Chapter 5: More on the Selection Structure

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Chapter 5 More on the Selection Structure
Programming with Microsoft Visual Basic .NET,
Second Edition
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Nested, If/ElseIf/Else, and Case Selection
StructuresLesson A Objectives

• 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

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Nested, If/ElseIf/Else, and Case Selection
StructuresLesson A Objectives (continued)

• Code an If/ElseIf/Else selection structure
• Include a Case selection structure in pseudocode
  and in a flowchart
• Code a Case selection structure
• Write code that uses the Is, TypeOfIs, and Like
  comparison operators

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Nested Selection Structures

• When a selection structures true path or its
  false path contains another selection structure,
  the inner selection structure is referred to as a
  nested selection structure

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Nested Selection Structures (continued)
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Nested Selection Structures (continued)
Part of Figure 5-3 Pseudocode showing the nested
selection structure in the true path
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Nested Selection Structures (continued)
Part of Figure 5-5 Pseudocode showing the nested
selection structure in the false path
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Logic Errors in Selection Structures

• Common logic errors with selection structures
• Using a logical operator when a nested selection
  structure is needed
• Reversing the primary and secondary decisions
• Using an unnecessary nested selection structure

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Logic Errors in Selection Structures (continued)

• An algorithm is the set of step-by-step
  instructions that accomplish a task
• Desk-checking means that you use sample data to
  walk through each of the steps in the algorithm
  manually, just as if you were the computer
• Desk-checking is also called hand-tracing

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Logic Errors in Selection Structures (continued)
Figure 5-7 A correct algorithm for the vacation
procedure
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Logic Errors in Selection Structures (continued)
Figure 5-8 Results of desk-checking the correct
algorithm shown in Figure 5-7
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Using a Logical Operator Rather Than a Nested
Selection Structure
Figure 5-9 Correct algorithm and an incorrect
algorithm containing the first logic error
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Reversing the Primary and Secondary Decisions
Figure 5-11 Correct algorithm and an incorrect
algorithm containing the second logic error
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Using an Unnecessary Nested Selection Structure
Figure 5-13 Correct algorithm and an inefficient
algorithm containing the third logic error
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The If/ElseIf/Else Form of the Selection Structure

• Figure 5-16 shows two versions of the Visual
  Basic .NET code for the grade procedure

Figure 5-15 Letter grades and messages
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The If/ElseIf/Else Form of the Selection
Structure (continued)
Figure 5-16 Two versions of the Visual Basic
.NET code for the grade procedure
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The Case Form of the Selection Structure
Figure 5-18 Syntax and an example of the Select
Case statement
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Desk-Checking the Grade Procedure
Figure 5-19 Results of desk-checking the grade
procedure shown in Figure 5-18
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Using To and Is in an ExpressionList

• To keyword specifies a range of minimum and
  maximum values
• Case 1 To 5
• Is keyword specifies a range of values when you
  know only one value, either the minimum or the
  maximum
• Case Is gt 10

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The Is, TypeOfIs, and Like Comparison Operators

• The Is operator determines whether two object
  references refer to the same object
• An object reference is a memory address within
  the computers internal memory it indicates
  where in memory the object is stored

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The Is, TypeOfIs, and Like Comparison Operators
(continued)

• The TypeOfIs operator determines whether an
  object is a specified type
• The Like operator uses pattern matching to
  determine if one string is equal to another string

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The Is Comparison Operator

• Use the Is operator to determine whether two
  object references refer to the same object
• An object reference is a memory address within
  the computers internal memory
• Syntax objectReference1 Is objectReference2
• The Is operator evaluates to True if
  objectReference1 is the same as objectReference2
  otherwise it evaluates to False

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The TypeOfIs Comparison Operator

• Use the TypeOfIs operator to determine whether
  an object is a specified type
• Syntax TypeOf object Is objectType
• The TypeOfIs operator evaluates to True if the
  type of object is the same as objectType
  otherwise it evaluates to False

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The Like Comparison Operator

• The Like operator allows you to use pattern
  matching to determine whether one string is equal
  to another string
• Syntax string Like pattern
• Both string and pattern must be String
  expressions
• pattern can contain one or more of the
  pattern-matching characters
• The Like operator evaluates to True if string
  matches pattern otherwise it evaluates to False

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The Like Comparison Operator (continued)
Figure 5-24 Pattern-matching characters
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The Math Practice ApplicationLesson B Objectives

• Include a group of radio buttons in an interface
• Designate a default radio button
• Include a check box in an interface
• Create a user-defined Sub procedure
• Generate random numbers using the Random object
  and the Random.Next method

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The Math Practice ApplicationLesson B Objectives
(continued)

• Call a user-defined Sub procedure
• Invoke a radio button controls Click event
  procedure from code
• Process code when a form is first loaded into the
  computers memory

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Completing the User Interface

• The application should display the addition or
  subtraction problem on the screen, then allow the
  student to enter the answer, and then verify that
  the answer is correct
• If the students answer is not correct, the
  application should give the student as many
  chances as necessary to answer the problem
  correctly

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Completing the User Interface (continued)
Figure 5-28 Sketch of the Math Practice
applications user interface
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Adding a Radio Button to the Form

• RadioButton control
• Limits the user to one choice in a group of
  options
• Should have a unique access key, which allows the
  user to select the button using the keyboard

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Adding a Radio Button to the Form (continued)

• The selected button is called the default radio
  button
• The selected button is either the radio button
  that represents the users most likely choice or
  the first radio button in the group
• Designate a radio button as the default radio
  button by setting the buttons Checked property
  to the Boolean value True

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Adding a Check Box Control to the Form

• Checkbox controls used for adding a check box
  control to the interface
• Use check box controls to allow the user to
  select any number of choices from a group of one
  or more independent and nonexclusive choices
• Any number of check boxes on a form can be
  selected at the same time

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Coding the Math Practice Application

• The Click event procedures for seven of the
  controls and the Load event for the MathForm need
  to be coded
• In this lesson, you code all but the Click event
  procedures for the uiExitButton control and the
  uiCheckAnswerButton and uiSummaryCheckBox
  controls (which you code in Lesson C)

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Creating a User-Defined Sub Procedure

• A user-defined Sub procedure is a collection of
  code that can be invoked from one or more places
  in an application
• Enter Private Sub SubName() just above the End
  Class statement
• The End Sub will automatically be generated

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Creating a User-Defined Sub Procedure (continued)

• The rules for naming a user-defined Sub procedure
  are the same as for naming variables
• Sub procedure names should be entered using
  Pascal case
• Capitalize the first letter in the name and the
  first letter of each subsequent word in the name

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Generating Random Numbers

• Visual Studio .NET provides a pseudo-random
  number generator
• Produces a sequence of numbers that meet certain
  statistical requirements for randomness
• Dim GeneratorRandom As New Random()
• Methods
• Next(minValue, maxValue)
• NextDouble()

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Coding the uiGrade1RadioButton and
uiGrade2RadioButton Click Event Procedures

• Use the Visual Basic .NET Call statement to call
  (invoke) a user-defined Sub procedure
• Syntax Call procedurename(argumentlist)
• The square brackets in the syntax indicate that
  the argumentlist is optional

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Coding the uiAdditionRadioButton and
uiSubtractionRadioButton Click Event Procedures

• When the user clicks the uiAdditionRadioButton
  control or the uiSubtractionRadioButton control,
  the controls Click event procedure should
• Display the appropriate mathematical operator
  (either a plus sign or a minus sign) in the
  uiOperatorPictureBox control
• Generate and display two random numbers in the
  uiNum1Label and uiNum2Label controls

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Coding the Forms Load Event Procedure

• Instructions entered in the forms Load event
  procedure are processed when the application is
  started and the form is loaded into memory
• To automatically display an addition problem when
  the Math Practice interface first appears, enter
  one of the following statements in the MathForms
  Load event procedure
• Call GenerateAndDisplayNumbers()
• Me.uiAdditionRadioButton.PerformClick()

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Completing the Math Practice ApplicationLesson C
Objectives

• Select the existing text in a text box control
• Code a check box controls Click event procedure
• Display and hide a control

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Coding the uiCheckAnswerButton Click Event
Procedure

• Need to code the Click event procedures for the
  uiCheckAnswerButton and the uiDisplaySummaryCheckB
  ox controls
• Figure 5-49 shows the pseudocode for the
  uiCheckAnswerButton controls Click event
  procedure

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Coding the uiCheckAnswerButton Click Event
Procedure (continued)
Figure 5-49 Pseudocode for the
uiCheckAnswerButton controls Click event
procedure
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Coding the uiSummaryCheckBox Click Event Procedure

• The uiSummaryCheckBox controls Click event
  procedure is responsible for both displaying and
  hiding the uiSummaryGroupBox control
• The procedure should
• Display the group box control when the user
  selects the check box
• Hide the group box control when the user
  deselects the check box

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Summary

• To create a selection structure that evaluates
  both a primary and a secondary decision, place
  (nest) the selection structure for the secondary
  decision within either the true path or false
  path of the selection structure for the primary
  decision
• To code a multiple-path (or extended) selection
  structure, use either the IfThenElse statement
  or the Select Case statement

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Summary (continued)

• To limit the user to only one choice in a group
  of two or more related and mutually exclusive
  choices, add a radio button control
• To allow the user to select any number of choices
  from a group of one or more independent and
  nonexclusive choices, add a check box control
• To call (invoke) a user-defined Sub procedure,
  use the Call statement

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Summary (continued)

• To process code when the form is loaded into
  memory, enter the code in the forms Load event
  procedure
• To display or hide a control, set the controls
  Visible property
• To code a check box controls Click event
  procedure, use a selection structure to determine
  whether the check box was either selected or
  deselected by the user