Chapter 05

Microsoft Visual Basic 2010: Reloaded Fourth 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 a nested selection structure• Include a multiple-alternative selection structure in

pseudocode and in a flowchart• Code a multiple-alternative selection structure • Include radio buttons in an interface

Microsoft Visual Basic 2010: Reloaded, Fourth Edition 2

Objectives (cont'd.)

• Display a message in a message box• Prevent the entry of invalid characters in a text box

Microsoft Visual Basic 2010: Reloaded, Fourth Edition

Making More Than One Decision

• Nested selection structure: when a selection structure’s true or false path contains another selection structure

Figure 5-1: Selection structures containing nested selection structures

Nested Selection Structures (cont'd.)

Figure 5-1: Selection structures containing nested selection structures (cont’d.)

The Voter Eligibility Application

Figure 5-2: Problem specification for the Voter Eligibility application

Figure 5-3: Sample run of the Voter Eligibility application

The Voter Eligibility Application (cont’d.)

Figure 5-4: Flowchart showing the nested selection structure in the true path

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

Figure 5-6: Code for the flowcharts in Figures 5-4 and 5-5

The Voter Eligibility Application (cont’d.)

Figure 5-6: Code for the flowcharts in Figures 5-4 and 5-5 (cont’d.)11

Multiple-Alternative Selection Structures

• Multiple-alternative selection structures: select structures that can choose from several alternatives

Figure 5-7: Problem specification for the Yardley Theater application

Multiple-Alternative Selection Structures (cont'd.)

Figure 5-8: Sample run of the Yardley Theater application

Multiple-Alternative Selection Structures (cont'd.)

Figure 5-9: Pseudocode for the Display button’s Click event procedure

Figure 5-9: Flowchart for the Display button’s Click event procedure

Figure 5-10: Two versions of the code corresponding to Figures 5-9 and 5-10

Figure 5-10: Two versions of the code corresponding to Figures 5-9 and 5-10 (cont’d.)

The Select Case Statement

• Select Case statement: – Used when there are many paths from which to

choose– Simpler and clearer than using several If…Then…

Else statements– Begins with Select Case, which specifies the

value to be matched– Ends with End Select– Has one Case clause for each possible path– Case Else is optional but must be the last clause

in the statement

Figure 5-12: How to use the Select Case statement

The Select Case Statement (cont'd.)• Case clause may have more than one value,

separated by commas– Only one value must be matched to process the

code in this Case clause• Processing of a Case clause code stops when the

next Case clause is encountered• If no values in Case clauses are matched, the Case Else clause is processed

Specifying a Range of Values in a Case Clause

• To and Is keywords: used to 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 5-13: How to specify a range of values in a Case clause

Using Radio Buttons in an Interface

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

• Radio button 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

Using Radio Buttons in an Interface (cont'd.)

• 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 buttonMicrosoft Visual Basic 2010: Reloaded, Fourth Edition 24

Figure 5-14: Gentry Supplies application’s interface

Using Radio Buttons in an Interface (cont'd.)

Figure 5-15: The Display button’s Click event procedure

Figure 5-15: The Display button’s Click event procedure (cont'd.)

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 5-16: How to use the MessageBox.Show method

Figure 5-16: How to use the MessageBox.Show method (cont’d.)

The MessageBox.Show Method (cont'd.)

Figure 5-17: Message box displayed by the code in Example 1 in Figure 5-16

The MessageBox.Show Method (cont'd.)

Figure 5-18: Message box displayed by the code in Example 2 in Figure 5-14

Figure 5-19: How to use the MessageBox.Show method’s return value

Using 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; set it to True to discard the character

Using the KeyPress Event (cont’d.)

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

• Line continuation character: the underscore• Allows you to split a line of code into two lines in

the code editor

Figure 5-20: How to use the KeyPress event to cancel invalid characters

Programming Tutorial 1

Figure 5-23: MainForm for the Rock, Paper, Scissors Game application

Programming Tutorial 2

Figure 5-34: MainForm for the Charleston Cable Company application

Programming Example

Figure 5-40: MainForm in the CD Emporium application

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…Then…Else or Select Case statements for multiple-alternative selection structures

• Diamond symbol represents the condition in a multiple-alternative selection structures– Each flowline represents a possible path

Summary (cont'd.)

• Each Case clause in Select Case statement represents a possible path

• 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

• Use a container to isolate groups of radio buttons

Summary (cont'd.)

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

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

• Use the KeyPress event of a text box to prevent it from accepting an inappropriate character– Set the e parameter’s Handled property to True

constant to discard the character

Chapter 05

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nested selection

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