Chapter 9, Slide 1Starting Out with Visual Basic 3 rd Edition Chapter 9 Files, Printing, and Structures.

Chapter 9, Slide 1 Starting Out with Visual Basic 3rd Edition

Chapter 9

Files, Printing, and Structures


Chapter 9Introduction


Chapter 9 Topics 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


Section 9.1Using Files

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 openedIf 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 in 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


Establishing StreamWriter Objects A StreamWriter object is used to write to a

sequential text file in the following way:• Declare a variable of type StreamWriter

• Create a StreamWriter object and assign it to the StreamWriter variable using either the

• CreateText method for new files

• AppendText method for existing files

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

Dim phoneFile As System.IO.StreamWriter

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

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


Using a String Variable as File Name Filename can be a string literal as already

shown but a string variable is more flexible• User can select the file they wish to edit• What if Notepad could only edit “textfile.txt”?

Example with string variable as filename

Can allow the user to enter the filename• Substitute txtFile.text for “customer.txt” • User can then enter filename in a text box

Dim custFile As System.IO.StreamWriterDim fileName as StringfileName = “customer.txt”custFile = System.IO.file.AppendText(fileName)


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()


Closing a StreamWriter Object

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

the file The Close method of a StreamWriter object

clears the buffer and closes the fileObjectVar.Close()

• Streamwriter object identified by ObjectVar


Writing Data to a File Example

Dim studentFile As System.IO.StreamWriterstudentFile = System.IO.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

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


Importing a Namespace System.IO is referred to as a namespace

• A group of logically related classes• System.IO contains StreamWriter and other

file related classes Can shorten references to such classes by

importing the namespace in your codeImports System.IO

• Allows us to use Dim custFile As StreamWriter

• Instead of Dim custFile As System.IO.StreamWriter


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, you’d use

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


The StreamWriter Write Method

The Write method does not place a newline character after each data item

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)


Write Method Example

Dim name As String = "Jeffrey Smith"Dim idNum As Integer = 47895Dim phone As String = "555-7864"

outputFile.Write(name)outputFile.Write(" ")outputFile.Write(idNum)outputFile.Write(" ")outputFile.WriteLine(phone)

Jeffrey Smith 47895 555-7864

TheResultingFile


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

Dim ObjectVar As System.IO.StreamReaderObjectVar = System.IO.File.OpenText(Filename)

Dim phoneFile As System.IO.StreamReaderphoneFile = System.IO.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 System.IO.StreamReadercustFile = System.IO.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

Imports System.IODim scoresFile As StreamReaderDim input As StringscoresFile = File.OpenText("Scores.txt")Do Until scoresFile.Peek = -1

input = 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

Imports System.IODim textFile As StreamReaderDim input As StringtextFile = File.OpenText("names.txt")Do While textFile.Peek <> -1

input &= 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 input

Imports System.IODim textFile As StreamReaderDim input As StringtextFile = File.OpenText("names.txt")input = textFile.ReadToEndtextFile.Close()


Write Then Read an Entire Array

Imports System.IODim intValues(9)------------------------------------------------Dim outputFile as StreamWriteroutputFile = File.CreateText("values.txt")For count = 0 To (intValues.Length – 1)

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

intValues(count) = Val(inputFile.ReadLine)Next countinputFile.Close()


Section 9.2The OpenFileDialog,

SaveFileDialog, FontDialog,and ColorDialog Controls

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• Provides users the ability to 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


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

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 usercdColor.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.

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

lblMessage.Font = fdFont.FontEnd If


Section 9.3The PrintDocument Control

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 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 event handler• Basic format of event handler is as follows:


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

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"

Sizes are specified with a number• 12

Styles are specified with provided constants• FontStyle.Regular• FontStyle.Bold• FontStyle.Underline


Sample PrintPage Event ProcedurePrivate Sub pdPrint_PrintPage(ByVal sender As System.Object, _

ByVal e As System.Drawing.Printing.PrintPageEventArgs) _Handles pdPrint.PrintPage

Dim inputFile As System.IO.StreamReaderDim x As Integer = 10 ‘Horizontal PositionDim y As Integer = 10 ‘Vertical Position

inputFile = System.IO.File.OpenText(filename)Do While inputFile.Peek <> -1

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

y += 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, 10}{1, 10}{2, 10}", 50, "Arg 1", 6)

Specifiesthe argument

numberSpecifies field width for arg

negative - left justifiedpositive - right justified

Argument 0Argument 1

Argument 2

Results in the following output:

50 Arg 1 6

10 spaces 10 spaces 10 spaces


Section 9.4Structures

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 empNumber As IntegerDim firstName As StringDim lastName As StringDim hours As SingleDim payRate As DecimalDim grossPay 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.empNumber = 1101deptHead.firstName = "Joanne"deptHead.lastName = "Smith"deptHead.hours = 40deptHead.payRate = 25deptHead.grossPay = deptHead.hours * deptHead.payRate

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 StudentRecordname As StringtestScores() As Single

End Structure

Dim student As StudentRecordReDim student.TestScores(4)student.name = "Mary McBride"student.testScores(0) = 89student.testScores(1) = 92student.testScores(2) = 84student.testScores(3) = 96student.testScores(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


Section 9.5Modifying the Demetris

Leadership Center Application

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

Chapter 9, Slide 1Starting Out with Visual Basic 3 rd Edition Chapter 9 Files, Printing, and Structures.

Documents

file slide

file data

data data

objectvar slide

file objectvar

control slide

writeline slide

rd edition chapter