1. Programming with Microsoft Visual Basic 2010 5th Edition
Chapter Six
The Repetition Structure

2. Previewing the Shopper’s Haven Application
Open the Shoppers.exe file
The Shopper’s 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 2010, 5th Edition

3. Previewing the Shopper’s Haven Application (cont’d.)
Figure 6-1 Discount and discounted price shown in the interface
Programming with Microsoft Visual Basic 2010, 5th Edition

4. Lesson A Objectives After studying Lesson A, you should be able to:
Differentiate between a looping condition and a loop exit condition
Explain the difference between a pretest loop and a posttest loop
Include pretest and posttest loops in pseudocode and a flowchart
Write a Do…Loop statement

5. Lesson A Objectives (cont’d.)
Stop an infinite loop
Utilize counters and accumulators
Explain the purpose of the priming and update reads
Abbreviate assignment statements using the arithmetic assignment operators
Code a counter-controlled loop using the For…Next statement

6. Repeating Program Instructions
Repetition structure (or loop)
Repeatedly processes instructions until condition is met
Example: “Make hay while the sun shines”
Looping condition
The requirement for repeating the instructions
In above example: “while the sun shines”
Loop exit condition
The requirement for not repeating the instructions
Example: “until the sun stops shining”

7. Repeating Program Instructions (cont’d.)
Pretest loop
Evaluates condition prior to processing instructions
Posttest loop
Evaluates condition after processing instructions

8. The Do…Loop Statement Do…Loop statement Two variations of syntax
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

9. The Do…Loop Statement (cont’d.)
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

10. Figure 6-7 Syntax versions and examples of the Do...Loop statement

11. Figure 6-8 Processing steps for the loop examples from Figure 6-7 (continues)

12. Figure 6-8 Processing steps for the loop examples from Figure 6-7 (cont’d.)

13. Figure 6-9 Flowcharts for the loop examples from Figure 6-7

14. The Do…Loop Statement (cont’d.)
Infinite loop
Also called endless loop
The condition to end the loop is never met
To stop an infinite loop:
Click Debug on the menu bar
Then click Stop Debugging

15. Counters and Accumulators
Used to calculate subtotals, totals, and averages
Counter: Numeric variable used for counting
Accumulator: Numeric variable for accumulating (adding together) something
Initializing
Assigning beginning value
Updating (incrementing)
Adding a number to accumulator’s value
Number can be positive or negative
Done within the loop body
Programming with Microsoft Visual Basic 2010, 5th Edition

16. The Sales Express Company Application
Objective: Calculate average sales amount entered by sales manager
Application uses pretest loop
Priming read
Use to prime (prepare or set up) loop by performing first action prior to entering loop
Update read
Allows user to update value that controls loop’s condition
Programming with Microsoft Visual Basic 2010, 5th Edition

17. Figure 6-16 Pseudocode for the Average button’s Click event procedure
Programming with Microsoft Visual Basic 2010, 5th Edition

18. Arithmetic Assignment Operators
Used to abbreviate an assignment statement:
Containing an arithmetic operator
Format
variableName = variableName arithmeticOperator value
Example
intAge = intAge + 1 can be abbreviated
intAge += 1

19. Figure 6-20 Syntax and examples of the arithmetic assignment operators (continues)

20. Figure 6-20 Syntax and examples of the arithmetic assignment operators (cont’d.)

21. The For…Next Statement
Used to code a counter-controlled loop
Processes instructions precise number of times
Condition tested before processing (pretest loop)
Must specify start value, end value, and step value
Start value and end value provide looping range
Step value increments or decrements counter
For clause represented by a hexagon in flowchart

22. Figure 6-22 For…Next statement’s syntax, examples, and processing tasks (continues)

23. Figure 6-22 For…Next statement’s syntax, examples, and processing tasks (cont’d.)

24. Figure 6-23 Processing steps for Example 1 in Figure 6-22

25. The Monthly Payment Calculator Application
Figure 6-24 Problem specification for the Monthly Payment Calculator application
Figure 6-25 Sample run of the Monthly Payment Calculator application
Programming with Microsoft Visual Basic 2010, 5th Edition

26. The Monthly Payment Calculator Application (cont’d.)
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

27. The Monthly Payment Calculator Application (cont’d.)
Figure 6-26 Pseudocode and flowchart for the Calculate button’s Click event procedure (cont’d.)

28. Figure 6-26 Pseudocode and flowchart for the Calculate button’s Click event procedure

29. Comparing the For…Next and Do…Loop Statements
Either can be used to code counter-controlled loop
For…Next is more convenient
When using a Do…Loop statement:
Must declare and initialize the counter variables
And update the counter variable
Include the appropriate comparison in the Do clause
When using For…Next
Declaration, initialization, update, and comparison
Handled by the For clause

30. Lesson A Summary
Repetition structure (loop): Repeats set of instructions until some condition is met
To stop an infinite loop
Click Debug on the menu bar and then click Stop Debugging
Counters and accumulators must be:
Initialized
Updated within loop
Arithmetic assignment operators
Used to abbreviate an assignment statement

31. 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 2010, 5th Edition

32. 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 second hands of clock can be compared to loops
Outer loop corresponds to minute hand
Inner loop corresponds to second hand

33. Nested Repetition Structures (cont’d.)
Figure 6-31 Logic used by a clock’s minute and second hands

34. The Refresh and Sleep Methods
Refresh method: Ensures that computer processes any previous lines of code that affect interface appearance
Syntax: Me.Refresh()
Me refers to current form
Sleep method: Delays program execution
Syntax:
System.Threading.Thread.Sleep(millisecond s)
Millisecond: 1/1000 of second

35. The Refresh and Sleep Methods (cont’d.)
Figure 6-33 Refresh and Sleep methods added to the procedure
Programming with Microsoft Visual Basic 2010, 5th Edition

36. Revisiting the Monthly Payment Calculator Application
Objective: Calculate and display car payments
Use nested loops in btnCalc control’s Click event procedure
Outer loop
Control interest rates ranging from %
Increment rates at each iteration by 1%
Inner loop
Controls terms from years

37. Figure 6-35 Outer and inner nested loops entered in the procedure
Figure 6-34 Modified problem specification for the Monthly Payment Calculator application
Figure 6-35 Outer and inner nested loops entered in the procedure
Programming with Microsoft Visual Basic 2010, 5th Edition

38. Figure 6-36 Monthly payments shown in the interface
Programming with Microsoft Visual Basic 2010, 5th Edition

39. Lesson B Summary To nest repetition structure: To refresh interface:
Place entire inner loop within outer loop
To refresh interface:
Use Refresh method
Syntax: Me.Refresh()
To pause program execution:
Use Sleep method
Syntax: System.Threading.Thread.Sleep(milliseconds)

40. Lesson C Objectives After studying Lesson C, you should be able to:
Include a list box on a form
Select a list box item from code
Determine the selected item in a list box

41. Creating the Shopper’s Haven Application
Application requirements
User enters price and discount rate
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

42. Figure 6-39 TOE chart for the Shopper’s Haven application
Programming with Microsoft Visual Basic 2010, 5th Edition

43. Including a List Box in an Interface
Displays list of choices from which user can select zero or more choices
SelectionMode property
Controls number of choices that can be selected
ListBox tool
Used to add list box to interface
List box can be made any size you want
Windows standard: Three to eight choices

44. 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
First item has index of zero
Items collection’s Add method
Used to add item to list box
Usually entered in form’s Load event procedure

45. Figure 6-41 Syntax and examples of the Items collection’s Add method
Programming with Microsoft Visual Basic 2010, 5th Edition

46. Adding Items to a List Box (cont’d.)
Figure 6-42 Result of processing the code from Figure 6-41
Programming with Microsoft Visual Basic 2010, 5th Edition

47. Adding Items to a List Box (cont’d.)
Sorted property of list box
Determines order of displayed items
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

48. Coding the Shopper’s Haven Application
Enter the appropriate Add methods:
In the form’s Load event procedure
Follow instructions on page 378

49. The SelectedItem and SelectedIndex Properties
SelectedItem property
Contains value of selected item
Contains empty string when no item is selected
SelectedIndex property
Contains value of selected item’s index
Contains -1 when no item is selected
Default list box item
Appears when application is first loaded
Chosen by setting either SelectedItem or SelectedIndex property

50. The SelectedItem and SelectedIndex Properties (cont’d.)
Figure 6-45 Second item selected in the list box

51. Figure 6-46 Examples of the list box’s SelectedItem and SelectedIndex properties
Programming with Microsoft Visual Basic 2010, 5th Edition

52. Figure 6-47 Examples of selecting the default list box item
Programming with Microsoft Visual Basic 2010, 5th Edition

53. Figure 6-48 Default item selected in the list box
Programming with Microsoft Visual Basic 2010, 5th Edition

54. The SelectedValueChanged and SelectedIndexChanged Events
SelectedValueChanged event and SelectedIndexChanged event
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 2010, 5th Edition

55. Coding the btnCalc Control’s Click Event Procedure
Figure 6-49 Pseudocode for the btnCalc control’s Click event procedure

56. Coding the btnCalc Control’s Click Event Procedure (cont’d.)
Figure 6-50 Calculated amounts shown in the interface
Programming with Microsoft Visual Basic 2010, 5th Edition

57. 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

58. Lesson C Summary (cont’d.)
To perform tasks when a different item is selected in a list box:
Add code to SelectedValueChanged or SelectedIndexChanged procedures