Chapter 4: The Selection Structure

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Chapter 4 The Selection Structure
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The IfThenElse Statement Lesson A Objectives

• Write pseudocode for the selection structure
• Create a flowchart to help you plan an
  applications code
• Write an If...Then...Else statement
• Write code that uses comparison operators and
  logical operators

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The IfThenElse Statement Lesson A Objectives
(continued)?

• Change the case of a string
• Determine whether a text box contains data
• Determine the success of the TryParse method

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The Selection Structure

• Condition expression evaluating to true or false
• Selection (decision) structure
• Chooses one of two paths based on a condition
• Example
• If employee works over 40 hours, add overtime pay
• Four selection structures in Visual Basic
• If, If/Else, If/ElseIf/Else, and Case

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Writing Pseudocode for If and If/Else Selection
Structures

• If selection structure
• Contains only one set of instructions
• Instructions are processed if the condition is
  true
• If/Else selection
• Contains two sets of instructions
• True path instruction set following true
  condition
• False path instruction set following false
  condition

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Writing Pseudocode for If and If/Else Selection
Structures
Figure 4-4 Examples of the If and If/Else
selection structures written in pseudocode
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Flowcharting the If and If/Else Selection
Structures

• Flowchart
• Set of standardized symbols showing program flow
• Oval the start/stop symbol
• Rectangle the process symbol
• Parallelogram the input/output symbol
• Diamond selection/repetition symbol

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Flowcharting the If and If/Else Selection
Structures (continued)?
Figure 4-5 Examples of the If and If/Else
selection structures drawn in flowchart form
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Coding the If and If/Else Selection Structures

• Syntax
• If condition Then statement block for true
  path Else statement block for
  false path End If
• condition must be a Boolean expression
• The Else clause is optional

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Comparison Operators

• Comparison (relational) operators
• Test two items for equality or types of
  non-equality
• Rules for comparison operators
• Cause an expression to evaluate to true or false
• Have lower precedence than arithmetic operators
• Are evaluated from left to right
• Example 5 -2 gt 1 2 ? 3 gt 3 ? False

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Comparison Operators (continued)?
Figure 4-7 Listing and examples of commonly used
comparison operators (continued)?
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Comparison Operators (continued)?
Figure 4-7 Listing and examples of commonly used
comparison operators
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Using Comparison OperatorSwapping Numeric Values
Figure 4-11 The If selection structure shown in
the xDisplayButtons Click event procedure
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Using Comparison OperatorsDisplaying the Sum or
Difference
Figure 4-16 The If/Else selection structure
shown in the xCalcButtons Click event procedure
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Logical Operators

• Logical (Boolean) operators
• Operators that create compound conditions
• Types And, Or, Not, AndAlso, OrElse, Xor
• Example If hours gt 0 And hours lt 40 Then
• Truth tables
• Short circuit evaluation
• Operators using technique AndAlso, OrElse

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Logical Operators (continued)?
Figure 4-18 Listing and examples of logical
operators (partial)?
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Logical Operators (continued)?
Figure 4-19 Truth tables for the logical
operators (continued)?
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Logical Operators (continued)?
Figure 4-19 Truth tables for the logical
operators
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Using the Truth Tables

• Scenario calculate a bonus for a salesperson
• Bonus condition A rating and sales gt 10,000
• Appropriate operators And, AndAlso (more
  efficient)?
• Both conditions must be true to receive bonus
• Sample code rating "A" AndAlso sales gt 10000
• Precedence of logical operators
• Lower than that of arithmetic or comparison
  operators

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Using Truth Tables (continued)?
Figure 4-20 Order of precedence for arithmetic,
comparison, and logical operators
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Using Logical Operators Calculating Gross Pay
Figure 4-22 AndAlso and OrElse logical operators
in the If...Then...Else statement (continued)?
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Using Logical Operators Calculating Gross Pay
(continued)?
Figure 4-22 AndAlso and OrElse logical operators
in the If...Then...Else statement
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Comparing Strings Containing Letters

• Scenario
• Display Pass if P is entered in
  xLetterTextBox
• Display Fail if F is entered in
  xLetterTextBox
• One of the possible implementations
• Dim letter As String
  letter Me.xLetterTextBox.Text
  If letter
  "P" OrElse letter "p Then
  Me.xResultLabel.Text Pass Else
  Me.xResultLabel.Text
  "Fail End if

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Converting a String to Uppercase or Lowercase

• String comparisons are case sensitive
• ToUpper method converts string to upper case
• ToLower method converts string to lower case
• Example If letter.ToUpper() "P" Then

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Comparing Boolean Values
Figure 4-29 Syntax and examples of the
String.IsNullOrEmpty method
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Comparing Boolean Values (continued)?
Figure 4-31 Syntax and an example of using the
Boolean value returned by the TryParse method
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Summary Lesson A

• A Boolean condition evaluates to true or false
• Selection structures choose an instruction path
  based on a condition
• If...Then...Else statement selection structure
  with a true path and a false path
• Operator precedence arithmetic, comparison,
  logical
• ToUpper and ToLower modify case of input text

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The Monthly Payment Calculator ApplicationLesson
B Objectives

• Group objects using a GroupBox control
• Calculate a periodic payment using the
  Financial.Pmt method
• Create a message box using the MessageBox.Show
  method
• Determine the value returned by a message box

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Monthly Payment Calculator
Figure 4-34 Sketch of the Monthly Payment
Calculator user interface
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Coding the Monthly Payment Calculator Application

• Procedures required according to TOE chart
• Click event code for the two buttons
• TextChanged, KeyPress, Enter code for text boxes
• Procedures that are already coded
• xExitButtons Click event and TextChanged events
• Procedure to code in Lesson B
• xCalcButtons Click event procedure

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Coding the xCalcPayButton Click Event Procedure
Figure 4-39 Pseudocode for the xCalcButtons
Click event procedure
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Using the Financial.Pmt Method

• Calculates periodic payment on loan or investment
• Syntax Financial.Pmt(Rate, NPer, PV, FV, Due)?
• Rate interest rate per period
• NPer total number of payment periods (the term)?
• PV present value of the loan or investment
• FV future value of the loan or investment
• Due due date of payments

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The MessageBox.Show Method

• Displays message box with text, button(s), icon
• Syntax MessageBox.Show(text, caption, buttons,
  icon, defaultButton)?
• text text to display in the message box
• caption text to display in title bar of message
  box
• buttons buttons to display in the message box
• icon icon to display in the message box
• defaultButton automatically selected if Enter
  pressed

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The MessageBox.Show Method (continued)?
Figure 4-48 Completed xCalcButtons Click event
procedure
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The MessageBox.Show Method (continued)?
Figure 4-50 Message box created by the
MessageBox.Show method
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Summary Lesson B

• Group box control treats components as one unit
• Add a group box using the GroupBox tool
• Financial.Pmt method calculates loan or
  investment payments
• MessageBox.Show method displays a message box
  with text, one or more buttons, and an icon

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Completing the Monthly Payment Calculator
ApplicationLesson C Objectives

• Specify the keys that a text box will accept
• Select the existing text in a text box

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Coding the KeyPress Event Procedures

• KeyPress event
• Occurs when key pressed while a control has focus
  
• Character corresponding to key is stored
• Stored value sent to KeyPress events e parameter
  
• One popular use of the KeyPress event
• Prevents users from entering inappropriate
  characters
• Selection structure used to test entered
  character

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Coding the KeyPress Event Procedures (continued)?
Figure 4-54 Completed CancelKeys procedure
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Coding the Enter Event Procedure

• Enter event
• Occurs when the text box receives the focus
• Responsible for selecting contents of text box
• User can replace existing text by pressing a key
• SelectAll method syntax Me.textbox.SelectAll()?
• Selects all text contained on a text box
• Using the SelectAll method
• Add to each text boxs Enter event procedure

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Coding the Enter Event Procedure (continued)
Figure 4-57 Existing text selected in the
xPrincipalTextBox
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Summary Lesson C

• KeyPress event procedure responds to the user
  pressing a key
• One use of a KeyPress event cancel a key entered
  by the user
• Enter event occurs when text box receives focus
• SelectAll method used to select all contents of
  a text box

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