Chapter 9 Files, Printing, and Structure Topics Using Files The OpenFileDialog, SaveFileDialog, FontDialog, and ColorDialog Controls The PrintDocument.

Chapter 9 Files, Printing, and Structure

Topics

•Using Files•The OpenFileDialog, SaveFileDialog, FontDialog, and ColorDialog Controls•The PrintDocument Control•Structures

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 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 cassette 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 StreamWriterDim phoneFile As StreamWriter

– Call CreateText method passing the filenamephoneFile = File.CreateText("phonelist.txt")

– This 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

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 StreamWriterDim phoneFile As StreamWriter

– Call AppendText method passing the filenamephoneFile = File.AppendText("phonelist.txt")

– This Method AppendText 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

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 fileObjectVar.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 deleted– 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 file

dataVar = 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 End of 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()

Detecting End of File (Cont’d)

• The EndofStream method also tests if you’ve reached end of file (no more characters to read)– Format is objectvar.EndOfStream– If no more characters, the value True is returned

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

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

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

Write Then Read Entire Array

Dim intValues(9) As Integer------------------------------------------------Dim outputFile as StreamWriteroutputFile = File.CreateText("values.txt")For intCount = 0 To intValues.Length – 1

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

intValues(intCount) = CInt(inputFile.ReadLine)NextinputFile.Close()

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 methodControlName.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 (*.*)|*.*"

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

lblMessage.Font = fdFont.FontEnd If

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

Other OpenFileDialog Properties

• InitialDirectory property specifies folder to use– Default if not specified is the 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

Structures

A Visual Basic programmer may also create new data types, called user-defined data types (UDT).

Two kinds of UDTs: structure and class.

This chapter discusses the use of structure and Chapter 12 the use of class.

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.intEmpNumber = 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 dynamic 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