The Repetition Process in Visual Basic

The Repetition Process

• The capability to repeat one or more statements as many times as necessary is what really sets a computer apart from other devices

• All loops have two parts: – the body of the loops (the statements being

repeated)– a termination condition that terminates the loop

• Failure to have a valid termination condition can lead to an endless (indefinite)loop

Types of Loops

• There are three types of loops:• Event-driven loops are repeated by the user

causing an event to occur, such as by clicking a command button

• Determinate loops repeat a known number of times

• Indeterminate loops repeat an unknown number of times

• Variables should be initialized to zero before being used in a loop

Summing Vs. Counting

• Summing - adding some value to an existing sum– Example 2+3+5=10

• Counting - adding one to a counter– Example - 1+1+1+1=4

Variable Scope and Local Variables

• Event-driven loops are repeated by user causing an event to occur--clicking a button

• Variable scope is important for loops.• Variables in event procedures are local i.e,

specific to a command button, and are reset to zero when procedure terminates. Other events do not know about this variable

Form Level Variable

• Variables defined at form level are known to all procedures on the form and retain their value between events****

• Form-level variables are declared in the Declarations procedure of the General object

• Should initialize form level variables to 0 with this syntax

• Private SubForm_Load

intSum=0

intNumValues=0

End Sub

Form-level Variables

Static Variable and Project Level Variable

• Static variables retain their value between events but are local to event procedure****

• Declared with Static keywordSyntax: Static Variable as Type, Variable As Type

Static intNumValue as Integer, curCost as Currency

• Project variables are known to the whole project, with multiple forms and are contained in a module of their own (Chapter 7)****

Pseudocode for Calculate Button(EvntDrvn.vbp)

Sum and count procedureInput ValueAdd Value to SumIncrement Number of values by 1Display Sum and Number of valuesAdd value to list box

End procedure

VB Code Box 5-2Code for Calculate Button (EvntDrvn.vbp)

Private Sub cmdCalc_Click()’Declare variables and assign values

Dim intTheValue As IntegerintTheValue = CInt(txtTheValue.text)

’Calculate New Sum and Number of Values enteredintSum = intSum + intTheValue’New sum summing statement)intNumValues = intNumValues + 1’# entries(count)txtSum.Text = Str(intSum)txtNumValues.text = Str(intNumValues)

’Add value to list box and covert to string lstEntries.AddItem str(TheValue)txtTheValue.text = "" ’Clear text box txtTheValue txtTheValue.SetFocus ’Set focus back to txtTheValue

End Sub

Use of AddItem and Clear Methods

• The AddItem method is used to add items to a list box at run time, instead of design time

• Form of AddItem method

lstEntries.Additem str(variable)

• Always add a string to list box since it contains text

• The Clear method clears a list box and a combo box

lstEntries.Clear

VB Code Box 5-3Code for cmdClear Button

Private Sub cmdClear_Click()’ Clear text boxes

txtTheValue.Text = ""txtSum.Text = ""txtNumValues.Text = ""

’Set form variables back to 0intSum = 0intNumValues = 0lstEntries.Clear ’Clear list box

’Set focus back to txtTheValue txtTheValue.SetFocus

End Sub

Determinate Loops Using For-Next Loop

Best way to create a determinate loop is to use a For-Next Loop (used when # of repetitions known in advance****)

Form of For-Next Loop:For variable = start value to end value Step change value

statements that compose body of loopNext variablewhere variable = the counter variable in the loopstart value = the beginning value of the counter variableend value = the ending value of the counter variable

change value = the amount the counter variable changes each time through the loop(if omitted is 1)

Next variable = the end of the For loop

Example of For-Next Loop

For intCounter = 1 to 10 Step 1

Counter Variable

Beginning Value

Ending Change Value Value

intSum = intSum + intCounter

Body of LoopNext intCounter

For Statement

Next Statement

VB Code Box 5-5Code for cmdCalc Event Procedure (ForNext.vbp)

Private Sub cmdCalc_Click()’Declare variables and set Sum variable to 0Dim intTheValue as Integer, intSum as IntegerDim intNumValues as Integer, intCounter as IntegerintSum = 0 If txtNumValues.Text = "" Then ’Number not entered

Msgbox "Please enter number of values to be summed"Exit Sub ’Do not go into loop

End ifintNumValues = CInt(txtNumValues.Text)’stor text entryFor intCounter = 1 to intNumValues

intTheValue = CInt(InputBox("Enter next value"))intSum = intSum + intTheValue ’Calculate sum lstEntries.AddItem Str(intTheValue) ’List box value

NexttxtSum.Text = Str(intSum)’ integer to text/displays sumlstEntries.AddItem "Sum is " & str(intSum)

End Sub

VB Code Box 5-6Code for cmdClear Event Procedure

ForNext.vbp

Private Sub cmdClear_Click()’Clear text box

txtNumValues.text = ""txtSum.Text = ""lstEntries.Clear ’Clear list box’Set focus to txtNumValues txtNumValues.SetFocus

End Sub

Printing a List

• To print the contents of listbox, use the Listcount and List() properties of the list box

• Listcount is the number of items in list box• List(Counter) is equal to the contents of the

corresponding list box Lst() property runs from 0 to Listcount -1

• Code to print contents of list box to Immediate Window:

Dim intCounter as IntegerFor intCounter = 0 to lstEntries.ListCount - 1

Debug.Print lstEntries.List(Counter)Next

VB Code Box 5-7Code to Print Contents of List Box to Printer

Dim intCounter as Integer

For intCounter = 0 to lstEntries.ListCount - 1 Printer.Print lstEntries.List(Counter)

Next

Printer.EndDoc

Indeterminate Loops

• Indeterminate loops run for an unknown number of repetitions until a condition becomes true or while a condition is true

• Four types of indeterminate loops– Until loop with termination condition before body of loop – While loop with termination condition before body of loop– Until loop with termination condition after body of loop– While loop with termination condition after body of loop

• Pre-Test loops have termination condition before loop body

• Post-test loops have termination condition after loop body

Form of Pre - and Post-Test Loops

• The form of the pre-test loops is:Do Until (or While) condition

body of loopLoop

• The form of the post-test loops is:Do

body of loopLoop Until (or While) condition

Pre and Post-Test Loops

Do While condition statement1 statement2 (continued)Loopnext statement

True Condition (loop continues) False Condition (loop ends)

Do While condition statement1 statement2 (continued)Loopnext statement

Do While Loop

Do Until Loop

Do Until condition statement1 statement2 (continued)Loopnext statement

True Condition (loop ends) False Condition (loop continues)Do Until condition statement1 statement2 (continued)Loopnext statement

Pre-test Loop ProcessingDo While Loops

• In a Do While Loop, if condition being tested is True, then control is transferred to the body of the loop for another repetition of the loop

• In a Do While Loop, if condition being tested is False, control is transferred out of the loop and to the statement immediately following the Loop statement

Pre-test Loop ProcessingDo Until Loops

• In a Do Until Loop, if the condition being tested is False, then control is transferred to the body of the loop for another repetition of the loop

• In a Do Until Loop, if condition being tested is TRUE, control is transferred out of the loop and to the statement immediately following the Loop statement

Post-test Loop ProcessingDo While Loops

• In a post-test loop, the body of the loop is executed and then the condition is tested.

• In a post-test, Do While Loop, if condition being tested in the Loop While statement is True, then control is transferred back to the Do statement at the top of the loop and then to the body of a loop for another repetition of the loop.

• In a post-test Do While Loop, if condition being tested in the Loop While statement is False, then control is transferred out of the loop and to the statement immediately following the Loop While statement

Post-test Loop ProcessingDo Until Loops

• In a post-test, Do Until Loop, if condition being tested in the Loop Until statement is False, then control is transferred to the Do statement at the top of the loop and then to the body of the loop for another repetition of the loop

• In a post-test Do Until Loop, if condition being tested in the Loop Until statement is True, then control is transferred out of the loop and to the statement immediately following the Loop Until Statement

Using the Pretest Do While /Do Until Loops to Count Backwards

intValue = 10Do While intValue > 0 MsgBox "Value = " & str(intValue)

intValue = intValue - 2Loop ORintValue = 10Do Until intValue <=0 MsgBox "Value = " & str(intValue)

intValue = intValue - 2Loop

Using the Post Test Do Loop While /Do Until Loops to Count Backwards

intValue = 10Do

MsgBox "Value = " & str(intValue) intValue = intValue -2Loop While intValue > 0 ORintValue = 10Do

MsgBox "Value = " & str(intValue) intValue = intValue -2Loop Until intValue <=0

Processing an Unknown Number of Values from a File

• A data file is a collection of data stored on magnetic or optical secondary storage in the form of records.

• A record is a collection of one or more data items that are treated as a unit.

• Files are identified by the computer’s operating system with filenames assigned by the user.

• Files are important to processing data into information because they provide a permanent method of storing large amounts of data that can be input whenever needed for processing into information

Three Types of Files

• Sequential access files – files from which the data records must be input in the same order that they were placed on the file.

• Direct access files – files that can be input in any order

• Database files – entered with a database management system and not input directly into a VB project

Using Sequential Access Files

• Sequential access files must be read in same order as they are created so they replicate the action of entering data from a keyboard.

• The number of records on a sequential access file is often unknown, but there is an invisible binary marker at the end of the file called the EOF (end of file) marker. ****

• Use a Do While loop or a Do Until loop to input data from sequential access file.

• Loops can input data until the EOF marker is encountered

(or while it has not been encountered). • Create sequential access files by using the Notepad text

editor because it saves in.txt format automatically

Opening and Inputting Data from a File

• Files must be opened with the Open statement:Open “filename" for Input as #n

filename also includes the drive and foldern is file number between 1 and 511

Example: Open “a:\chapter 5\SumData.txt” for Input as #10

• To input data from a file once it has been opened , use the Input #n, statement

Input #n, list of variables#n -must match the file number in the Open

Statementlist of variables - names for which data is enteredExample: Input #10, intTheValue

Using a Do Loop and Closing Sequential Files

• Using an Until loop to input data from fileDo Until EOF(n)

Input #n, list of variables

Loop

• Opened Files must be closed at end of procedure Close #n

Example: Close #10

VB Code Box 5-10 Code to Input and Sum Values from File

Private Sub cmdCalc_Click()Dim intTheValue As Integer, intSum As IntegerDim intNumValues As IntegerOpen "a:\Chapter5\SumData.txt" For Input As #10 intSum = 0’set variable to 0intNumValues = 0 ’set variable to 0 Do Until EOF(10) ’Input to end of file Input #10, intTheValue ’inputs data file intSum = intSum + intTheValue ’sums intNumValues = intNumValues + 1’counts items lstEntries.AddItem str(TheValue)Loop’# of entries in data filetxtNumValues.text = Str(intNumValues) txtSum.Text = Str(intSum)’display sum in text boxlstEntries.AddItem "Sum is " & str(intSum)Close #10 ’Close data file

End Sub

VB Code Box 5-11Code for cmdClear Event Procedure

Private Sub cmdClear_Click()txtNumValues.Text = ""txtSum.Text = ""lstEntries.Clear

End Sub

The Combo Box• A combo box is a combination of a text box and a

list box. It typically a drop-down list box• It has a text box portion that is displayed at all times

and a drop-down list of items that can be displayed by clicking on the down arrow.

• One property of note for the combo box is the Style property, which can be set at design time to Drop Down combo (the default), Simple Combo, or Drop Down list. Its prefix is cbo .

• The combo box has all the properties and methods of the list box including the AddItem, Listcount, and list properties.

More on Combo Boxes• The default event for the Combo box is the Change event--to

use the Click event, you must change to it.

• The Sorted property is a useful property for the combo and list boxes that arranges the items in the box.

– If property is True names are in order last name , first name

• Items can be added by using .AddItem method and a text box and if sorted property is true name is added in appropriate order

– cboMembers.Additem txtCustName.text

• Items can be removed from a combo or list box with the RemoveItem method which requires that number of the item to be remove be given. However, to remove a selected item, use the ListIndex property, eg,

cboMembers.RemoveItem cboMembers.ListIndex

Application to Vintage Video

Pseudocode for Filling Combo Box

Open fileRepeat Until end of file encountered

Input NameTransfer name to combo box

end of loopClose file

VB Code Box 5-13

Code for Form_Load Event

Private Sub Form_Load()’Declare variable

Dim strName As StringlstVideos.AddItem "Welcome to Vintage Videos"

’Open data file #1Open "a:\members.txt" For Input As #1Do Until EOF(1) ’Input names and add to combo box Input #1, Name cboMembers.AddItem NameLoopClose #1’Close data file #1

End Sub

Code to Add Name to List(cmdAdd Command Button)

Private Sub cmdAdd_Click()

If txtCustName.txt <> “” Then

cboMembers.AddItem txtCustName.Text

End If

VB Code Box 5-16Code to Delete Name from List

Private Sub cmdDelete_Click()’ Check to see if no name selected or combo box is empty If cboMembers.ListIndex > -1 Then cboMembers.RemoveItem cboMembers.ListIndex Else’Display message box MsgBox "No name selected or list is empty" End IfEnd Sub

VB Code Box 5-14 Partial Revised Code for End of cmdCalc curPrice = CCur(txtVideoPrice)’Find this line!!! curTotalCost = curTotalCost + curPrice 'Add price to total cost curTaxes = curTotalCost * curTaxRate 'Compute taxes on total cost curAmountDue = curTotalCost +cur Taxes 'Compute amount due txtTotalCost.Text = Format(curTotalCost, "Currency") txtTaxes.Text = Format(curTaxes, "currency") txtAmountDue.Text = Format(curAmountDue, "currency") txtVideoPrice.Text = Format(curPrice, "currency") lstVideos.AddItem txtVideoName.Text & _ " " & txtVideoPrice.Text lstTypes.Text = "" txtVideoName.Text = "" txtVideoPrice.Text = "" txtVideoName.SetFocus 'Set Focus to video name text boxEnd Sub

VB Code Box 5-15Code for cmdPrint Event Procedure

Private Sub cmdPrint_Click() Dim intNumber As Integer, intCounter As Integer lstVideos.AddItem "Total video price " + _ txtTotalCost.Text ’Total price & lstVideos.AddItem "Taxes " + txtTaxes.Text’Taxes lstVideos.AddItem "Total price = " + _ txtAmountDue.Text ’Total price $ lstVideos.AddItem "Thanks for your business!" intNumber = lstVideos.ListCount For intCounter = 0 To Number - 1 Debug.Print lstVideos.List(intCounter) NextEnd Sub

Saving Updates to List

• Uses an Open statement also– Open ” filename” for Output as #n

• filename includes drive and path• #n =number of file

– Example: – Open “a:\chapter5\members.txt” for Output as #10

• Use a Write statement to save changes– Example Write #n, variable list– Write #10, MemberName

VB Code Box 5-17Code for cmdExit Event Procedure

Private Sub cmdExit_Click() Dim intNumMembers As Integer, strMemberName As String Dim intCounter As Integer Open "a:\chapter5\members.txt" For Output As #10 intNumMembers = cboMembers.ListCount For intCounter = 0 To intNumMembers - 1 strMemberName = cboMembers.List(Counter) Write #10, strMemberName Next Close #10 EndEnd Sub

Creating an Executable File

• An executable file is one that can be executed on any computer on which it is installed.

• Creating an executable file in VB is accomplished with the File|Make filename.exe menu selection.

• Once an executable file is created, a shortcut to it can be created by right-clicking the file name in Windows Explorer and selecting Create Shortcut.

• Drag the shortcut to the desktop.

Nested Loops• A Nested loop is a loop within a loop. Must complete the inner

loop within the outer loop.• Nested For-Next loops have a For-Next loop within a For-Next

loop in which the inner loop will go through all its values for each value of the outer loop.

• Three key programming rules to remember about using nested For-Next loops:

• Always use different counter variables for the outer and inner For-Next loops.****

• Always have the Next statement for the inner For-Next loop before the Next statement for the outer For-Next loop.

• Always include the counter variable in the Next statements to distinguish between the loops.

Debugging Loops

• Debug a loop by inserting a Debug.Print command in the loop to print to the Immediate Window.

• Add a Quick Watch by locating the pointer on a variable and clicking the eyeglass icon on the Debug Toolbar. The values for this variable will be shown in the Watch Window.

• Use the Locals window to display the values of variables local to a procedure.

• Use the Toggle Breakpoint icon to pause execution at a designated line in the code and then use the various windows to view the values for variables.