microsoft visual basic 2008: reloaded third edition chapter five more on the selection structure

Microsoft Visual Basic 2008: Reloaded Third Edition

Chapter FiveMore on the Selection Structure

Objectives

After studying this chapter, you should be able to:

• Include a nested selection structure in pseudocode and in a flowchart

• Code an If/ElseIf/Else selection structure

• Include a Case selection structure in pseudocode and in a flowchart

• Code a Case selection structure

• Include radio buttons in an interface

Microsoft Visual Basic 2008: Reloaded, Third Edition 2

Objectives (continued)

• Display a message in a message box

• Prevent the entry of unwanted characters in a text box



Nested Selection Structures

• Nested selection structure: a selection structure that is completely contained within another selection structure

• Primary decision: decision made by the outer selection structure

• Secondary decision: decision made by the inner selection structure

Nested Selection Structures (continued)


Figure 5-1: Sample run of the Voter Eligibility application


Figure 5-2: Pseudocode and code showing the nested selection structure in the true path


Figure 5-3: Flowchart showing the nested selection structure in the true path


Figure 5-4: Pseudocode and code showing the nested selection structure in the false path


Figure 5-5: Flowchart showing the nested selection structure in the false path

The If/ElseIf/Else Selection Structure

• Example: a procedure to display a message based on a letter grade


The If/ElseIf/Else Selection Structure (continued)


Figure 5-6: Sample run of the Grade Message application

The If/ElseIf/Else Selection Structure (continued)


Figure 5-7: Two versions of the messageButton’s Click event procedure


Figure 5-7: Two versions of the messageButton’s Click event procedure (continued)


The Case Selection Structure

• Case selection structure: – Used when there are many paths from which to

choose– Simpler and clearer than using If/ElseIf/Else– Flowchart symbol is the same as the symbol for If,

If/Else, and If/ElseIf/Else


The Case Selection Structure (continued)

Figure 5-8: Pseudocode showing the Case selection structure



• Case selection structure in a flowchart:– Uses the diamond symbol– One flowline into the diamond, but many flowlines

out of the diamond– Each flowline represents a possible path

• Case selection structure evaluates an expression to determine which path to take

• Uses the Select Case statement:– Begins with Select Case– Ends with End Select– Has one Case clause for each possible value



Figure 5-9: Flowchart showing the Case selection structure


Figure 5-10: How to use the Select Case statement


Specifying a Range of Values in an ExpressionList

• To and Is keywords: specify a range of values in a Case clause’s expression list

• To: – When you know both the upper and lower bounds of

the range

• Is: – When you know only one end of the range– Used with a comparison operator


Figure 5-11: Example of using the To and Is keywords in a Select Case statement

Using Radio Buttons

• RadioButton control: allows the user to select only one of a group of two or more choices

• RadioButton choices are related but mutually exclusive; only one can be selected

• Container control:– Isolates a group of radio buttons– Includes GroupBox, Panel, and TableLayout controls

Microsoft Visual Basic 2005: Reloaded, Second Edition 21

Using Radio Buttons (continued)


Figure 5-12: Sample run of the Gentry Supplies application


• Minimum number of radio buttons in a group is two– Must select a radio button to deselect another

• Recommended maximum number in a group: seven

• Windows standard is to set one as the default radio button– Shows as selected when the screen appears– Should be the most likely selection or the first radio

button in the group

• Set the Checked property to True to make it the default radio button



Figure 5-13: The displayButton’s Click event procedure



Figure 5-13: The displayButton’s Click event procedure (continued)


The MessageBox.Show Method

• MessageBox.Show method: – Displays a message box with text, one or more

buttons, and an icon

• When a message box is displayed, the program waits until the user selects a button

• MessageBox.Show returns an integer value indicating which button the user selected

• DialogResult values include:– Windows.Forms.DialogResult.Yes– Windows.Forms.DialogResult.No


Figure 5-14: How to use the MessageBox.Show method

The MessageBox.Show Method (continued)


Figure 5-15: Message box displayed by Example 1 in Figure 5-14

The MessageBox.Show Method (continued)


Figure 5-16: Message box displayed by Example 2 in Figure 5-14


Figure 5-17: How to use the value returned by the MessageBox.Show method

Coding the KeyPress Event


• Can prevent a text box from accepting an inappropriate character by coding the text box’s KeyPress event

• KeyPress event: occurs each time the user presses a key while the control has the focus• Use the e parameter’s KeyChar property to

determine the pressed key• Use the e parameter’s Handled property to

cancel the key if it is inappropriate• Use ControlChars.Back constant to represent

the Backspace key on the keyboard

Coding the KeyPress Event (continued)


Figure 5-18: How to use the KeyPress event

Programming Tutorial


Figure 5-20: User interface

Programming Example


Figure 5-31: User interface


Summary

• Selection structures can be nested in either the true or false path of another selection structure

• Primary decision is made by the outer selection structure, while the secondary decision is made by the inner (nested) selection structure

• Use If/ElseIf/Else or Case structures when there are several possible alternative outcomes

• Select Case statement is used to code the Case selection structure


Summary (continued)

• Use To keyword to specify a range of valid values when both the lower and upper bounds are known

• Use Is keyword with a comparison operator to specify a lower or upper bound but not both

• Use radio buttons to limit the user to one choice from a group of two or more related but mutually exclusive choices

• MessageBox.Show method allows an application to communicate with the user

Summary (continued)

• MessageBox.Show method returns an integer indicating which button was chosen by the user

• Use sentence capitalization for the text message in the MessageBox.Show method, but book title capitalization for the caption

• Use the KeyPress event of a text box to prevent it from accepting an inappropriate character

• Use the ControlChars.Back constant to represent the Backspace key on the keyboard


microsoft visual basic 2008: reloaded third edition chapter five more on the selection structure

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