Chapter 6: The Repetition
Structure
Programming with Microsoft Visual Basic
.NET, Second Edition
The Repetition Structure (Looping)
Lesson A Objectives
• Code the repetition structure using the For...Next
and Do...Loop statements
• Write pseudocode for the repetition structure
• Create a flowchart for the repetition structure
• Initialize and update counters and accumulators
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The Repetition Structure
• Use the repetition structure to repeatedly process
one or more program instructions until some
condition is met, at which time the repetition ends
• The repetition structure is referred to as a loop
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The Repetition Structure (continued)
• Pretest loop: evaluation occurs before the
instructions within the loop are processed
• Posttest loop: evaluation occurs after the
instructions within the loop are processed
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The For…Next Loop
• Use the For…Next statement to code a loop that
repeats for a specific number of times
Figure 6-2: Syntax and examples of the For...Next statement
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The For…Next Loop (continued)
Figure 6-2: Syntax and examples of the For...Next statement
(continued)
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The For…Next Loop (continued)
• counter is a numeric variable that keeps track of
how many times the loop instructions are
repeated
• startvalue, endvalue, and stepvalue
– Must be numeric
– Can be positive or negative, integer or non-integer
– Default stepvalue is 1
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The For…Next Loop (continued)
Figure 6-4: Pseudocode and flowchart for the first example shown
in Figure 6-2
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The For…Next Loop (continued)
For…Next loop examples:
Dim count As Integer
For count = 0 to 3 Step 1
Debug.WriteLine(count)
Next count
Dim count As Integer
For count = 3 to 0 Step -1
Debug.WriteLine(count)
Next count
Dim count As Integer
For count = 0 to 10 Step 2
Debug.WriteLine(count)
Next count
Dim loc As Single
For loc = 0.5 To 15 Step 0.5
Debug.WriteLine(loc)
Next loc
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The Do…Loop Statement
• Unlike the For…Next statement, the Do…Loop
statement can be used to code both a pretest
loop and a posttest loop
• The Do…Loop statement begins with the Do
clause and ends with the Loop clause
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The Do…Loop Statement
(continued)
Figure 6-7: Syntax and examples of the Do...Loop statement
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The Do…Loop Statement
(continued)
Figure 6-7: Syntax and examples of the Do...Loop statement
(continued)
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The Do…Loop Statement
(continued)
Figure 6-9: Flowcharts
for the examples shown
in Figure 6-7
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The Do…Loop Statement
(continued)
Figure 6-9: Flowcharts for
the examples shown in
Figure 6-7 (continued)
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Using Counters and Accumulators
• Counters and accumulators are used within a
repetition structure to calculate subtotals, totals,
and averages
• A counter is a numeric variable used for counting
something and is typically updated by 1
• An accumulator is a numeric variable used for
accumulating and is updated by an amount that
varies
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Using Counters and Accumulators
(continued)
• Initializing: assigning a beginning value to the
counter or accumulator
• Updating (incrementing): adding a number to the
value stored in the counter or accumulator
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Nested Repetition Structures
Lesson B Objectives
• Nest repetition structures
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Nesting Repetition Structures
• In a nested repetition structure, one loop,
referred to as the inner loop, is placed entirely
within another loop, called the outer loop
• A clock uses nested loops to keep track of the
time
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Nesting Repetition Structures
(continued)
Figure 6-16: Nested loops used by a clock
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The Grade Calculator Application
• Professor Arkins needs an application that allows
him to assign a grade to any number of students
• Each student’s grade is based on three test
scores, with each test worth 100 points
• The application should total the test scores and
then assign the appropriate grade, using the
table shown on the next slide
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The Grade Calculator Application
(continued)
Total points earned
Grade
270–300
A
240–269
B
210–239
C
180–209
D
below 180
F
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The Grade Calculator Application
(continued)
• uiAssignGradeButton’s Click event procedure
– Allows Professor Arkins to enter each student’s
test scores, and then assign the appropriate grade
– Contains two loops, one nested within the other
– A For...Next statement controls the inner loop
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The Grade Calculator Application
(continued)
• uiAssignGradeButton’s Click event procedure
(continued)
– A Do...Loop statement controls the outer loop
– The inner loop is a pretest loop
– The outer loop is a posttest loop
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The Grade Calculator Application
(continued)
Figure 6-20: Sample run of the application that contains the
procedure
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Coding the Shoppers Haven
Application
Lesson C Objectives
• Select the existing text in a text box
• Prevent a form from closing
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Shoppers Haven
• The manager of Shoppers Haven wants an
application that the store clerks can use to
calculate the discounted price of an item, using
discount rates from 10% through 30% in
increments of 5%
• The clerks will enter the item’s original price
• The application should display the discount rates
and the discounted prices in the interface
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Shoppers Haven (continued)
Figure 6-21: User interface for the Shoppers Haven application
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Shoppers Haven (continued)
Figure 6-22: TOE chart for the Shoppers Haven application
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Shoppers Haven (continued)
Figure 6-23: Pseudocode for the Calculate button’s Click event
procedure
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Shoppers Haven (continued)
Figure 6-27: Discounted prices shown in the Shoppers Haven
application
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Selecting the Existing Text in a Text
Box
• Use the SelectAll method to select all of the text
contained in a text box
• Syntax: textbox.SelectAll()
• textbox is the name of the text box whose text
you want to select
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Selecting the Existing Text in a Text
Box (continued)
• Enter the SelectAll method in a text box control’s
Enter event
• A text box control’s Enter event occurs when the
user tabs to the control, and when the Focus
method is used in code to send the focus to the
control
• The uiOriginalTextBox control’s Enter event is
responsible for highlighting the existing text in the
control
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Selecting the Existing Text in a Text
Box (continued)
Figure 6-29: Text selected in the Shoppers Haven application
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Coding the TextChanged Event
Procedure
• A control’s TextChanged event occurs when the
contents of a control’s Text property change
• Use the uiOriginalTextBox’s TextChanged event
to clear the contents of the uiDiscPricesLabel
when the user changes the original price
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Coding the ShoppersForm’s Closing
Event Procedure
• A form’s Closing event occurs when a form is
about to be closed
• In the Shoppers Haven application, the Closing
event procedure is responsible for:
– Verifying that the user wants to exit the application
– Taking an action based on the user’s response
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Coding the ShoppersForm’s Closing
Event Procedure (continued)
Figure 6-31: Pseudocode for the ShoppersForm’s Closing event
procedure
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Coding the ShoppersForm’s Closing
Event Procedure (continued)
Figure 6-33: Message box displayed by the form’s Closing event
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Summary
• Repetition structure (loop): the computer repeats
a set of instructions until some condition is met
• Code a repetition structure in Visual Basic .NET
using one of the following statements: For...Next,
Do...Loop, and For Each…Next
• The For...Next statement is pretest loops only
• The Do...Loop statement can code pretest and
posttest loops
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Summary (continued)
• To use a counter or accumulator:
– Initialize, if necessary
– Update using an assignment statement in a
repetition structure
• To nest a repetition structure, place the entire
inner loop within the outer loop
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Summary (continued)
• To process code when the user tabs to a control,
or when the Focus method is used in code to
send the focus to the control, enter the code in
the control’s Enter event procedure
• To process code when a form is about to be
closed, enter the code in the form’s Closing event
procedure
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