Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 9- 1 STARTING...

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 9- 1

STARTING OUT WITH

Visual Basic 2008FOURTH EDITION

Tony GaddisHaywood Community College

Kip IrvineFlorida International University

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter

Files, Printing, and Structures9


Introduction

Saving data to sequential text files Reading the data back into an application Using the OpenFileDialog, SaveFileDialog,

ColorDialog, and FontDialog controls Using the PrintDocument control to print reports

from your application Packaging units of data together into structures


Using Files9.1

A File Is a Collection of Data Stored on a Computer Disk

Information Can Be Saved to Files and Later Reused


The Life Span of Data

Thus far, all of our data has been stored in controls and variables existing in RAM

This data disappears once the program stops running

If data is stored in a file on a computer disk, it can be retrieved and used at a later time


Three Steps in Using a File

1. The file must be opened

If it does not yet exist, it will be created

2. Data is read from or written to the file

3. The program closes the file


Reading and Writing to a File Data must be retrieved from disk and put in

memory for an application to work with it Data is transferred from disk to memory by:

Reading it from an input file Placing it in variables or control properties

Data is transferred from memory to disk by: Writing it to an output file Getting it from variables or control properties

Data is frequently placed in the text property of a control


File Types/Access Methods Text file type

Character based text Contents can be viewed by Notepad

Binary file type Pure binary form Contents cannot be viewed with a text editor

Access Methods Sequential access – a continuous stream of data written

and read as a whole from beginning to end Random access – access in any order with data written

to or read from specific places in the file Like the difference between a casette tape and a CD


Creating Files with StreamWriter Objects Add Imports System.IO before class declared

Makes StreamWriter classes available in code A StreamWriter object is used to create a

sequential text file in the following way: Declare an object variable of type StreamWriter

Call CreateText method passing the filename

Method returns a StreamWriter object Object is assigned to a StreamWriter variable

Variable phoneFile now defines a stream of data that can be written to phonelist.txt

Dim phoneFile As StreamWriter

phoneFile = File.CreateText(“phonelist.txt”)


Appending Text with StreamWriter A StreamWriter object is used to append data to a

sequential text file in the following way: Declare an object variable of type StreamWriter

Call AppendText method passing the filename

Method returns a StreamWriter object Object is assigned to a StreamWriter variable

Variable phoneFile now defines a stream of data that can be added to the end of phonelist.txt

Dim phoneFile As StreamWriter

phoneFile = File.AppendText(“phonelist.txt”)


File Paths Filename can include the file path

Can be a complete file path with drive letter“C:\WordProc\memo.txt"

Refer to a file in the default drive root directory"\pricelist.txt"

Or include no path information at all"mytext.txt“

If no path information specified, the bin folder of the current project is used


Writing Data to a File

The WriteLine method of a StreamWriter object actually writes data to the file

ObjectVar.WriteLine(Data) Streamwriter object identified by ObjectVar The method’s Data argument consists of

constants or variables with data to be written WriteLine appends an invisible newline character

to the end of the data Omit argument to write a blank line to a file

ObjectVar.WriteLine()


Writing Data to a File Example

Dim studentFile As StreamWriterstudentFile = File.CreateText("StudentData.txt")studentFile.WriteLine("Jim")studentFile.WriteLine(95)studentFile.WriteLine("Karen")studentFile.WriteLine(98)studentFile.WriteLine("Bob")studentFile.WriteLine(82)studentFile.Close()

Jim95Karen98Bob82

TheResultingFile,StudentData.txt


The StreamWriter Write Method

The Write method writes an item of data without writing a newline character

Usually need to provide some sort of delineation or delimiter between data items A blank space could be used Comma is a more common delimiter

ObjectVar.Write(Data)


Closing a StreamWriter Object

Should close files when finished with them Avoids losing data Data is initially written to a buffer Writes unsaved data from the buffer to the file

The Close method of a StreamWriter object clears the buffer and closes the file

ObjectVar.Close() Streamwriter object identified by ObjectVar

Tutorial 9-1 provides an example of an application that writes data to a file


Appending to a File

If opening an existing file with CreateText Existing contents are removed New text overwrites the old text

If opening an existing file with AppendText Existing contents are retained New text adds on to the end of the old text

If adding a new friend to friendFile, use:

friendFile = File.AppendText("MyFriends.txt")


Appending a File Example

‘Declare an object variableDim friendFile as StreamWriter‘Open the filefriendFile = File.AppendText(“MyFriends.txt”)‘Write the datafriendFile.WriteLine(“Bill Johnson”)friendFile.WriteLine(30)friendFile.WriteLine(“36 Oak Street”)‘Close the filefriendFile.Close()

friendFile“After”

Jim Weaver30P.O. Box 124Mary Duncan2447 Elm StreetKaren Warren2824 Love Lane

friendFile “Before”

Jim Weaver30P.O. Box 124Mary Duncan2447 Elm StreetKaren Warren2824 Love LaneBill Johnson3036 Oak Street


StreamReader Objects Use StreamReader objects to read from a file Define and open similar to StreamWriter:

Sample code:

Variable phoneFile now defines a stream of data that can be read from phonelist.txt

Must have Imports System.IO before class declaration as was done with StreamWriter

Dim ObjectVar As StreamReaderObjectVar = File.OpenText(Filename)

Dim phoneFile As StreamReaderphoneFile = File.OpenText(“phonelist.txt")


Reading Data from a File The ReadLine method of a StreamReader object

actually reads data from the filedataVar = ObjectVar.ReadLine()

Streamwriter object identified by ObjectVar The result of the method, the data read from

the file, is assigned to string variable dataVar Sample code:

Dim custFile As StreamReadercustFile = File.OpenText("customer.txt")custName = custFile.ReadLine()

custName holds the data read from the file StreamReader also has a Close method


Determining Whether a File Exists The File.OpenText method issues a runtime

error if the file does not exist Avoid this by using the File.Exists method

Format is File.Exists(filename) Returns a boolean result that can be tested:

Tutorial 9-2 shows how to read text file data

If System.IO.File.Exists(filename) Then' Open the file.inputFile = System.IO.File.OpenText(filename)

ElseMessageBox.Show(filename & " does not exist.")

End If


Detecting the End of a File The Peek method tests if you’ve reached end of

file (no more characters to read) Format is objectvar.Peek If no more characters, the value -1 is returned

Tutorial 9-3 demonstrates the Peek method

Dim scoresFile As StreamReaderDim strInput As StringscoresFile = File.OpenText("Scores.txt")Do Until scoresFile.Peek = -1

strInput = scoresFile.ReadLine()lstResults.Items.Add(input)

LoopscoresFile.Close()


Read Method Read method returns the integer code of the next

character in the file Chr function converts integer code to character

This loop appends one character at a time to input until no more characters are in the file

Dim textFile As StreamReaderDim strInput As String = String.EmptytextFile = File.OpenText("names.txt")Do While textFile.Peek <> -1

strInput &= Chr(textFile.Read)LooptextFile.Close()


ReadToEnd Method ReadToEnd method returns the rest of the file

from the current read position to end of file Functions differently from ReadLine method

ReadToEnd method ignores line delimiters The statement

input = textFile.ReadToEnd reads the file contents and stores it in strInput

Dim textFile As StreamReaderDim strInput As StringtextFile = File.OpenText("names.txt")strInput = textFile.ReadToEndtextFile.Close()


Write Then Read an Entire ArrayDim intValues(9)------------------------------------------------Dim outputFile as StreamWriteroutputFile = File.CreateText("values.txt")For intCount = 0 To (intValues.Length – 1)

outputFile.WriteLine(intValues(intCount))Next intCountoutputFile.Close()------------------------------------------------Dim inputFile as StreamReaderinputFile = File.OpenText("values.txt")For intCount = 0 To (intValues.Length – 1)

intValues(intCount) = CInt(inputFile.ReadLine)Next intCountinputFile.Close()


The OpenFileDialog,SaveFileDialog, FontDialog,and ColorDialog Controls

9.2

Visual Basic Provides Dialog Controls That Equip Your Applications With Standard Windows Dialog

Boxes for Operations Such As Opening Files, Saving Files, and Selecting Fonts and Colors


OpenFileDialog and SaveFileDialog Windows has a standard method of allowing a

user to choose a file to open or save These methods let users browse for a file

The OpenFileDialog and SaveFileDialog controls provide this capability in VB

To use the OpenFileDialog control Double click on this tool in the Toolbox Appears in component tray Use ofd as standard prefix when naming

SaveFileDialog is used in a similar way


Displaying an Open Dialog Box Display control with the ShowDialog method

ControlName.ShowDialog() Method returns a value indicating which dialog

box button the user selects, either DialogResult.OK, or DialogResult.Cancel

For example:If ofdOpenfile.Showdialog() = DialogResult.OK Then

MessageBox.Show(ofdOpenFile.FileName)Else

MessageBox.Show(“You selected no file”)End If


Dialog Box Filter Property

FileDialog controls have a Filter property Limits files shown to specific file extensions Specify filter description shown to user first Then specify the filter itself Pipe symbol (|) used as a delimiter

Following Filter property lets user choose: Text files (*.txt), displays all .txt files All files (*.*), displays all file extensions

ofdOpenFile.Filter = "Text files (*.txt)|*.txt|" & _ "All files (*.*)|*.*"


Other OpenFileDialog Properties InitialDirectory property specifies folder to use

Default if not specified is current folder To set dialog box initial directory to C:\Data:

ofdOpenFile.InitialDirectory = “C:\Data”

Title property specifies the text on the title bar Default title is Open if not specified

ofdOpenFile.Title = “Select a File to Open”

Filename property returns file selected from dialog box by user, in this case to selectedFile

selectedFile = ofdOpenFile.Filename


Open Dialog Box Example

' Configure the Open dialog box and display it.With ofdOpenFile

.Filter = "Text files (*.txt)|*.txt|" & _"All files (*.*)|*.*"

.InitialDirectory = "C:\Data"

.Title = "Select a File to Open"If .ShowDialog() = DialogResult.OK Then

inputFile = System.IO.File.OpenText(.Filename)End If

End With

User may choose to display .txt files or all files Files from Data folder of hard drive are shown Dialog box title shows Select a File to Open Variable inputFile holds file selected by user


Open Dialog Box Example InitialDirectory property Title property Filter property


SaveFileDialog Control SaveFileDialog uses the same methods:

ShowDialog() The same properties:

Filter InitialDirectory Title Filename

And the same result constants: DialogResult.OK DialogResult.Cancel

Tutorial 9-4 uses these controls in a text editor


ColorDialog Control Displays a typical Windows color dialog box

Provides users the ability to choose a color


ColorDialog Control To use the ColorDialog control

Double click the tool in the Toolbox Appears in component tray Use cd as standard prefix when naming

The following code sets the text in control lblMessage to the color selected by the user

cdColor.ShowDialog()If cdColor.ShowDialog() = DialogResult.OK Then

lblMessage.ForeColor = cdColor.ColorEnd If


FontDialog Control Displays a Windows font selection dialog box

Allows users to choose font, font size, etc.


FontDialog Control To use the FontDialog control

Double click the tool in the Toolbox Appears in component tray Use fd as standard prefix when naming

The following code sets the text in control lblMessage to the font selected by the user

fdFont.ShowDialog()If fdFont.ShowDialog() = DialogResult.OK Then

lblMessage.Font = fdFont.FontEnd If


The PrintDocument Control9.3

The PrintDocument Control Allows You to Print Data to the Printer


PrintDocument Control

Allows you to send output to the printer To use the PrintDocument control

Double click the tool in the Toolbox Appears in component tray Use pd as standard prefix when naming

PrintDocument control has a Print method This method starts the printing process Format is:

PrintDocumentControl.Print() This triggers a PrintPage event


PrintPage Event Handler

Private Sub pdPrint_PrintPage(ByVal sender As System.Object, _ByVal e As System.Drawing.Printing.PrintPageEventArgs) _Handles pdPrint.PrintPage

‘Your print code inserted here

End Sub

The code in the PrintPage event handler performs the actual printing Double click PrintDocument control in tray This creates the PrintPage event handler Insert your print code inside the event handler Basic format of event handler shown below:


DrawString Method

The DrawString method is used inside the PrintPage event to: Specify data to send to the printer in string Set font, font size, and font style Determine horizontal position (HPos) of text Determine vertical position (VPos) of text Brushes.Black specifies output in black

DrawString method is formatted as follows:e.Graphics.DrawString(String, _

New Font(FontName, Size, Style), _Brushes.Black, HPos, VPos)


Specifying Fonts, Sizes, Styles

Fonts are specified with the string which names the font to be used "Times New Roman" “Arial" , etc.

Sizes are specified with a number 10, 12, etc.

Print effects are specified with provided constants FontStyle.Regular FontStyle.Bold FontStyle.Underline


Sample PrintPage Event Procedure

Private Sub pdPrint_PrintPage(ByVal sender As System.Object, _ByVal e As System.Drawing.Printing.PrintPageEventArgs) _Handles pdPrint.PrintPage

Dim inputFile As StreamReaderDim intX As Integer = 10 ‘Horizontal PositionDim intY As Integer = 10 ‘Vertical Position

inputFile = File.OpenText(strFilename)Do While inputFile.Peek <> -1

e.Graphics.DrawString(inputFile.ReadLine, _New Font("Courier", 10, FontStyle.Regular), _Brushes.Black, intX, intY)

intY += 12 ‘Increment Vert PosLoopinputFile.Close()End Sub

Tutorial 9-5 adds a print feature to Tutorial 9-4


Printing Column Based Reports Business reports typically contain a:

Report header printed at the top of the page Report body with the data, usually in columns Optional footer, often totalling certain columns

Report header usually has column headings Monospaced font used for column reports

Each character takes same amount of space This allows columns to be aligned

String.Format used to align data along column boundaries


String.Format Example

String.Format("{0, 7}{1, -10}{2, 7}", 50, "Arg1", 6)

Specifiesthe argument

numberSpecifies field width for arg

negative - left justifiedpositive - right justified

Argument 0Argument 1

Argument 2

Results in the following output:

50Arg 1 6

7 spaces 10 spacesLeft Justified

7 spaces


Structures9.4

Visual Basic Allows You to Create Your Own Data Types, in Which You May

Group Multiple Data Fields


Structures vs. Arrays

Arrays: Multiple fields in one array All of the same data type Distinguished by a numerical index

Structures Multiple fields in one structure Can be of differing data types Distinguished by a field name


Syntax for Declaring a Structure

StructureName is a name that identifies the structure itself

FieldDeclarations are the declarations of the individual fields within the structure

[AccessSpecifier] Structure StructureNameFieldDeclarations

End Structure


Structure Declaration Example

Structure EmpPayDataDim intEmpNumber As IntegerDim strFirstName As StringDim strLastName As StringDim sngHours As SingleDim decPayRate As DecimalDim decGrossPay As Decimal

End Structure

Following declares a structure with six fields intended to record employee payroll data

Structure name is EmpPayData


Creating and Initializing a Structure

Dim deptHead As EmpPayData

deptHead.strEmpNumber = 1101deptHead.strFirstName = "Joanne"deptHead.strLastName = "Smith"deptHead.sngHours = 40deptHead.decPayRate = 25deptHead.decGrossPay = CDec(deptHead.sngHours) * _

deptHead.decPayRate

Using the EmpPayData structure just defined Define variable deptHead of type EmpPayData deptHead contains the six fields in the structure Access each field using varName.fieldName


Passing Structure Variables to Procedures and Functions

Structures can be passed to procedures and functions like any other variable

The data type to use in the specification is the name of the structure

Sub CalcPay(ByRef employee as EmpPaydata)‘ This procedure accepts an EmpPayData variable‘ as its argument. The employee’s gross pay‘ is calculated and stored in the grossPay‘ field.With employee

.decGrossPay = .sngHours * .decPayRateEnd With

End Sub


Structures Containing Arrays

Structure StudentRecordDim strName As StringDim sngTestScores() As Single

End Structure

Dim student As StudentRecordReDim student.sngTestScores(4)student.strName = "Mary McBride"student.sngTestScores(0) = 89Student.sngTestScores(1) = 92Student.sngTestScores(2) = 84Student.sngTestScores(3) = 96Student.sngTestScores(4) = 91

Structures can contain arrays Must ReDim after declaring structure variable


Can declare an array of structures Example below declares employees as an array

of type EmpPayData with 10 elements Can refer to each field using the format

arrayName(index).fieldName

Tutorial 9-6 examines an application with a structure

Arrays Containing Structures

Dim employees(9) As EmpPayData

' Refer to the empNumber of the first employeeemployees(0).empNumber = 1101


Modifying the Demetris Leadership Center Application

9.5

Modify this application to include the ability to save and retrieve data,

use an array of structure variables instead of parallel arrays, print the sales report


Changes to Demetris Application

Slide 9- 54

Replace parallel arrays with an array of structure variables

Add option to print sales report Add options to save & retrieve units sold

Structure ProductDataDim strName As String ' Item nameDim strDesc As String ' DescrDim intProdNum As Integer ' Item nbrDim decPrice As Decimal ' Unit priceDim intUnitsSold As Integer ' Units soldEnd StructureConst intMAX_SUB As Integer = 8' Array of ProductDataDim products(intMAX_SUB) As ProductData

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Slide 9- 1 STARTING...

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