Chapter 5: More on the Selection Structure

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Chapter 5 More on the Selection Structure
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Third 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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Negation of boolean expression

• Not (a And b) ? (Not a) Or (Not b)?
• Not (a Or b) ? (Not a) And (Not b)?
• ? is equivalent
• You can replace And and Or with AndAlso and
  OrElse.
• Verify these equations by using the truth tables

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Previewing the Completed Application
(VB2005\Chap05)?
Figure 5-1 Math Practice applications user
interface
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Nested Selection Structures
Figure 5-3 Pseudocode showing the nested
selection structure
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Nested Selection Structures (continued)?
Figure 5-4 Flowchart showing the nested
selection structure
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Logic Errors in Selection Structures

• Common logic errors with selection structures
• Using a logical operator instead of nested
  structure
• Reversing the primary and secondary decisions
• Using an unnecessary nested selection structure
• Algorithm
• Desk-checking, hand-tracing, run by hand

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Logic Errors in Selection Structures (continued)?
codesalesperson code. Only salespeople with code
x will get the bonus.
Figure 5-10 A correct algorithm for the bonus
procedure
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Logic Errors in Selection Structures (continued)?
Figure 5-11 Results of desk-checking the correct
algorithm shown in Figure 5-10
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Using a Logical Operator Rather Than a Nested
Selection Structure (continued)?
Figure 5-11 Correct algorithm and an incorrect
algorithm containing the first logic error
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Reversing the Primary and Secondary Decisions
(continued)?
Figure 5-14 Correct algorithm and an incorrect
algorithm containing the second logic error
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Using an Unnecessary Nested Selection Structure
(continued)?
Figure 5-16 Correct algorithm and an inefficient
algorithm containing the third logic error
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The If/ElseIf/Else Selection Structure

• Extended (multiple path) selection structures
• If/ElseIf/Else
• Select Case
• If/ElseIf/Else
• Allows for selection among more than two paths
• Provides a more readable version of selection
  logic
• Example display message based on grades A F

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The If/ElseIf/Else Selection Structure
(continued)?
Figure 5-18 Two versions of xDisplayMsgButtons
Click event procedure (continued)?
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The If/ElseIf/Else Selection Structure
(continued)?
Figure 5-18 Two versions of xDisplayMsgButtons
Click event procedure
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The Case Selection Structure

• Alternative to If/ElseIf/Else multiple path
  selection
• Select Case statement
• Begins with Select Case, ends with End Select
  clause
• Each Case represents a different instruction path
• Optional Case Else clause is the default case
• selectorExpression is evaluated to determine path
• Each case, except Case Else, has an
  expressionList
• How the computer determines the instruction path
• selectorExpression value matches expressionList
  value

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The Case Selection Structure (continued)?
short-circuiting
Figure 5-22 Syntax and an example of the Select
Case statement (continued)?
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The Case Selection Structure (continued)?
Figure 5-21 Flowchart showing the Case selection
structure
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The Case Selection Structure (continued)?
Figure 5-22 Syntax and an example of the Select
Case statement
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Desk-Checking the xDisplayMsgButtons Click Event
Procedure

• Desk check with the letters C, F, and X

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Desk-Checking the xDisplayMsgButtons Click Event
Procedure (continued)?
Figure 5-23 Results of desk-checking the
xDisplayMsgButtons Click event procedure shown
in Figure 5-22
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Specifying a Range of Values in an ExpressionList

• Specifying range of minimum and maximum values
• To keyword use if upper and lower bounds are
  known
• Is keyword use if only upper or lower bound is
  known
• Example with To Case 1 To 5
• Example with Is Case Is gt 10
• Relational operators used with Is , lt, lt, gt,
  gt, ltgt

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The Is, TypeOfIs, and Like Comparison Operators
Those work independent of the select statement

• Is operator
• Determines if two object references are the same
• TypeOf...Is
• Determines whether an object is a specified type
• Like
• Determines whether a string matches a given
  pattern

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The Is Comparison Operator

• Is operator
• Determines if two object references are the same
• Object reference
• Memory address within computers internal memory
• Syntax objectReference1 Is objectReference2
• Evaluates to True if object references are the
  same
• Otherwise evaluates to False

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The TypeOfIs Comparison Operator

• TypeOfIs operator
• Determines whether an object is a specified type
• Syntax TypeOf object Is objectType
• Evaluates to True if type of object equals
  objectType
• Otherwise evaluates to False

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The TypeOfIs Comparison Operator (continued)?
Figure 5-29 Syntax and an example of the
TypeOf...Is operator
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The Like Comparison Operator
Figure 5-31 Syntax and examples of the Like
operator
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The Like Comparison Operator (continued)?
Figure 5-31 Syntax and examples of the Like
operator
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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 and call an independent Sub procedure

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The Math Practice ApplicationLesson B Objectives
(continued)?

• Generate random numbers
• Invoke a radio buttons Click event procedure
  from code

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The Math Practice Application
Figure 5-39 Display summary check box added to
the interface
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Lesson C Objectives

• Code a check box's Click event procedure
• Display and hide a control