Component 10a, Slide 1 CP2030 Copyright © University of Wolverhampton CP2030 Visual Basic for C++ Programmers v Component 10 – Working with Binary Files.

Component 10a, Slide 1CP2030 Copyright © University of Wolverhampton

CP2030Visual Basic for C++ Programmers

Component 10

– Working with Binary Files

– Error handling

– Working with other applications


Aims and objectives

Understand the structure of Binary Files Use the various techniques available in

Visual Basic to write to binary files. Understand how to “launch” and interact

with other applications from within V.B.


VB data files

VB supports three kinds of data files

– Sequential files store data as ACII text. Characters are read from and stored to the file character by character

– Random-access files store data in special VB internal formats. Such files require rigid file structure

– Binary files store data as individual bytes. No file structure exists, and no special internal formats are assumed

we will look at Binary files this week


Binary files

Sequential and random access files have a certain degree of organisation of the data stored within them. Whereas VB treats binary files as just a file consisting of a stream of bytes

Treating files as binary files is useful to handle and interpret files created in formats alien to VB such as spreadsheets or wordprocessors, as you can read, or alter any bytes including ones that are control characters.

VB only sees the data as bytes and so any interpretation of what the each Byte means must be written into the program


Three steps to process a Binary File

Use the following three steps to process binary files

1 open the File For Binary

2 Use a record variables in Get and Put instructions to read and write the data

3 Close the file


1st step - opening the file

The syntax is as follows:

Open filename For BinaryAs #filenum

where filename is the filename and path

#filenum is the filehandle number

e.g. applying this to a WORD document

Open “c:\temp\letter.doc” For Binary As #1


2nd step - writing or reading to the file

We can use the Put statement to write to a binary file record and the Get statement to read from the file

It has the form

Get #filenum , startbyte , variable

Put #filenum , startbyte , variable

Put #1 , 12 , iCount

- startbyte is a numeric expression specifying the byte in the file where i/o begins - variable is any variable used to transmit or receive the data values


File position pointer

A file position pointer is associated with each binary file

identifies which byte is to be written to or read from

– first byte is position 1, 2nd is number 2 and so on

can be moved to any byte in the file then read or write as many bytes as you want

If we omit startbyte with Get or Put statements then the file position pointer establishes a default value, successive Get and Put instructions adjust this value appropriately


Get and Put

Get reads the necessary number of Bytes to satisfy the length of the variable e.g 2 bytes for a single, 8 for a currency and so on.

For a fixed length string or a user defined type, Get reads the number of bytes to satisfy the length of the variable. A variable-length string variable will have the same number of bytes read into it as being stored in the variable at that time.

Put writes the number of bytes equal to the length of the variable


3rd step - closing the file

As with all file handling work it is important to Close the file when all i/o activity has finished

format

Close #filenum

e.g.

Close #1


Seek and Loc functions

With binary files Seek and Loc functions return a file position in number of bytes from the beginning of the file. Seek returns the current file pointer and Loc returns the number of the last byte read.

E.g.

Put #4, 26, iMyAgePrint “Byte number file pointer is at is ” ; Seek (4)Print “Last byte written was number” ; Seek (4)

which gives the outputByte number file pointer is at is 28last byte written was number 27


Error Handling - On Error

Probably the best way to find if a file exists is to use the ‘On Error’ statement, which catches all file related errors

On Error { GoTo line | Resume Next | GoTo 0 }

Goto Line : is either a line number or a label– e.g. On Error Goto ErrorHandler:– e.g. On Error Goto 20

Resume Next : causes next line of code to be executed, ignoring the error

Goto 0 : turns off the error handling


Error Handling - Code Structure

Sub subname

declarations

code not having error handling if any .......

On Error Goto ErrorHandlingRoutineLabel

code having error handling

On Error Goto 0 ‘[Optional] turn off error handling, if required

code not having error handling if any .......

Exit Sub

ErrorHandlingRoutineLabel:

Error handling code

Resume Next ‘Resume at next line after where the error occured

End Sub


Error Handling - Resume Statement

The Resume statement must be in the Error Handling routine or else an error occurs

Resume { [0] | Next | line}

Resume 0 : Program execution resumes at the line causing the error

Resume Next : Program execution resumes at the line immediately after the one causing the error

Resume Line : Either a line number or label in the main procedure where code is to resume executing


Error Handling - Example

Here when the file or drive is incorrect an error occurs

Dim sMsg As String ' Declare variables.On Error GoTo ErrorHandler ' Set up error handler.Open “C:\CISFILE.TXT" For Input As #1 ' Try to open file.

‘statements.......Close #1 ' Close the file.Exit Sub ' Exit before entering

ErrorHandler: ' Error handlerMsgbox “Error in Opening File”, ,”File Access Error”Resume Next ' Resume procedure.

End Sub


Error Handling - Err and Error()

The ‘Err’ statement is the error number returned from the last error to occur

For full list see “Trappable Errors” on Help File errors in range 52-76; e.g.

– 53 File not found– 61 Disk full

Other common errors– 9 Subscript out of range– 11 Division by zero

The ‘Error(ErrorNumber) statement returns a message describing the specified error number


Improved Error Handling

An improved message for the message box in the error handling routine could then be

ErrorHandler: ' Error handlerMsgbox Error(Err), ,”File Access Error”Resume Next ' Resume procedure.

End Sub

Here the message could be ‘file not found’ or ‘File already open’ etc.


Error Handling, continued

For a comprehensive error handling section:

ErrorHandler: Select Case Err Case 58: MsgBox Error(Err),,”File Access Error” …….. ‘Code to allow user to

“browse”for filename …….. ‘ then open valid file. Case 61: MsgBox Error(Err),,”Replace with new

disk” …….. End Select


Use the power of existing applications…...

Why write ‘Edit’ or ‘Print’ functionality into your VB programwhen Microsoft have already done it for you withNOTEPAD (or WORD).

Similarly, why re-invent the ‘Spreadsheet’ ??

BUT…. several pitfalls…..requires some experimentation to get it right. Try it !…………………… . . . . .


A simple way to use other applications…

Shell ( commandstring [,windowstyle] )egShell (NOTEPAD)orShell (NOTEPAD , 2)

Commandstring = name of executable, batchfile etc .exe assumed but could be .bat .com .pif not case sensitive, BUT must be EXACT name of package

Shell starts package ASYNCHRONOUSLY…care!!


windowstyle …..

1 = Normal with focus2 = Minimised with focus (default)etc….see help for full list.


…but then you often give the shelled application the focus…….

AppActivate applicationhandleeg

AppActivate applicationhandle

an example should help (see later)…………..


When application running…SENDKEYS

SendKeys keystring [ , wait ]egSendKeys “Computing”or SendKeys “ABC”, TRUE

wait can be true or false (default=false). Specifiesthat execution of calling procedure will not continueUNTIL ‘other’ application/package has consumed thesekeys.


‘Special’ keys each have unique name eg

{UP} is up arrow key, {ESC} is escape etcNote braces{}…….see help for full list

Shifted keys are achieved by using ‘prefix’ codes, eg

SendKeys +{ESC} sends Shift plus EscapeNote… + = shift ^ = control %=alt etc


Repeated keys are easy………

SendKeys {key repeats}eg

Sendkeys {LEFT 40} sends 40 presses of Left arrow key


A couple of examples…...

nfile = File1.ListCount Rem assume your app. uses a FILELIST control crlf$ = Chr$(13) + Chr$(10) Rem put required text into clipboard Clipboard.Clear For a = 0 To nfile - 1 txt$ = File1.List(a) Clipboard.SetText (Clipboard.GetText() + crlf$ + txt$) Next a continued next slide………….


txt$ = "Notepad" MyAppID = Shell(txt$, 1): Rem start NOTEPAD Rem crude way of giving it time to execute Rem DOEVENT would be better (see help) t = Timer: Do: Loop Until Timer > t + 0.5 Or Timer < 10 AppActivate MyAppID: Rem give NOTEPAD the focus

SendKeys "%E": Rem send ALT+E ie select 'EDIT' menu SendKeys "P": Rem send 'P' ie 'PASTE' text currently in clipboard

done.


An example NOT using clipboard….

Rem start NOTEPAD with a specified ASCII file txt$ = "Notepad \MYFILE.TXT" MyAppID = Shell(txt$, 1) t = Timer: Do: Loop Until Timer > t + 0.5 Or Timer < 10 AppActivate MyAppID


Summary

We have investigated– Binary files

the file structure of the binary file type how we can map reading and writing to a

binary file using the Get and Put statement the use of the Seek and Loc functions to

determine the present or last byte position of the file position pointer.

– How to trap and handle errors

– Interacting with other applications

Component 10a, Slide 1 CP2030 Copyright © University of Wolverhampton CP2030 Visual Basic for C++ Programmers v Component 10 – Working with Binary Files.

Documents

file slide

binary file v

structure of binary

binary file record

file character

week slide

program slide

binary files error handling