Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Six Repeating Program Instructions.
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Transcript of Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Six Repeating Program Instructions.
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 2
Objectives
After studying this chapter, you should be able to:
• Differentiate between a pretest and a posttest loop
• Include pretest and posttest loops in pseudocode and in a flowchart
• Write a Do…Loop statement
• Utilize counters and accumulators
• Display a dialog box using the InputBox function
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 3
Objectives (cont'd.)
• Include a list box in an interface
• Enable and disable a control
• Refresh the screen
• Delay program execution
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 4
The Repetition Structure
• Repetition structure (or loop): a structure that repeatedly processes one or more program instructions until a condition is met
• Looping condition: the requirement for repeating the instructions
• Loop exit condition: the requirement for not repeating the instructions
The Repetition Structure (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 5
Figure 6-1: Example of a looping condition and a loop exit condition
The Repetition Structure (cont’d.)
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Figure 6-2: Problem specification for the Getting to a Million Club application
Figure 6-3: Sample run of the Getting to a Million Club application
The Repetition Structure (cont’d.)
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Figure 6-4: Pseudocode containing only the sequence structure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 8
The Repetition Structure (cont’d.)
• Pretest loop – The condition is evaluated before the instructions
within the loop are processed– The instructions may be processed zero or more
times• Posttest loop
– The condition is evaluated after the instructions within the loop are processed
– The instructions are always processed at least once
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 9
Figure 6-5: Modified problem specification and pseudocode containing a loop
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 10
The Repetition Structure (cont'd.)
• Repetition statements in Visual Basic– Do...Loop– For...Next– For Each...Next
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 11
The Do...Loop Statement
• Do...Loop statement: codes both a pretest and posttest loop
• Loop body: the instructions between the Do and the Loop clauses
• Use While or Until to code the condition for the loop
• Repetition symbol in a flowchart is the diamond
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Figure 6-6: How to use the Do…Loop statement
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 13
The Do...Loop Statement (cont'd.)
Figure 6-7: Flowchart for the pretest loop example from Figure 6-6
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 14
The Do...Loop Statement (cont'd.)
Figure 6-8: Flowchart for the posttest loop example from Figure 6-6
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 15
Using Counters and Accumulators
• Counter: a numeric variable used for counting
• Accumulator: a numeric variable used for accumulating (adding together)
• Initializing: assigning a beginning value to a counter or accumulator variable
• Updating (or incrementing): adding a number to the value of a counter or accumulator variable
• Counters are always incremented by a constant value, usually 1
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Figure 6-9: Modified pseudocode and code for the calcButton’s Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 17
Figure 6-9: Modified pseudocode and code for the calcButton’s Click event procedure (cont’d.)
Using Counters and Accumulators (cont’d.)
Figure 6-10: Sample run of the modified Getting to a Million Club application
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The Sales Express Company Application
• Requirements: display the average amount the company sold during the prior year
• Input: the amount of each salesperson’s sales• Multiline property: if True, allows multiple lines
of text in a text box• ReadOnly property: if True, prevents a user from
changing the text box contents during run time• ScrollBars property: specifies whether a text
box has no scroll bars, horizontal or vertical, or both
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 19
The Sales Express Company Application (cont'd.)
Figure 6-11: Problem specification for the Sales Express Company application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 20
The Sales Express Company Application (cont'd.)
Figure 6-12: Sample run of the Sales Express Company application
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Figure 6-13: Pseudocode for the Average button’s Click event procedure
The Sales Express Company Application (cont'd.)
The Sales Express Company Application (cont'd.)
• Priming read: used to obtain the first input
• Update read: allows the user to update the value of an input item
• Infinite (or endless) loop: a loop that has no way to end
• Must verify that a variable does not contain the value 0 before using it as a divisor
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Microsoft Visual Basic 2010: Reloaded, Fourth Edition 23
Figure 6-14: Flowchart for the Average button’s Click event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 24
The InputBox Function
• InputBox function: displays a predefined dialog box that allows the user to enter data– Contains a text message, an OK button, a Cancel
button, and an input area
• InputBox function returns:– The user’s entry if the user clicks the OK button– An empty string if the user clicks the Cancel button
or the Close button on the title bar
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The InputBox Function (cont'd.)
Figure 6-15: Example of an input dialog box
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The InputBox Function (cont'd.)
• InputBox function arguments:– prompt: the message to display inside the dialog box– title: the text to display in the dialog box’s title bar– defaultResponse: a prefilled value for the user input
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 27
Figure 6-16: How to use the InputBox function
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 28
The InputBox Function (cont'd.)
Figure 6-16: How to use the InputBox function (cont'd.)
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Figure 6-17: Code associated with the pseudocode and flowchart shown in Figures 6-13 and 6-14, respectively
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 30
Figure 6-17: Code associated with the pseudocode and flowchart shown in Figures 6-13 and 6-14, respectively (cont’d.)
The InputBox Function (cont'd.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 31
Including a List Box in an Interface
• List box: displays a list of choices from which the user can select zero or more choices
• SelectionMode property: controls the number of choices a user can select– None: user can scroll but not select anything– One: user can select one item– MultiSimple and MultiExtended: user can select
multiple items
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 32
Adding Items to a List Box
• Items collection: a collection of the items in a list box
• Collection: a group of one or more individual objects treated as one unit
• Add method: adds an item to the list box’s Items collection– Items to be added must be converted to String– String Collection Editor window can be used to
specify list items during design time
• Load event of a form: occurs when an application is started and the form is displayed for the first time
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 33
Adding Items to a List Box (cont'd.)
Figure 6-18: String Collection Editor window
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Adding Items to a List Box (cont'd.)
Figure 6-19: How to use the Items collection’s Add method
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Adding Items to a List Box (cont'd.)
Figure 6-20: Add methods entered in the MainForm’s Load event procedure
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 36
Adding Items to a List Box (cont'd.)
Figure 6-21: Result of processing the Add methods in Figure 6-20
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 37
Adding Items to a List Box (cont'd.)
Figure 6-22: Sample run of the Jasper’s Food Hut application
Figure 6-23: Add to List button’s Click event procedure in the Jasper’s Food Hut application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 38
Adding Items to a List Box (cont'd.)
Figure 6-24: Sample run of the Clark’s Chicken application
Figure 6-25: Add to List button’s Click event procedure in the Clark’s Chicken application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 39
Adding Items to a List Box (cont'd.)
• Sorted property: – Determines if the list box items are sorted – Sort order is dictionary order
Figure 6-26: Examples of the list box’s Sorted property
Accessing Items in a List Box
• Index: – A unique number that identifies an item in a
collection– Is zero-relative: the first item has index of 0
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 40
Accessing Items in a List Box (cont'd.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 41
Figure 6-27: How to access an item in a list box
Determining the Number of Items in a List Box
• Items.Count property: stores the number of items in a list box– Count value is always one higher than the highest
index in the list box
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Microsoft Visual Basic 2010: Reloaded, Fourth Edition 43
Figure 6-28: How to determine the number of items in a list box
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 44
The SelectedItem and SelectedIndex Properties
• SelectedItem property: – Contains the value of the selected item in the list– If nothing is selected, it contains the empty string
• SelectedIndex property: – Contains the index of the selected item in the list– If nothing is selected, it contains the value -1
• Default list box item: the item that is selected by default when the interface first appears
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 45
The SelectedItem and SelectedIndex Properties (cont'd.)
Figure 6-29: Item selected in the animalListBox
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Figure 6-30: How to use the SelectedItem and SelectedIndex properties
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 47
The SelectedItem and SelectedIndex Properties (cont'd.)
Figure 6-31: How to select the default list box item
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 48
The SelectedValueChanged and SelectedIndexChanged Events
• SelectedValueChanged and SelectedIndexChanged events: – Occur when a user selects an item in a list box– Occur when a code statement selects an item in a list
box
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Figure 6-32: Code showing the SelectedValueChanged and SelectedIndexChanged event procedures
The SelectedValueChanged and SelectedIndexChanged Events
(cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 50
Figure 6-32: Code showing the SelectedValueChanged and SelectedIndexChanged event procedures (cont’d.)
The SelectedValueChanged and SelectedIndexChanged Events
(cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 51
Figure 6-32: Code showing the SelectedValueChanged and SelectedIndexChanged event procedures (cont’d.)
The SelectedValueChanged and SelectedIndexChanged Events
(cont’d.)
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Figure 6-33: Result of processing the code shown in Figure 6-32
The SelectedValueChanged and SelectedIndexChanged Events
(cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 53
The Product Finder Application
• Allows the user to enter a product ID• Searches for the ID in a list box• If found, highlights the ID• If not found, ensures that no ID is highlighted in the
list box
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Figure 6-34: Sample run of the application when the produce ID is found
Figure 6-35: Sample run of the application when the product ID is not found
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 55
The Product Finder Application (cont'd.)
Figure 6-36: Pseudocode for the Find button’s Click event procedure
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Figure 6-37: Product Finder application’s code
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 57
Figure 6-37: Product Finder application’s code (cont’d.)
The Color Viewer Application
• Enabled property: used to enable or disable a control– When False, the control appears dimmed (grayed
out), indicating it is not available for use
• Refresh method: ensures that the computer processes any previous lines of code that affect the interface’s appearance
• Sleep method: delays program execution– Argument is specified in milliseconds
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 58
The Color Viewer Application (cont’d.)
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 59
Figure 6-38: MainForm in the Color Viewer application
The Color Viewer Application (cont’d.)
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Figure 6-39: View Colors button’s Click event procedure
Programming Tutorial 1
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Figure 6-41: MainForm for the Roll ‘Em Game application
• Creating the Roll ‘Em Game
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 62
Programming Tutorial 2
Figure 6-52: MainForm for the Just Birthdays application
• Coding the Just Birthdays Application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 63
Programming Example
• Grade Calculator Application
Figure 6-63: MainForm in the Grade Calculator application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 64
Summary
• The three programming structures are sequence, selection, and repetition
• Repetition structure (or loop): repeatedly processes a set of instructions
• Pretest loop tests the condition before the instructions are processed
• Posttest loop tests the condition after the instructions are processed
• Do...Loop statement: codes a pretest or posttest loop
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 65
Summary (cont'd.)
• Use a While or Until condition in a Do...Loop
• Flowchart symbol for repetition is a diamond
• Counter and accumulators: variables that calculate subtotals, totals, and averages
• Priming read gets the first value prior to the loop
• Update read gets remaining values from the user• InputBox function: allows user input
• Verify that a variable does not contain a value of 0 before using it as a divisor
• List box: displays a list of choices
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 66
Summary (cont'd.)
• List box’s Items collection: contains the items in the list box
• Use String Collection Editor window to add list items during design time
• Items.Add method: adds an item to the list during run time
• Form’s Load event occurs before the form appears
• List box item’s index is used to access the item• Items.Count property stores the number of items
Microsoft Visual Basic 2010: Reloaded, Fourth Edition 67
Summary (cont'd.)
• SelectedItem property of a list box: contains the value of the selected item in the list
• SelectedIndex property of a list box: contains the index position of the selected item in the list
• SelectedValueChanged and SelectedIndexChanged events occur when an item in a list box is selected
• Enabled property: enables or disables a control• Sleep method: delays program execution• Me.Refresh: refreshes (redraws) the form