1. Programming with Microsoft Visual Basic 2008 Fourth Edition
Chapter Six
The Repetition Structure

2. Previewing the Shoppers Haven Application
Open the Shoppers.exe file
The Shoppers Haven application:
Allows user to enter an item’s original price and its discount rate
Calculates and displays amount of discount and discounted price
Programming with Microsoft Visual Basic 2008, Fourth Edition

3. Previewing the Shoppers Haven Application (continued)
Figure 6-1: Discount and discounted price shown in the interface
Programming with Microsoft Visual Basic 2008, Fourth Edition

4. Lesson A Objectives After studying Lesson A, you should be able to:
Code the repetition structure using the Do...Loop statement
Include the repetition structure in pseudocode
Include the repetition structure in a flowchart
Initialize and update counters and accumulators
Code the repetition structure using the For...Next statement
Programming with Microsoft Visual Basic 2008, Fourth Edition

5. The Repetition Structure
Repetition structure (or loop): Repeatedly processes instructions until condition is met
Example: Calculate net pay for each employee
Pretest loop: Evaluates condition prior to processing instructions
Posttest loop: Evaluates condition after processing instructions
Programming with Microsoft Visual Basic 2008, Fourth Edition

6. The Do…Loop Statement
Do…Loop statement: Used to code both pretest loop and posttest loop
Two variations of syntax:
One for pretest loop and one for posttest loop
While keyword: Indicates that instructions should be processed while condition is true
Until keyword: Indicates that instructions should be processed until condition becomes true
Programming with Microsoft Visual Basic 2008, Fourth Edition

7. The Do…Loop Statement (continued)
Begins with Do clause, ends with Loop clause
Instructions to be repeated are placed between Do and Loop clauses
Use While or Until keyword before condition
Condition must evaluate to Boolean True or False
Location of {While|Until} condition:
Pretest loop: Appears in Do clause
Posttest loop: Appears in Loop clause
Diamond: Represents loop condition in flowchart
Programming with Microsoft Visual Basic 2008, Fourth Edition

8. The Do…Loop Statement (continued)
Figure 6-2: Syntax versions and examples of the Do...Loop statement
Programming with Microsoft Visual Basic 2008, Fourth Edition

9. The Do…Loop Statement (continued)
Figure 6-3: Processing steps for the pretest loop example from Figure 6-2
Programming with Microsoft Visual Basic 2008, Fourth Edition

10. The Do…Loop Statement (continued)
Figure 6-4: Processing steps for the posttest loop example from Figure 6-2
Programming with Microsoft Visual Basic 2008, Fourth Edition

11. The Do…Loop Statement (continued)
Figure 6-5: Pseudocode and flowchart for the pretest loop example shown in Figure 6-2
Programming with Microsoft Visual Basic 2008, Fourth Edition

12. The Do…Loop Statement (continued)
Figure 6-6: Pseudocode and flowchart for the posttest loop example shown in Figure 6-2
Programming with Microsoft Visual Basic 2008, Fourth Edition

13. Counters and Accumulators
Used to calculate subtotals, totals, and averages
Counter: Numeric variable used for counting
Accumulator: Variable used to tally various amounts
Initializing: Assigning initial value to counter or accumulator
Updating (incrementing or decrementing):
Changing value stored in counter or accumulator
Update statement must be within repetition structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

14. The Sales Express Application
Objective: Display average annual sales of company
Application uses pretest loop
Priming read: Use to prime (prepare or set up) loop by performing first action prior to entering loop
Programming with Microsoft Visual Basic 2008, Fourth Edition

15. The Sales Express Application (continued)
Figure 6-7: Pseudocode for the bntCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

16. The For…Next Statement
Processes instructions specific number of times
Condition tested before processing (pretest loop)
Also called counter controlled loop
Must specify start value, end value, and step value
Start value and end value provide looping range
Step value increments or decrements counter
Hexagon: Flowchart symbol
Programming with Microsoft Visual Basic 2008, Fourth Edition

17. The For…Next Statement (continued)
Figure 6-11: Pseudocode and flowchart for Example 1 in Figure 6-9
Programming with Microsoft Visual Basic 2008, Fourth Edition

18. The Monthly Payment Calculator Application
Task of btnCalc control’s Click event procedure:
Calculate and display monthly car payments
Use term of five years and rates from 5 – 10%
Basic structure of For…Next statement
Use procedure level variable, dblRate, as counter
Set starting and ending values to 0.05 and 0.1
Set step value to 0.01
Calculate monthly payment for current rate
Display interest rate and corresponding payment
Programming with Microsoft Visual Basic 2008, Fourth Edition

19. The Monthly Payment Calculator Application (continued)
Figure 6-11: Additional code entered in the btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

20. The Monthly Payment Calculator Application (continued)
Figure 6-12: Monthly payments shown in the interface
Programming with Microsoft Visual Basic 2008, Fourth Edition

21. Lesson A Summary
Repetition structure (loop): Repeats set of instructions until some condition is met
Use Do...Loop statement to code pretest and posttest loops
Counters are used to track count of something
Accumulators are used to tally up an amount
Counters and accumulators must be:
Initialized
Updated within loop
Programming with Microsoft Visual Basic 2008, Fourth Edition

22. Lesson A Summary (continued)
Use For...Next statement to code pretest loops
Use hexagon to show a For…Next loop in flowchart
Programming with Microsoft Visual Basic 2008, Fourth Edition

23. Lesson B Objectives After studying Lesson B, you should be able to:
Nest repetition structures
Refresh the screen
Delay program execution
Programming with Microsoft Visual Basic 2008, Fourth Edition

24. Nested Repetition Structures
Inner loop placed entirely within outer loop
Inner loop is referred to as nested loop
Clocks use nested loops to keep track of time
Minute and hour hands of clock can be compared to loops:
Outer loop corresponds to hour hand
Inner loop corresponds to minute hand
Programming with Microsoft Visual Basic 2008, Fourth Edition

25. Nested Repetition Structures (continued)
Figure 6-16: Logic used by a clock’s hour and minute hands
Programming with Microsoft Visual Basic 2008, Fourth Edition

26. The Refresh and Sleep Methods
Refresh method: Ensures that any code appearing before it that affects interface’s appearance is processed
Syntax: Me.Refresh()
Me refers to current form
Sleep method: Delays program execution
Syntax:
System.Threading.Thread.Sleep(milliseconds)
Millisecond: 1/1000 of second
Programming with Microsoft Visual Basic 2008, Fourth Edition

27. The Refresh and Sleep Methods (continued)
Figure 6-18: Refresh and Sleep methods added to the procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

28. Monthly Payment Calculator Application—Nested For...Next Statements
Objective: Calculate and display car payments
Use nested loops in btnCalc control’s Click event
Outer For…Next statement:
Control interest rates ranging from %
Increment rates at each iteration by 1%
Inner For…Next statement:
Controls terms from years
Programming with Microsoft Visual Basic 2008, Fourth Edition

29. Figure 6-19: Outer and inner (nested) loops entered in the procedure
Monthly Payment Calculator Application—Nested For...Next Statements (continued)
Figure 6-19: Outer and inner (nested) loops entered in the procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

30. Figure 6-20: Monthly payments shown in the interface
Monthly Payment Calculator Application—Nested For...Next Statements (continued)
Figure 6-20: Monthly payments shown in the interface
Programming with Microsoft Visual Basic 2008, Fourth Edition

31. Lesson B Summary
To nest repetition structure, place entire inner loop within outer loop
To refresh interface, use Refresh method
To pause program execution, use Sleep method
Programming with Microsoft Visual Basic 2008, Fourth Edition

32. Lesson C Objectives After studying Lesson C, you should be able to:
Include a list box in an interface
Select a list box item from code
Determine the selected item in a list box
Programming with Microsoft Visual Basic 2008, Fourth Edition

33. Coding the Shoppers Haven Application
Objective: Allow entry of price and discount rate
Application requirements:
Discount rates range from 10% through 30%
Discount rate should be incremented by 5%
Calculate discount amount and discounted price
Display discount amount and discounted price
Programming with Microsoft Visual Basic 2008, Fourth Edition

34. Coding the Shoppers Haven Application (continued)
Figure 6-23: TOE chart for the Shoppers Haven application
Programming with Microsoft Visual Basic 2008, Fourth Edition

35. Coding the Shoppers Haven Application (continued)
Figure 6-24: Partially completed user interface for the Shoppers Haven application
Programming with Microsoft Visual Basic 2008, Fourth Edition

36. Including a List Box in an Interface
List box: Displays list of choices from which user can select 0 or more items
SelectionMode property:
Controls number of choices that can be selected
Values: None, One, MultiSimple, or MultiExtended
ListBox tool: Used to add list box to interface
List box can be made any size you want
Programming with Microsoft Visual Basic 2008, Fourth Edition

37. Adding Items to a List Box
Collection: Group of objects treated as one unit
Items collection: Refers to group of items in list box
Index: Unique number that identifies each item in collection
Is zero-relative (starts with 0)
Items collection’s Add method: Used to add item to list box
Implemented in form’s Load event procedure to prepare list box before form is displayed
Programming with Microsoft Visual Basic 2008, Fourth Edition

38. Adding Items to a List Box (continued)
Figure 6-26: Syntax and examples of the Add method
Programming with Microsoft Visual Basic 2008, Fourth Edition

39. Adding Items to a List Box (continued)
Figure 6-27: Result of processing the code shown in Figure 6-26
Programming with Microsoft Visual Basic 2008, Fourth Edition

40. Adding Items to a List Box (continued)
Sorted property of list box: Determines order in which items are displayed in list box
If true, newly added item is placed in its proper position
If false, newly added item is placed at end of list
Uses dictionary sort order
Programming with Microsoft Visual Basic 2008, Fourth Edition

41. The SelectedItem And SelectedIndex Properties
SelectedItem property: Contains value of item selected in list box
Contains empty string when no item is selected
SelectedIndex property: Contains index of item selected in list box
Contains -1 when no item is selected
Default list box item: Appears when application is first loaded
Can be chosen by setting either SelectedItem or SelectedIndex property
Programming with Microsoft Visual Basic 2008, Fourth Edition

42. The SelectedItem And SelectedIndex Properties (continued)
Figure 6-30: Examples of the SelectedItem and SelectedIndex properties
Programming with Microsoft Visual Basic 2008, Fourth Edition

43. The SelectedItem And SelectedIndex Properties (continued)
Figure 6-31: Default item selected in the list box
Programming with Microsoft Visual Basic 2008, Fourth Edition

44. The SelectedValueChanged and SelectedIndexChanged Events
SelectedValueChanged and SelectedIndexChanged events: Occur when user or code statement selects item in list box
Can use these events to process instructions when selection is made
Programming with Microsoft Visual Basic 2008, Fourth Edition

45. Coding the btnCalc Control’s Click Event Procedure
Figure 6-32: Pseudocode for the btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

46. Coding the btnCalc Control’s Click Event Procedure (continued)
Figure 6-33: Discount and discounted price shown in the interface
Programming with Microsoft Visual Basic 2008, Fourth Edition

47. Lesson C Summary A list box displays list of items
Use SelectionMode property to set number of items that can be selected in list box
Use Items.Add method to add items to list box
Use Sorted property to sort list box items in dictionary order
Use SelectedItem or SelectedIndex property to determine which item was selected
Programming with Microsoft Visual Basic 2008, Fourth Edition

48. Lesson C Summary (continued)
Use SelectedValueChanged or SelectedIndexChanged events to perform tasks when an item in list box is selected
Programming with Microsoft Visual Basic 2008, Fourth Edition