File and Error Handling

7/29/2019 File and Error Handling

1/69

-

Programming in Visual Basic 6.0

2002 The McGraw-Hill Companies, Inc. All rights reserved.

Update Edition

Chapter 10

Data Files


2/69

Programming in Visual Basic 6.0 Update Edition


2

-

Data Files

Files stored on disk device

Contain actual data

Records ==> Rows or lines

Fields ==> Data elements within row Usually stored in an organized manner

Sorted by one of the fields

Key Field, unique data item for each record


3/69



3

-

Sequential File Organization

Records end with CR, File ends with EOF

Fields separated by comma delimiters

Variable length

Strings enclosed in quotes

Numbers not enclosed in quotes Read only in the order it was written

To read a field you must read all preceding fields of all

preceding records

"Start at the beginning and continue to the end"


4/69



4

-

Sequential File Example

"Lynne","Weldon","999 Wide Way","Aiken","SC","29803"

"Jim","Buck","1 Cow Lane","Aiken","SC","29801" "Tom","Thumb","PO Box 200","Aiken","SC","29802-200"


5/69



5

-

Random File Organization

Records are fixed length and have a record

number for reference

Fields are fixed length and position

Less data then length will be padded with spaces

More data than length will be truncated Read or written in any order

Think of the structure being "like a table"


6/69



6

-

Random File Example

Lynne Weldon 803-649-9999

Jim Buckner 803-652-1111

Tom Thumb 803-593-1234


7/69



7

-

Processing Data Files

Open file

Read or Write

Close file


8/69



8

-

Open Statement

Usually coded in Form_Load

Tip: Instead of specifying entire path, always store the sequential

files in the same directory as your VB Project and use App.Path to

specify the path

Open "fully qualified path for filename" For {Input|

Output|Append|Random} As

#FileNumber[Len=RecLength]

{ } indicates required - pick one

[ ] indicates optionalFileNumber = 1 to 511, RecLength Max = 32,767


9/69



9

-

Open For

Input

reads a previously stored file from disk Output

writes file to disk beginning at BOF, overwritingprevious records

Append Writes file to disk beginning at existing EOF

Random Reads or writes random files


10/69



10

-

Open Statement Examples

Sequential

Open "C:\VB6\myfile.txt" For Input As #6

Open "A:\myfile.txt" For Output As #1

Open App.Path & \myfile.txt " For Append As #2

Random

Open "A:\myfile.txt" For Random As #3 Len=60

P i i Vi l B i 6 0 U d t Editi


11/69



11

-

Actions Occurring Upon Open

Directory is checked for file

Does not exist - thencreate unless opened for input where itmust already exist

Buffer in memory established Len determines buffer size

When buffer is full VB writes data to disk File Pointer is created for current location

Located at BOF for Input, Output, and Random

Located at EOF for Append

File Number established Must be unique while file open

P i i Vi l B i 6 0 U d t Editi 12


12/69



12

-

Locating a File

Problem: Open statement requires fully qualified

path for filename

Alternative 1: Require that all data files be saved in the same

directory as the VB project

Use App.Path to specify the path as follows,Dim strPath as StringstrPath=App.Path & "\Filename.extension"Open strPath for Input as #4

Alternative 2: Use Common Dialog with ShowOpen method to

display Open dialog for user to locate file



13/69



13

-

Locating a File-Using Common Dialog

Dim strFileName as String

With dlgCommon.DialogTitle = "Open".CancelError = False.Filter = "All Files (*.*)|*.*"

.ShowOpenIf Len(.FileName) = 0 ThenExit Sub

End IfstrFileName=.FileName

End WithOpen strFileName for Input as #1



14/69



14

-

Close Statement

Terminates processing of a disk file

Usually coded in Form_Unload,

mnuFileSave, mnuFileSaveAs, mnuFileExit

Close [#FileNumber]

[] indicates optional

If FileNumber is omitted then all open files are closed

Programming in Visual Basic 6 0 Update Edition 15


15/69



15

-

Close Statement Examples

Close #6

Close #1, #3

Close



16/69



16

-

Actions Occurring Upon Close

Physically writes the last partially filled buffer to

disk for sequential files (Write statement hasplaced data in the buffer)

Writes EOF

Releases the Buffer

Releases the FileNumber

Note: END closes all open files but is not the best way toclose files. You should close files explicitly!



17/69



17

-

Getting Next Available FileNumber

FreeFile Function

System assigns next available number

Advantages

Never have FileNumber conflicts

Don't have to keep track of FileNumbers

Dim intFileNumber as Integer

intFileNumber=FreeFile

Open "Names.txt" For Output As #intFileNumber


18/69-



Update Edition

Sequential Files



19/69



19

-

Reading a Sequential File

Open an existing file for Input

Use Input Statement to read the records



20/69



20

-

Input Statement


If populating a listbox or combo use a Do

Until EOF to read data

Input #FileNumber,List of variables

Input #2,strCourseNum, strClass, intHours



21/69



21

-

Input Statement (cont.)

If populating a listbox or combo use a

Do Until EOF to read in Form_Load

Dim strVideoTitle as String

Do Until EOF(3)

Input #3,strVideoTitle

cboVideoTitle.AddItem strVideoTitleLoop

Notice numbers match!



22/69



22

-

Write Statement

Use to place/save data in a sequential file

File must already be opened "For Output"

Place code in mnuFileSave, mnuFileCloseor Form_QueryUnload

Follow with a Close statement

Programming in Visual Basic 6.0 Update Edition 23


23/69



23

-

Write Statement (cont.)

Example

Write #FileNumber,List of variable or fields

Write #1,txtFirstName, txtLastName,

txtStreet, txtCity, txtxState, txtPhone

Write #2,strCourseNum, strClass,

intHours



24/69



24

-

Write Statement (cont.)

If populating a listbox or combo use a

For Next structure to write/save data

DimintIndexas Integer

Open "A:\videos.txt" For Output As #1

ForintIndex=0 tocboVideoTitle.ListCount-1

Write #1,cboVideoTitle.ListIndexNextintIndex

Close #1



25/69



25

-

Deleting a Sequential File

Kill statement

File cannot be open when Kill is used

Kill "fully qualified path for filename"

Kill "A:\Names.txt"



26/69

g g p


26

-

Renaming a Sequential File

Name statement

File cannot be open when Name is used

Name "oldfully qualified path for filename"

As "newfully qualified path for filename"

Name "A:\Names.txt" As "A:\People.txt"



27/69

g g p

2002 The McGraw-Hill Companies, Inc. All rights reserved.-

Updating/Overwriting a Sequential File

By updating we mean "saving changes made" Open the original file for Input

Open a temp file for Output

Input a record from the original into text boxes or other

controls on a form that the user can change Write out the record to the temp file after the user changes

the data

Close both files

Kill the original file

Rename the temp file as the original file



28/69

g g p


Inserting Records At the EOF

Open the file for Append

Allow the user to enter new data using

textboxes or other controls on a form

Write the data once a complete record of

data has been entered

Close the file after all new records have

been inserted



29/69

g g p


Creating a New File

Open the file for Output

If the filename does not exist VB will create

the file

Write data to the file from textboxes or

other controls the user enters data into

Close the file



30/69


Saving Changes to a File

Standard practice is to ask the user if they want to

save the changes before program termination Set up a module level Boolean variable to identify if

changes have been made

In Form_QueryUnload event

Check module level Boolean variable to see if changes havebeen made

If TRUE ask the user if they want to save

If user responds "yes", save

Code example p 403


31/69-



Update Edition

Trapping Program Errors



32/69


Error Handling

Trap run-time errors rather than having user

deal with them or program terminating Some problems cannot be avoided - BUT

they must be anticipated

Drive or printer not functioningImproperly formatted disk

File not found



33/69


Most Common Run-Time Errors

11 Division by 0

13 Type mismatch***482 Printer error

53 File not found

61 Disk full

68 Device unavailable

71 Disk not ready

75 Path/file access error

76 Path not found



34/69


When a run-time error occurs

VB generates an error number

VB checks the number against a table of

known error codes

Programmer can intercept the error codeand take action before VB terminates the

project



35/69


Error Trapping Steps

Turn on the error-handling feature using OnError statement in subprocedure

Create error handling code routines

Set them off from other code with line labels

Write code to continue after the error is"handled"



36/69


On Error Statement

Use this statement at the beginning of a

procedure to activate error trapping Designate a line label in the same procedure

to go to if an error occurs

Line labels - begin in column 1, end with colon:

On Error GoTo ErrorHandlerRefers to

line label



37/69


Error Handling Code

Precede with Exit Sub statement (or Exit

Function) Check the error number(s)

Single error number -- If structure

Multiple error numbers -- Select Case

Inform user if necessary Designate next line of code to execute

Resume

Resume Next

Resume line label



38/69


Resume

What line of code should be executed after the

error has been handled?

Resume - line of code that caused error

Resume Next - line of code that would logically

be executed after the line of code that caused error

Resume line label- line with indicated label



39/69


Err Object

Intrinsic VB object (like Printer object you used

in 1st semester VB class) Properties

Number - error number, 0 to 65,535

Source - object or application that caused error

Description

Method

Raise - set an error number and/or cause it to occur



40/69


Error Handling Standards

Use Resume if you identify the problem and the

user could correct it Use Resume Next if you identify the problem

and execution can proceed without running the

error generating line of code

Raise the error again (Err.Raise Err) if you

cannot identify the problem so VB will handle it

and generate a system error message



41/69


Error Handling Standards (cont.)

Use Resume line labelif you want to exit the

procedure

Call you exit procedure (perhaps, mnuFileExit)

to end without displaying any error message

Turn off error handling/trapping with:

On Error GoTo 0



42/69


Handling One Error Number

Private Sub Whatever( )On Error GoTo ErrorHandler

code to do whatever this subprocedure does

Exit Sub

ErrorHandler:

If Err.Number=71

msgbox to inform user of error for

correction

Resume

ElseErr.Raise Err

End If

End Sub



43/69


Handling Multiple Error Numbers

Private Sub Whatever( )

On Error GoTo ErrorHandlercode to do whatever this subprocedure

does

Exit Sub

ErrorHandler:

Select Case Err.NumberCase 71

Msgbox

Case 53, 76

Msgbox

Case Else

Err.Raise Err

End Select

Resume

End Sub


44/69

-



Update Edition

Random Files


R d Fil O i i


45/69


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

Read or written in any order

Think of the structure being "like a table"


R d Fil E l


46/69


Random File Example

Lynne Weldon 803-649-9999

Jim Buckner 803-652-1111

Tom Thumb 803-593-1234


D fi i th R d St t


47/69


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 statementEx: Dim strFName as String * 20

Number variables do not require explicit

length


T /E d T


48/69


Type/End Type

Private Type Person

intEmpNum AsInteger

strFName AsString* 20

strLName AsString*30strPhone AsString*12

curRate AsCurrency

End Type

Dim mudtPersonRecord As PersonOpen App.Path & "\Names.dat"For Random as #1

Len=Len (mudtPersonRecord)

Note: mudt prefix for User Defined Type


O St t t


49/69


Open Statement

Once the random file is opened it can beused for both input and output unlikesequential files!

If you open a file that does not exist, VBwill create it as an empty file

Once opened, data are available forread/write operations one record at a time


R di R d Fil


50/69


Reading a Random File

Open an existing file for Random

Use Get Statement to read the records


G t St t t


51/69


Get Statement


If populating a listbox or combo use a Do

Until reach last record to read data

Get #FileNumber, [RecordNumber],RecordName

Get #2, 4, mudtPersonRecordGet #2,intRecordNumber, mudtPersonRecord

[ ] indicates optional, ifRecordNumber is omitted, the next record is read


Put Statement


52/69


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


P t St t t E l


53/69


Put Statement Example

Put #FileNumber, [RecordNumber],RecordName

Put #2,4, mudtPersonRecord

Put #2,intRecordNumber, mudtPersonRecord

[ ] indicates optional, ifRecordNumber is omitted, the next

record is read


Accessing the Fields


54/69


Accessing the Fields

Get and Put operate on an entire record

Reference the individual fields using dot

notation for the User Defined Type


Accessing the Fields ( t )


55/69


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


LOF Function


56/69


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)


Seek Function


57/69


Seek Function

Returns the current location of the pointer =

the next record in the file to be processed

Seek(FileNumber)

intNextRecord=Seek(3)


Trim Functions


58/69


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)


Reading/Retrieving Records Sample Code


59/69


Reading/Retrieving Records Sample Code

Sub GetRecord(lngRecNum as Long)

Get #1, lngRecNum, mudtPersonRecordWith mudtPhoneRecordtxtEmpNum.Text = .intEmpNumtxtFName.Text = RTrim(.strFName)txtLName.Text = Rtrim(.strLName)txtPhone.Text = Rtrim(.strPhone)txtRate.Text = .curRate

End WithEnd Sub


Writing Records Sample Code


60/69


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 WithPut #1, lngRecNum, mudtPersonRecord

End Sub


Using ListBox to Store Random File Key Field (p 413)


61/69


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, readits ItemData property to retrieve the desired

record


Updating a Random File


62/69


Updating a Random File

Create routines to

Edit existing records

Add new records

Delete existing records


Edit Existing Records


63/69


Edit Existing Records

Display the record

Disable all command buttons except Save

and Cancel

Lock text boxes for fields containing data

you do not want the user to modify


Add New Records


64/69


Add New Records

Clear all text boxes on form

Disable all command buttons except Save

and Cancel

When user clicks Save write the new record

at the end of the file

If using a list box to store record numbers,

update listbox by adding new data and

record number


Delete Existing Records


65/69


Delete Existing Records

No way to delete a record in place

Various methods are used to indicate that a record is to betreated as "deleted"

Write a special character in an existing field to indicate deleted

Include a "Delete Code" field in the record description and mark it

True to indicate deleted

If using a list box to store record numbers, update listbox

by deleting the record's associated data and record number


66/69

-



Update Edition

Programming Hints


InputBox Function


67/69


InputBox Function

Used to request simple input from user

Similar in syntax and usage to the MsgBoxfunction

Includes

prompt to inform user of desired input

text box for user to enter input

OK and Cancel command buttons


InputBox Syntax


68/69


InputBox Syntax

VariableName= InputBox("Prompt"[, "Title"]

[,Default] [, XPos] [, YPos])

Where: Prompt - displays in the dialog to inform the user of what to

input

Title - displays in the title bar of the dialog

Default - default value than displays in the text box of the

dialog XPos - Horizontal position of the top left corner of the dialog

YPos - Vertical position of the top left corner of the dialog


InputBox Examples


69/69

InputBox Examples

strName= InputBox ("Enter Your Name")

intQuan= InputBox ("How many do you want?", _

"Order Quantity", 1)

File and Error Handling

Documents

Transcript of File and Error Handling