Chapter 5: More on the Selection Structure

Programming with Microsoft Visual Basic 2012

Chapter 5: More on the Selection Structure

Previewing the Modified Covington Resort Application

Programming with Microsoft Visual Basic 2012 2

Figure 5-1 Interface showing the calculated amounts Figure 5-2 Recalculated amounts shown in the interface

Lesson A Objectives

After studying Lesson A, you should be able to:• Include a nested selection structure in pseudocode and

in a flowchart• Code a nested selection structure• Desk-check an algorithm• Recognize common logic errors in selection structures• Include a multiple-alternative selection structure in

pseudocode and in a flowchart• Code a multiple-alternative selection structure


• Nested selection structure– When either a selection structure’s true path or its false

path contains another selection structure– The inner selection is called a nested selection


Nested Selection Structures

Figure 5-4 A problem that requires a nested selection structure


Nested Selection Structures (cont.)

Figure 5-5 A problem that requires two nested selection structures


Flowcharting a Nested Selection Structure

Figure 5-6 Problem specification and a correct solution for the voter eligibility problem


Flowcharting a Nested Selection Structure (cont.)

Figure 5-7 Another correct solution for the voter eligibility problem


Coding a Nested Selection Structure

Figure 5-8 Code for the flowcharts in Figures 5-6 and 5-7

• Four common errors:1. Using a compound condition rather than a nested

selection structure2. Reversing the decisions in the outer and nested

selection structures3. Using an unnecessary nested selection structure4. Including an unnecessary comparison in a condition


Logic Errors in Selection Structures

• Algorithm– The step-by-step instructions for accomplishing a task

• Desk-checking– The process of reviewing the algorithm while seated at

your desk rather than in front of a computer– Also called hand-tracing

• You use a pencil and paper to follow the algorithm’s instructions by hand

• Desk-check to make sure you do not miss any instructions, and that existing instructions are correct and in the proper order


Logic Errors in Selection Structures (cont.)



Figure 5-11 Sample data and expected results for the algorithm shown in Figure 5-10

Figure 5-10 A correct algorithm for the golf fee procedure


First Logic Error: Using a Compound Condition Rather Than a Nested Selection Structure


Figure 5-12 Correct algorithm and an incorrect algorithm containing the first logic error


First Logic Error: Using a Compound Condition Rather Than a Nested Selection Structure (cont.)


Figure 5-12 Correct algorithm and an incorrect algorithm containing the first logic error

Figure 5-13 Results of desk-checking the incorrect algorithm from Figure 5-12


Second Logic Error: Reversing the Outer and Nested Decisions


Figure 5-14 Correct algorithm and an incorrect algorithm containing the second logic error

Figure 5-15 Results of desk-checking the incorrect algorithm from Figure 5-14


Third Logic Error: Using an Unnecessary Nested Selection Structure


Figure 5-16 Correct algorithm and an inefficient algorithm containing the third logic error

Figure 5-17 Results of desk-checking the inefficient algorithm from Figure 5-16


Fourth Logic Error: Including an Unnecessary Comparison in a Condition


Figure 5-18 Problem specification, a correct algorithm, and an inefficient algorithm

Figure 5-19 Results of desk-checking the algorithms from Figure 5-18

Multiple-Alternative Selection Structures


• When you need to choose from several different options, use a multiple-alternative selection structure– Also called an extended selection structure

Figure 5-20 Problem specification for the Allen High School problem

Multiple-Alternative Selection Structures (cont.)


Figure 5-21 Pseudocode and flowchart containing a multiple-alternative selection structure

Multiple-Alternative Selection Structures (cont.)


Figure 5-22 Two versions of the code containing a multiple-alternative selection structure

Figure 5-23 Excellent message shown in the interface

The Select Case Statement


• Select Case statement– Used when a multiple-alternative selection structure has

many paths from which to choose– It’s simpler and clearer to code the selection structure

rather than several If…Then…Else statements

The Select Case Statement (cont.)


Figure 5-24 Syntax and an example of the Select Case statement

The Select Case Statement (cont.)


Specifying a Range of Values in a Case Clause

Figure 5-25 Syntax and an example of specifying a range of values

Lesson A Summary

• To create a selection structure that evaluates both a primary and a secondary decision:– Place (nest) the secondary decision’s selection structure

within either the true or false path of the primary decision’s selection structure

• To verify that an algorithm works correctly:– Desk-check (hand-trace) the algorithm


Lesson A Summary (cont.)

• To code a multiple-alternative selection structure:– Use either If…Then…Else statements or the Select Case

statement• To specify a range of values in a Select Case statement’s

Case clause:– Use the To keyword when you know both the upper and

lower values in the range– Use the Is keyword when you know only one end of the

range• The Is keyword is used in combination with one of the

following comparison operators: =, <, <=, >, >=, <>


Lesson B Objectives

After studying Lesson B, you should be able to:• Include a group of radio buttons in an interface• Designate a default radio button• Include a check box in an interface• Compare Boolean values



Modifying the Covington Resort Application

Figure 5-36 Revised TOE chart for the Covington Resort application (continues)


Modifying the Covington Resort Application (cont.)

Figure 5-36 Revised TOE chart for the Covington Resort application

(continued)



Figure 5-37 Partially completed interface for the Covington Resort application



Adding a Radio Button to the Interface• Radio button

– Limits the user to only one choice from a group of two or more related but mutually exclusive choices

– The three-character ID for a radio button’s name is rad

Figure 5-38 Atrium radio button added to the View group box



Adding a Radio Button to the Interface (cont.)• The automatically selected radio button is called the

default radio button– Represents the user’s most likely choice – The first button in the group

• Checked property– Set to True to designate the default radio button



Adding a Check Box to the Interface• Unlike radio buttons, check boxes provide one or more

independent and nonexclusive items from which the user can choose

• Any number of check boxes on a form can be selected at the same time

• The three-character ID for a check box’s name is chk

Figure 5-39 Vehicle parking fee check box added to the interface



Figure 5-40 Correct TabIndex values


Modifying the Calculate Button’s Code

Figure 5-41 Modified pseudocode for the btnCalc_Click procedure


Modifying the Calculate Button’s Code (cont.)

Figure 5-42 Modified list of named constants and variables



Figure 5-44 Variables added to the procedure

Figure 5-43 Named constants added to the procedure



Figure 5-45 Examples of comparing Boolean values (continues)

Comparing Boolean Values



Figure 5-45 Examples of comparing Boolean values

(continued)


Modifying the ClearLabels Procedure

Figure 5-50 ClearLabels procedure

• CheckedChanged event– Occurs when the value in a control’s Checked property

changes


Figure 5-51 Covington Resort

application’s code at the end of Lesson B

(continues)

Modifying the ClearLabels Procedure (cont.)



(continued)



(continues)


(continued)




(continues)


(continued)





Lesson B Summary

• To limit the user to only one choice in a group of two or more related but mutually exclusive choices:– Use the RadioButton tool to add two or more radio buttons to

the form – To include two groups of radio buttons on a form, at least one

of the groups must be placed within a container, such as a group box

• To allow the user to select any number of choices from a group of one or more independent and nonexclusive choices:– Use the CheckBox tool to add one or more check box controls

to the form


Lesson B Summary (cont.)

• To determine whether a radio button or check box is selected or unselected:– Use the Checked property of the radio button or check

box– The property will contain the Boolean value True if the

control is selected; otherwise, it will contain the Boolean value False

• To process code when the value in the Checked property of a radio button or check box changes:– Enter the code in the radio button’s or check box’s

CheckedChanged event procedure

Lesson C Objectives

After studying Lesson C, you should be able to:• Determine the success of the TryParse method• Generate random numbers• Show and hide a control while an application is running


Using the TryParse Method for Data Validation

• TryParse method – Converts a string to a number of a specific data type– With successful conversion:

• The TryParse method stores the number in the variable specified in the method’s NumericVariableName argument

– With unsuccessful conversion:• TryParse stores the number 0 in the variable


Using the TryParse Method for Data Validation (cont.)


Figure 5-58 Syntax and an example of using the Boolean value returned by the TryParse method


Figure 5-59 Sample run of the original Click event procedure



Figure 5-60 Modified btnCalc_Click procedure


Generating Random Integers


• Pseudo-random number generator– A device that produces a sequence of numbers that meet

certain statistical requirements for randomness– Create a Random object by declaring it in a Dim

statement– Use the Random.Next method to generate random

integers

Generating Random Integers (cont.)


Figure 5-61 Syntax and examples of generating random integers


Figure 5-62 Roll ‘Em Game application’s interface

Figure 5-63 Pseudocode for the Roll the Dice button’s Click event procedure



Figure 5-64 Result of clicking the Roll the Dice button


Showing and Hiding a Control


• Hide objects by changing their Visible property from True to False

Figure 5-65 Resized form Figure 5-66 Interface with six of the picture boxes hidden

Showing and Hiding a Control (cont.)


Figure 5-67 Roll the Dice button’s Click event procedure


Lesson C Summary

• To determine whether the TryParse method converted a string to a number of the specified data type:– Use the syntax booleanVariable =

dataType.TryParse(string, numericVariableName)– The TryParse method returns the Boolean value True

when the string can be converted to the numeric dataType

– Otherwise, it returns the Boolean value False


Lesson C Summary (cont.)

• To generate random integers:– Create a Random object to represent the pseudo-random

number generator• The syntax for creating a Random object is

Dim randomObjectName As New Random– Use the Random.Next method to generate a random integer

• The method’s syntax is randomObjectName.Next(minValue, maxValue)

• The Random.Next method returns an integer that is greater than or equal to minValue, but less than maxValue

• The Random.Next method’s return value is assigned to a variable


Lesson C Summary (cont.)

• To show or hide a control while an application is running:– Set the control’s Visible property to the Boolean value

True to show the control during runtime– Set the control’s Visible property to the Boolean value

False to hide the control during runtime

Chapter 5: More on the Selection Structure

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