Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Six Repeating Program Instructions.

Microsoft Visual Basic 2010: Reloaded Fourth Edition

Chapter SixRepeating 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


Objectives (cont'd.)

• Include a list box in an interface

• Enable and disable a control

• Refresh the screen

• Delay program execution


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.)


Figure 6-1: Example of a looping condition and a loop exit condition



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



Figure 6-4: Pseudocode containing only the sequence structure



• 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


Figure 6-5: Modified problem specification and pseudocode containing a loop


The Repetition Structure (cont'd.)

• Repetition statements in Visual Basic– Do...Loop– For...Next– For Each...Next


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


Figure 6-6: How to use the Do…Loop statement


The Do...Loop Statement (cont'd.)

Figure 6-7: Flowchart for the pretest loop example from Figure 6-6


The Do...Loop Statement (cont'd.)

Figure 6-8: Flowchart for the posttest loop example from Figure 6-6


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


Figure 6-9: Modified pseudocode and code for the calcButton’s Click event procedure


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


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


The Sales Express Company Application (cont'd.)

Figure 6-11: Problem specification for the Sales Express Company application



Figure 6-12: Sample run of the Sales Express Company application


Figure 6-13: Pseudocode for the Average button’s Click event procedure



• 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



Figure 6-14: Flowchart for the Average button’s Click event procedure


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


The InputBox Function (cont'd.)

Figure 6-15: Example of an input dialog box



• 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


Figure 6-16: How to use the InputBox function



Figure 6-16: How to use the InputBox function (cont'd.)


Figure 6-17: Code associated with the pseudocode and flowchart shown in Figures 6-13 and 6-14, respectively


Figure 6-17: Code associated with the pseudocode and flowchart shown in Figures 6-13 and 6-14, respectively (cont’d.)



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


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


Adding Items to a List Box (cont'd.)

Figure 6-18: String Collection Editor window



Figure 6-19: How to use the Items collection’s Add method



Figure 6-20: Add methods entered in the MainForm’s Load event procedure



Figure 6-21: Result of processing the Add methods in Figure 6-20



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



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



• 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


Accessing Items in a List Box (cont'd.)


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



Figure 6-28: How to determine the number of items in a list box


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


The SelectedItem and SelectedIndex Properties (cont'd.)

Figure 6-29: Item selected in the animalListBox


Figure 6-30: How to use the SelectedItem and SelectedIndex properties


The SelectedItem and SelectedIndex Properties (cont'd.)

Figure 6-31: How to select the default list box item


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


Figure 6-32: Code showing the SelectedValueChanged and SelectedIndexChanged event procedures


(cont’d.)


Figure 6-32: Code showing the SelectedValueChanged and SelectedIndexChanged event procedures (cont’d.)


(cont’d.)


Figure 6-33: Result of processing the code shown in Figure 6-32


(cont’d.)


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


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


The Product Finder Application (cont'd.)

Figure 6-36: Pseudocode for the Find button’s Click event procedure


Figure 6-37: Product Finder application’s code


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


The Color Viewer Application (cont’d.)


Figure 6-38: MainForm in the Color Viewer application

The Color Viewer Application (cont’d.)


Figure 6-39: View Colors button’s Click event procedure

Programming Tutorial 1


Figure 6-41: MainForm for the Roll ‘Em Game application

• Creating the Roll ‘Em Game


Programming Tutorial 2

Figure 6-52: MainForm for the Just Birthdays application

• Coding the Just Birthdays Application


Programming Example

• Grade Calculator Application

Figure 6-63: MainForm in the Grade Calculator application


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


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


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


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

Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Six Repeating Program Instructions.

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