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Objectives

• Include the selection structure in pseudocode and
  in a flowchart
• Write an IfThenElse statement
• Write code that uses comparison operators and
  logical operators
• Create a variable having block scope
• Use the line continuation character

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Objectives (continued)

• Concatenate strings
• Use the ControlChars NewLine constant
• Change the case of a string
• Determine whether an expression can be converted
  to a number
• Display a message in a message box

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Objectives (continued)

• Include a nested selection structure in
  pseudocode, flowchart, and code
• Code an If/Elseif/Else selection structure
• Include a Case selection structure in pseudocode,
  flowchart and code

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The Selection Structure

• Also called decision structure
• Make a comparison and choose between 2 paths
  depending upon the result of comparison
  (condition)

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Writing Pseudocode for the If and If/Else
Selection Structures

• If selection structure - 1 set of instructions
  which are processed if condition is true
• If/Else - 2 sets of instructions the first
  when condition is true, the other when condition
  is false

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Flowcharting the If and If/Else Selection
Structures
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Coding the If and If/Else Selection Structures
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Coding the If and If/Else Selection Structures
(continued)

• Statement Block
• Set of instructions in true path
• (between keywords If and Then)
• Optional set of instructions in false path
• (between keywords Then and End If)
• Any items in square brackets are optional

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Comparison Operators

• Also referred to as relational operators

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Comparison Operators (continued)
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Using Comparison Operators Example 1
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Using Comparison Operators Example 1 (continued)
Note variable intTemp has block scope and can
only be used in block in which declared
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Using Comparison Operators Example 1 (continued)
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Using Comparison Operators Example 1 (continued)

• Line continuation character
• Space followed by an underscore
• Used to break up long instructions onto 2 lines
• Concatenation Operator
• The ampersand ( )
• Used to concatenate (connect or link) strings

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Using Comparison Operators Example 1 (continued)
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HOW TO
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Using Comparison Operators Example 2
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Using Comparison Operators Example 2 (continued)
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Using Comparison Operators Example 2 (continued)
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Using the ToUpper and ToLower Methods
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Using the ToUpper and ToLower Methods (continued)
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Using the ToUpper and ToLower Methods (continued)
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Logical Operators
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Logical Operators (continued)
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Truth Tables
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Truth Tables (continued)
Note short-circuit evaluation can occur with
And operator. Both conditions will not be
tested if first condition tests false
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Using the Truth Tables

• Understanding order of precedence is critical

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Using the Truth Tables (continued)
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Using Logical Operators in an IfThenElse
Statement
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Modifying the Skate-Away Sales Application
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Modifying the Skate-Away Sales Application
(continued)

• Modify to eliminate unwanted error message

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Modifying the Skate-Away Sales Application
(continued)
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The IsNumeric Function

• Function
• A predefined procedure that performs a specific
  task and returns a value after completing the
  task
• IsNumeric function checks if an expression can be
  converted to a number
• Returns Boolean true if able to convert to number
• Returns Boolean false if conversion cant be made

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The IsNumeric Function (continued)
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The MessageBox.Show method
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The MessageBox.Show method (continued)

• Arguments in the MessageBox.Show method
• Text - a string containing the text to display
• Caption - text displayed in title bar
• Buttons - buttons to display in message box
• Can be one of 6 different constants
• Icon icon displayed in title bar
• Defaultbutton button automatically selected if
  user presses the Enter key on keyboard

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The MessageBox.Show method (continued)
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The MessageBox.Show method (continued)
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HOW TO
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Nested Selection Structures

• When at least one side of the selection
  structures path contains another selection
  structure
• Inner structure is referred to as nested
• Else is paired up with nearest preceding If
  regardless of indentation of code
• Use when you want to make one decision and then
  choose between two options depending upon the
  result of the first decision

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Nested Selection Structures (continued)
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Nested Selection Structures (continued)
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Nested Selection Structures (continued)
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Nested Selection Structures (continued)
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The If/ElseIf/Else Selection Structure
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The Case Selection Structure

• PsuedoCode
• Get Grade
• Grade value
• Display Excellent
• Display Above Average
• Display Average
• D,F Display Below Average
• Other Display Error

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The Case Selection Structure (continued)
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HOW TO
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Using To and Is in an ExpressionList

• To allows range of values in Case expression
• Example
• Number of items ordered Price per item
• 1-5 25
• 6-10 23
• Is allows use of comparison operators
• More than 10 20

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Using To and Is in an ExpressionList (continued)
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Programming Example Fat Calculator

• Application allows user to enter the total number
  of calories and grams of fat contained in a
  specific food
• Calculate and display 2 values number of fat
  calories, and fat percentage
• Display appropriate message depending on level of
  fat percentage

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TOE Chart
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User Interface
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Objects, Properties, and Settings
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Objects, Properties, and Settings (continued)
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Tab Order
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Pseudocode

• btnExit Click event procedure
• close application
• btnCalc Click event procedure
• if the txtCalories and txtFatGrams controls
  contain numbers
• assign the numbers to variables
• calculate the fat calories by multiplying total
  calories by 9
• calculate fat percentage by dividing fat by
  total calories
• display fat calories and fat percentage in
  appropriate labels
• if fat percentage is over 30
• display This food is high in fat in
  message box
• else
• display This food is not high in fat in
  message box
• end if
• else
• display The calories and fat grams must be
  numbers
• end if

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Code
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Code (continued)
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Summary

• Selection structure allows program to make a
  decision and then select one of 2 paths depending
  upon the results of that decision
• Visual Basic .NET has 4 selection structures
• If If/Else
• If/ElseIf/Else Case
• Selection structures can be nested within other
  selection structures
• Outer structure contains primary decision
• Inner structure contains secondary decision

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Summary (continued)

• All expressions in a selection structure evaluate
  to either true or false
• Use line continuation character (space ) to
  break up long instruction into 2 physical lines
• Use concatenation operator () to connect (link)
  2 strings together
• String comparisons are case sensitive
• Use ToUpper and ToLower functions to convert case

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Summary (continued)

• ControlChars.NewLine constant advances insertion
  point to the next line in a control
• Use logical operators to create compound
  conditions (And, Or, AndAlso)
• IsNumeric function verifies expression can be
  converted to a number
• MessageBox.Show allows communication with user in
  the form of a message box
• Use If/ElseIf/Else and Case selection structures
  to choose between multiple alternatives