chapter 5 basics of file input and output reading, writing, and printing text files 5 exploring...

Chapter 5Basics of File

Input and Output

Reading, Writing, and Printing Text

Files

5

Exploring Microsoft Visual Basic 6.0Copyright © 1999 Prentice-Hall, Inc.By Carlotta Eaton

Exploring MS Visual Basic 6

Copyright 1999 Prentice-Hall, Inc. 2

Objectives...

1. Use Common Dialog control2. Display dialog boxes to open and save

files, set printer options, and select colors and fonts

3. Implement file I/O operations such as open, close, save and print

4. Write code to open a file and display it, then save changes to the file

5. Text, binary, RTF and HTML file formats



Objectives

6. Sequential vs. Binary vs. Random file access

7. Use File System Object (FSO) model to process files

8. Use string functions9. Print form and text from your project10. Write code to handle errors and avoid

runtime error crashes



Windows Dialog Boxes

Open Dialog BoxSave As Dialog BoxPrint Dialog BoxColor Dialog BoxFont Dialog Box



Common Dialog Control

CommonDialogName.MethodWhere

CommonDialogName is the name property assigned to the common dialog control

Method is one of the following:

ShowOpen displays the Open dialog box

ShowSave displays the Save As dialog box

ShowColor displays the Color dialog box

ShowFont display the Font dialog box

ShowPrinter displays the Print dialog box

ShowHelp starts the Windows Help engine



Open Dialog Box

Common Dialog control ShowOpen method



Save As Dialog Box

Common Dialog control ShowSave method



Print Dialog Box

Common Dialog control ShowPrinter method



Color Dialog Box

Common Dialog control ShowColor method



Font Dialog Box

Common Dialog control ShowFont method



The Common Dialog Control

Common dialog control provides access to Windows’ most commonly used dialog boxes

First add the Common dialog ActiveX control to the toolbox

Filter property selects the files to display based on file

extensions Example:

“Text file (*.txt) | *.txt”



The Sticky Pad Project



Create the Sticky Pad Project using the Common Dialog

Control

Hands-On Exercise 1 Create and test a new form Add the common dialog control Create the menus Add code for menus Display the Open dialog box Display the Save As dialog box Add and test code for the Close Note menu Add Banner comments



Instances and Classes

Class formal definition of an object Analogy - gingerbread man cookie

cutter

Instance one of the set of objects that belongs to

a class Analogy - any of the gingerbread men

cookies



Types of Files

File - block of information stored on disk or another media

Text file - file that contains lines of written information that can be sent directly to the screen or printer

Binary file - file that contains bits that do not necessarily represent printable text Examples: Word file, machine language file



Types of Files

RTF file (Rich Text Format) - standard developed by Microsoft to format documents

HTML (Hypertext Markup Language) - used to define formatting for web page files

Note: Rich Text Box control is only available in the Professional and Enterprise editions



File Access Methods

I/O (Input/Output) - refers to an operation or program that enters or extracts data from a computer

Access - read data from or write data to a storage device



File Access

Sequential access - used to read and write text files in continuous blocks

Random access - used to read and write text or binary files structured as fixed-length records

Binary access - used to read and write files that are structured according to an application’s specifications



Reading and Writing Files

Statements and FunctionsOpen statementInput functionInput # statementClose statementLOF (Length of File) functionPrint # statement



The Open Statement

Open pathname For mode As #filenumberWhere

pathname is a string that specifies a filename, and may include the folder and drive

mode must be one of the following:

Append, Binary, Input, Output, or Random

filenumber is a valid file number from 1 to 511

Note: The FreeFile function is used to obtain the next available file number

Example:

Open Filename For Input As #1



The Close Statement

Close #filenumberWhere

filenumber is any valid file number. If the filenumber is omitted all active files are closed.

Example:

Close #1



The LOF Function

LOF(filenumber)

Where

Filenumber is a valid file number

Example:

FileLength = LOF(1)



The Input Function

Input(charnumber, filenumber) Where

Charnumber is the number of characters to read

Filenumber is any valid file number

Example:

String = Input(Number, 1)



The Print # Statement

Print #filenumber, outputname Where

filenumber is any valid file number

outputname is the object to be printed

Example:

Print #1, txtNote



Open, Read and Write a File

Sub FileIO()Dim Pathname as StringDim FileLength As longOpen Pathname For Input As #1FileLength = LOF(1)txtFile = Input(Filelength, 1)Close #1‘Do other stuff hereOpen Pathname For Output As #1Print #1, txtNoteClose #1

End Sub



The File System Object

FSO - new feature of Visual Basic 6.0 Provides and object based model for

working with drives, folders, and files FileSystemObject object - primary object Drive object - info about a drive such as space Folder object - create, change, move and

delete Files object - create, delete, change and move TextStream object - enables you to read and

write text file



The File System Object

Useful Functions and Methods CreateObject function OpenTextFile method TextStream object methods and

properties



FSO Example Code Sub OpenTextFile()

Const ForReading = 1Const ForWriting = 2Const ForAppending = 3Dim fso ‘File System ObjectDim textfile ‘textstream file

Set fso = CreateObject(“Scripting.FileSystemObject”)Set textfile = fso.OpenTextFile(“C;\testfile.txt”,_

ForWriting, True, TristateFalse)textfile.Write “This is a test.”textfile.Close

End Sub



Handling I/O Errors

Bug - mistake in a computer programCompiler errors - occur when the rules of

the Visual Basic programming language were not followed

Runtime errors - occur when executing a program

Logic errors - occur when the program makes a mistake in understanding the problem or implements solution incorrectly



Handing I/O Errors

On Error GoTo LineName must be the first executable

statement in a procedure If error is encountered, the program

will execute the code immediately after the LineName statement

Place an Exit Sub statement immediately before LineName statement



Handling I/O ErrorsSub FileIO()‘Purpose: Error handling example

Dim Pathname As String On Error GoTo ErrorHandler ‘set up handler‘Insert code to open, save, or print file here‘No errors encountered hereExit Sub

ErrorHandler:‘Display a message box and exit procedureMsgBox “Cannot complete file operation”Exit Sub

End Sub



Add File I/O to Sticky Pad Project

Hands-On Exercise 2 Open the Sticky Pad project Add code to open a text file Add code to save a text file Add code to display the font dialog box Test the font dialog box Add code to display the color dialog box



Working with Strings

Len (Length of String) function - finds the length of a string

InStr (In String) function - searches a string for a substring

Left function - finds the leftmost characters of a string

Right function - finds the rightmost characters of a string

Mid function - finds characters in the middle of a string



The Len Function

Len (string | varname) Where

String is any valid string expression

Varname is any valid variable name

Either string or a varname is given, not both

Example:

If Len(Filename) = 0 Then Exit Sub



The InStr Function

InStr(startposition, string, substring)Where

startposition sets the starting position for the search

string is the string you are searching

substring is the substring you are looking for

Example:

Position = InStr(Pathname, “\”)



The Left Function

Left(string, length)Where

string is any valid string expression

length indicates the number of characters to return

Example:

FirstEight = Left(string, 8)



The Right Function

Right(string, length)Where



Example:

LastEight = Right(string, 8)



The Mid Function

Mid(string, startposition, length)

Where


startposition sets the starting position for extracting the substring


Example:

MiddleWord = Mid(string, start, 6)



Working with Printers

We can print 3 different ways We create a form, then print the form Send text and graphics to the default

printer using the system defaults Display the Print dialog box to offer

printing options, set the printer options, then send to the user’s choice of printers and print



The PrintForm Function

FormName.Printform

Where

FormName is a form object. If this object is omitted, the selected form is printed.

Example:

frmMain.PrintForm



Working with Printers

Print Form is a quick and easy way to print

Prints at screen resolution 72 - 96 dots per inch

Printers are capable of resolutions of300, 600, 1200 or more dots per inch

So, print out is low quality



The Printers Collection

Set as the default printer

Another printer

A fax card

A file stored on disk



Working with the Printer Object

Printer Object device-independent drawing space

that supports text and graphics Send signal to start print job Place text or graphics on pages Print the page Start a new page and continue until

finished



The Printer Object

Printer.method Where

Printer is the default print

Method is one of the followingPrint Objectname

New Page

End Doc

KillDoc



Working with the Printer Object

Private Sub mnuPrint_Click()‘Purpose: Print note using system defaults

On Error GoTo PrintErrorPrinter.Print ; ‘initialize printerPrinter.Print Me.Caption ‘send to printerPrint.Print vbCrLf; ‘send linefeedPrint.Print Me.txtNote ‘send textboxPrinter.EndDoc ‘print and endExit Sub

PrintError:MsgBox “Can’t print sticky note file “& Pathname

End Sub



Set Printer Object Properties

Printer.property Where

Printer is the default print

Property is one of the following

FontName, FontSize

FontBold, FontItalic

ScaleLeft

ScaleTopPage



Display the Print Dialog Box

Print Setup Print



Display the Print Dialog Box

Print #filenumber, outputname Where

filenumber is any valid file number

outputname is the object to be printed

Example:

Print #1, txtNote



Set the Flags Property

CommonDialogName.Flags FlagConstant Where


Flags is the method

FlagConstant is one or more of the following

cdlPDPrintSetup ‘display print setup

cdlPDHelpButton ‘display help button



Set More Properties for Printing

CommonDialogName. MethodWhere


Method is one of the following

Copies ‘number of copies to print

FromPage ‘page to start printing

ToPage ‘page to stop printing

hDC ‘selected printer

PrinterDefault = Setting

‘if true prints with current printer



Add Printing Capability to the Sticky Pad Project

Hands-On Exercise 3 Open the Sticky Pad project Display the Print Setup dialog box Display the Print Dialog box Add code and test the print header

code Add and test the print footer code Add and test the print text box code



Summary ...

Common Dialog Control gives us access to the Windows’ dialog boxes

Open and Save dialog boxes used with files Print Setup and Print dialog boxes used when

printing Color and Font dialog boxes used with fonts A class is the formal definition of an object An instance is one of the objects that belongs

to a class Text, binary, RTF and HTML file formats



Summary...Sequential access reads and writes

sequentially like a cassette tapeRandom access reads and writes much

like an audio CDBinary access reads and writes varying

record lengths, or no records at allOpen, Input, Input #, Close, Print #

statements and functions for reading and writing files



Summary

File System Object (FSO) is an object-based tool to work with drives, folders and files

Bugs can be compiler errors, runtime errors, or logic errors. Use error handling code to avoid runtime errors.

Printer Object is a device independent drawing space

Print Setup and Print dialog boxes give user more control over printing process



Practice with Visual Basic

1. Common Dialog Demo2. Print Demonstration3. ANSI Values Demo4. Clipboard Demo5. Using Extra Forms6. Printing Text7. Processing Text Demonstrations



Case Studies

Personal Text EditorThe Rich Text Box ControlAsk the ProsEnhance with Extra Forms

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