copyright © 2001 by wiley. all rights reserved. chapter 5: the repetition process in visual basic...

Introduction to Programming with Visual Basic 6.0 by McKeown and Piercy

1Copyright © 2001 by Wiley. All rights reserved.

Chapter 5:The Repetition Process in Visual Basic

Event Driven Loops

Determinate Loops

Indeterminate Loops

Sequential Access Files

Combo Boxes

Executable Files

Chapter 5:The Repetition Process in

Visual Basic




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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 loop (the statements being

repeated)– a termination condition that stops the loop

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




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Types of Loops• There are three types of loops

– event-driven– determinate – indeterminate

• Event-driven loops are repeated by the user causing an event to occur

• Determinate loops repeat a known number of times

• Indeterminate loops repeat an unknown number of times

• Variables should be initialized before being used in a loop




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Event-Driven Loops

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

• Variable scope is important for loops; Variables in event procedures are local and are reset to zero when procedure terminates

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

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

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

• Declared with Static keyword




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Form-level Variables




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Use of AddItem and Clear Methods

• The AddItem method is used to add items to a list box at run time

• Form of AddItem method

list1.Additem string

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

• The Clear method clears a list boxlist1.Clear




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Code to Sum a Series of Numbers

Private Sub cmdCalc_Click() Dim intTheValue As Integer intTheValue = CInt(txtTheValue.Text) intSum = intSum + intTheValue ‘intSum is form level intNumValues = intNumValues + 1 ‘form level txtSum.Text = Str(intSum) txtNumValues.Text = Str(intNumValues) lstEntries.AddItem str(intTheValue) txtTheValue.Text = "” txtTheValue.SetFocusEnd Sub




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Determinate Loops Using For-Next Loop

Best way to create a determinate loop is to use a For-Next Loop

Form of For-Next Loop:For variable = start value to end value Step change value statements that compose body of loopNext variable

where 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

Next variable = the end of the For loop




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Example of For-Next Loop

Counter Variable

Beginning Value

Ending Change Value Value

intSum = intSum + intCounter

Body of Loop

For intCounter = 1 to 10 Step 1For Statement

Next intCounter

Next Statement




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Code To Sum using For-Next Loop

Private Sub cmdCalc_Click() Dim intTheValue as Integer, intSum as Integer Dim intNumValues as Integer, intCounter as Integer intSum = 0 ‘Initialize Sum to Zero 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)For intCounter = 1 to intNumValues

intTheValue = CInt(InputBox("Enter next value")) intSum = intSum + intTheValue lstEntries.AddItem Str(intTheValue) ‘Add to listNexttxtSum.Text = Str(intSum)lstEntries.AddItem "Sum is " & Str(intSum)

End Sub




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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 but List() property runs from 0 to Listcount -1

• List(intCounter) is equal to the contents of the corresponding list box

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

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

Next




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Indeterminate Loops• Indeterminate loops run for an unknown number of

repetitions until a condition is 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




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Form of Pre- and Post-Test Loops

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

Loop

• The form of the post-test loops is:Dobody of loop

Loop Until (or While) condition




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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 file names 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.

• Three types of files: sequential access, database, and direct access files

• We will use sequential access files as input.




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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 a text editor such as Notepad.




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Opening and Inputting Data from a File

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

• To input data from a file, use the Input #n, statement

Input #n, list of variables

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

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

Input #n, list of variables

Loop




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

Code to Input and Sum values from File

Private Sub cmdCalc_Click() Dim intTheValue As Integer, intSum As Integer Dim intNumValues As Integer Open "a:\SumData.txt" For Input As #10 intSum = 0 intNumValues = 0 Do Until EOF(10) ’Input to end of file Input #10, intTheValue intSum = intSum + intTheValue intNumValues = intNumValues + 1 lstEntries.AddItem Str(intTheValue) Loop txtNumValues.Text = Str(intNumValues) txtSum.Text = Str(intSum) lstEntries.AddItem "Sum is " & str(intSum) Close #10End Sub




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

The Combo Box

• A combo box is a combination of a text box and a 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.




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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 alphabetically.

• 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. To remove a selected item, you can use the ListIndex property, eg,

cboMembers.RemoveItem cboMembers.ListIndex




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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. 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.




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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.




Event Driven Loops

Determinate Loops

Indeterminate Loops


Combo Boxes

Executable Files

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.

copyright © 2001 by wiley. all rights reserved. chapter 5: the repetition process in visual basic...

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