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

2. Component 10a, Slide 2 CP2030 Copyright © University of Wolverhampton Aims and objectives v Understand the structure of Binary Files v Use the various techniques available in Visual Basic to write to binary files. v Understand how to “launch” and interact with other applications from within V.B.

3. Component 10a, Slide 3 CP2030 Copyright © University of Wolverhampton VB data files v 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 v we will look at Binary files this week

4. Component 10a, Slide 4 CP2030 Copyright © University of Wolverhampton Binary files v 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 v 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. v VB only sees the data as bytes and so any interpretation of what the each Byte means must be written into the program

5. Component 10a, Slide 5 CP2030 Copyright © University of Wolverhampton 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

6. Component 10a, Slide 6 CP2030 Copyright © University of Wolverhampton 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

7. Component 10a, Slide 7 CP2030 Copyright © University of Wolverhampton 2nd step - writing or reading to the file v We can use the Put statement to write to a binary file record and the Get statement to read from the file v 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

8. Component 10a, Slide 8 CP2030 Copyright © University of Wolverhampton File position pointer v A file position pointer is associated with each binary file v identifies which byte is to be written to or read from – first byte is position 1, 2nd is number 2 and so on v can be moved to any byte in the file then read or write as many bytes as you want v 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

9. Component 10a, Slide 9 CP2030 Copyright © University of Wolverhampton Get and Put v 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. v 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. v Put writes the number of bytes equal to the length of the variable

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

11. Component 10a, Slide 11 CP2030 Copyright © University of Wolverhampton Seek and Loc functions v 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, iMyAge Print “Byte number file pointer is at is ” ; Seek (4) Print “Last byte written was number” ; Seek (4) which gives the output Byte number file pointer is at is 28 last byte written was number 27

12. Component 10a, Slide 12 CP2030 Copyright © University of Wolverhampton Error Handling - On Error v 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 } v Goto Line : is either a line number or a label – e.g. On Error Goto ErrorHandler: – e.g. On Error Goto 20 v Resume Next : causes next line of code to be executed, ignoring the error v Goto 0 : turns off the error handling

13. Component 10a, Slide 13 CP2030 Copyright © University of Wolverhampton 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

14. Component 10a, Slide 14 CP2030 Copyright © University of Wolverhampton Error Handling - Resume Statement v The Resume statement must be in the Error Handling routine or else an error occurs Resume { [0] | Next | line} v Resume 0 : Program execution resumes at the line causing the error v Resume Next : Program execution resumes at the line immediately after the one causing the error v Resume Line : Either a line number or label in the main procedure where code is to resume executing

15. Component 10a, Slide 15 CP2030 Copyright © University of Wolverhampton Error Handling - Example v 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 handler Msgbox “Error in Opening File”,,”File Access Error” Resume Next' Resume procedure. End Sub

16. Component 10a, Slide 16 CP2030 Copyright © University of Wolverhampton Error Handling - Err and Error() v The ‘Err’ statement is the error number returned from the last error to occur v 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 v The ‘Error( ErrorNumber ) statement returns a message describing the specified error number

17. Component 10a, Slide 17 CP2030 Copyright © University of Wolverhampton Improved Error Handling v An improved message for the message box in the error handling routine could then be ErrorHandler:' Error handler Msgbox Error(Err),,”File Access Error” Resume Next' Resume procedure. End Sub v Here the message could be ‘ file not found ’ or ‘ File already open’ etc.

18. Component 10a, Slide 18 CP2030 Copyright © University of Wolverhampton Error Handling, continued v 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

19. Component 10a, Slide 19 CP2030 Copyright © University of Wolverhampton Use the power of existing applications…... Why write ‘Edit’ or ‘Print’ functionality into your VB program when Microsoft have already done it for you with NOTEPAD (or WORD). Similarly, why re-invent the ‘Spreadsheet’ ?? BUT…. several pitfalls…..requires some experimentation to get it right. Try it !…………………….....

20. Component 10a, Slide 20 CP2030 Copyright © University of Wolverhampton A simple way to use other applications … Shell ( commandstring [,windowstyle] ) eg Shell (NOTEPAD) or Shell (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!!

21. Component 10a, Slide 21 CP2030 Copyright © University of Wolverhampton windowstyle ….. 1 = Normal with focus 2 = Minimised with focus (default) etc….see help for full list.

22. Component 10a, Slide 22 CP2030 Copyright © University of Wolverhampton …but then you often give the shelled application the focus……. AppActivate applicationhandle eg AppActivate applicationhandle an example should help (see later)…………..

23. Component 10a, Slide 23 CP2030 Copyright © University of Wolverhampton When application running…SENDKEYS SendKeys keystring [, wait ] eg SendKeys “Computing” or SendKeys “ABC”, TRUE wait can be true or false (default=false). Specifies that execution of calling procedure will not continue UNTIL ‘other’ application/package has consumed these keys.

24. Component 10a, Slide 24 CP2030 Copyright © University of Wolverhampton ‘Special’ keys each have unique name eg {UP} is up arrow key, {ESC} is escape etc Note braces{}…….see help for full list Shifted keys are achieved by using ‘prefix’ codes, eg SendKeys +{ESC} sends Shift plus Escape Note… + = shift ^ = control %=alt etc

25. Component 10a, Slide 25 CP2030 Copyright © University of Wolverhampton Repeated keys are easy……… SendKeys {key repeats} eg Sendkeys {LEFT 40} sends 40 presses of Left arrow key

26. Component 10a, Slide 26 CP2030 Copyright © University of Wolverhampton 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………….

27. Component 10a, Slide 27 CP2030 Copyright © University of Wolverhampton 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.

28. Component 10a, Slide 28 CP2030 Copyright © University of Wolverhampton 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

29. Component 10a, Slide 29 CP2030 Copyright © University of Wolverhampton Summary v We have investigated – Binary files u the file structure of the binary file type u how we can map reading and writing to a binary file using the Get and Put statement u 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