRecord txtEmpNum.Text = .intEmpNum txtFName.Text = RTrim(.strFName) txtLName.Text = Rtrim(.strLName) txtPhone.Text = Rtrim(.strPhone) txtRate.Text = .curRate End With End Sub

  46. Writing Records –Sample Code Sub PutRecord(lngRecNum as Long) With mudtPhoneRecord .intEmpNum=Val(txtEmpNum.Text) .strFName = txtFName.Text .strLName = txtLName.Text .strPhone = txtPhone.Text .curRate = Val(txtRate.Text) End With Put #1, lngRecNum, mudtPersonRecord End Sub

  47. Using ListBox to Store Random File Key Field (p 413) • When you Get or Put a record you need to know the record number • To keep track of record numbers store them in ItemData property of ListBox • When user selects a value from the list, read its ItemData property to retrieve the desired record

  48. Updating a Random File • Create routines to • Edit existing records • Add new records • Delete existing records