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Chapter 5: More on the Selection Structure

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Using a logical operator when a nested selection structure is needed ... An algorithm is the set of step-by-step instructions that accomplish a task ... – PowerPoint PPT presentation

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Title: Chapter 5: More on the Selection Structure


1
Chapter 5 More on the Selection Structure
Programming with Microsoft Visual Basic .NET,
Second Edition
2
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

3
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

4
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

5
Nested Selection Structures (continued)
6
Nested Selection Structures (continued)
Part of Figure 5-3 Pseudocode showing the nested
selection structure in the true path
7
Nested Selection Structures (continued)
Part of Figure 5-5 Pseudocode showing the nested
selection structure in the false path
8
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

9
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

10
Logic Errors in Selection Structures (continued)
Figure 5-7 A correct algorithm for the vacation
procedure
11
Logic Errors in Selection Structures (continued)
Figure 5-8 Results of desk-checking the correct
algorithm shown in Figure 5-7
12
Using a Logical Operator Rather Than a Nested
Selection Structure
Figure 5-9 Correct algorithm and an incorrect
algorithm containing the first logic error
13
Reversing the Primary and Secondary Decisions
Figure 5-11 Correct algorithm and an incorrect
algorithm containing the second logic error
14
Using an Unnecessary Nested Selection Structure
Figure 5-13 Correct algorithm and an inefficient
algorithm containing the third logic error
15
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
16
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
17
The Case Form of the Selection Structure
Figure 5-18 Syntax and an example of the Select
Case statement
18
Desk-Checking the Grade Procedure
Figure 5-19 Results of desk-checking the grade
procedure shown in Figure 5-18
19
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

20
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

21
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

22
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

23
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

24
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

25
The Like Comparison Operator (continued)
Figure 5-24 Pattern-matching characters
26
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

27
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

28
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

29
Completing the User Interface (continued)
Figure 5-28 Sketch of the Math Practice
applications user interface
30
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

31
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

32
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

33
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)

34
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

35
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

36
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()

37
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

38
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

39
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()

40
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

41
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

42
Coding the uiCheckAnswerButton Click Event
Procedure (continued)
Figure 5-49 Pseudocode for the
uiCheckAnswerButton controls Click event
procedure
43
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

44
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

45
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

46
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
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