Decision Making

1
Chapter 4

• Decision Making

2
Objectives

• Create constants to improve the readability of
  code
• Learn about the DateTime data type and perform
  operations on dates
• Create instances of check boxes, scroll bars,
  group boxes and radio buttons
• Group control instances
• Understand the Boolean data type
• Use If statements and Select Case statements to
  make decisions

3
Constants (1)

• User-defined constants like variables store a
  value while the solution runs
• Value of a constant does not change
• Similar to a literal value in that regard
• SyntaxPrivate Const constantname As type
  initexpr
• Const statement declares a constant
• As type clause defines the data type of the
  constant
• Required when Option Strict is enabled
• Initexpr defines the value of the constant
• You must assign a value to a constant when
  declaring it

4
Constants (2)

• Use constants to improve the readability of code
• Constants make a program more maintainable
  because a constants value can be changed in one
  place rather than many
• Use standard prefixes for the constants you
  declare to further improve readability

5
Constants (Example)

• Declare a constant named csngPI having a data
  type of Single and a value of 3.14159
• Private Const csngPI As Single 3.14159
• Declare a constant named csng2PI having a data
  type of Single. The value uses another constant
  and multiplied by the literal value 2
• Private Const csng2PI As Single csngPI 2

6
Constants (Errors)

• Constants cannot be declared using variables
  because a variable's value does not exist until
  run time
• The following constant declaration is illegal
  because the declaration uses a variable
• Private msngPI As Single 3.14159
• Private Const csng2PI As Single msngPI 2

7
Importing a Class

• The Imports statement allows you to call the
  methods of a class without specifying the full
  namespace hierarchy
• The following statement imports the
  System.Convert namespace
• Imports System.Convert
• Having imported the System.Convert namespace,
  both of the following statements will work
• mdatCurrent System.Convert.ToDateTime(txtDate.T
  ext)
• mdatCurrent ToDateTime(txtDate.Text)

8
Working with Dates

• The DateTime data type stores date values
• Literal date values are surrounded by pound signs
  ()
• The standard prefix for the DateTime data type is
  "dat"
• Remember to use IsDate to determine whether a
  value can be converted to a date
• Declaration examplePrivate mdatCurrent As
  DateTime 3/22/2002
• Type conversion example
• mdatCurrent _ System.Convert.ToDateTime(txtDat
  e.Text)

9
DateTime Structure (Properties)

• Now - returns current date and time
• Today - returns current date
• Day, Month, Year - returns day, month, and year,
  respectively
• These properties store Integer values
• Hour, Minute, Second - returns time values
• These properties store Integer values
• DayOfWeek, DayOfYear - returns an Integer
  containing the day of the week and the day of the
  year respectively

10
DateTime Structure (Example)

• Declare a DateTime variable
• Private mdatCurrent As DateTime
• Store the current date in the variable
• mdatCurrent System.DateTime.Today
• Store the current date and time in the variable
• mdatCurrent System.DateTime.Now

11
DateTime Structure (Methods)

• AddHourse, AddMinutes, and AddSeconds adds fixed
  time intervals to a date
• Methods return a DateTime structure
• AddYears, AddMonths, and AddDays adds fixed date
  intervals
• Methods return a DateTime structure
• ToString converts a date to string
• ToLongDateString and ToShortDateString converts a
  DateTime structure to a formatted date
• ToLongTimeString and ToShortTimeString converts
  time to formatted time
• Subtract subtracts a date from another date
• Returns a DateTime structure containing the time
  difference

12
DateTime Structure (Example)

• Convert the current date to a string
• lblDate.text Today.ToLongDateString
• Add one day to the current date
• Private datNextDay As DateTime
  Today.AddDays(1)
• Add one month to the current date
• Private datNextMonth As DateTime _
• Today.AddMonths(1)
• Subtract one day from the current date
• Private datLastDay As DateTime
  Today.AddDays(-1)

13
When VB .NET Loads a Form (Constructors)

• When VB.NET initializes a form, or any class, the
  New procedure executes
• The New procedure is called a constructor
• The Win Form Designer places code to initialize
  control instances in the New procedure
• By calling the procedure named InitializeComponent
  
• Add your own code to perform custom
  initialization after the call to
  InitializeComponent

14
Constructor (Example)

• The following shows a constructor with a
  statement added (shown in bold)
• Remember to add code after InitializeComponent()
• Public Sub New()
• MyBase.New()
• InitializeComponent()
• lblDate.Text Today.ToShortDateString
• End Sub

15
Performing Calculations on Dates (1)

• The DateDiff function computes number of
  intervals between two dates
• Syntax
• Result DateDiff( interval, date1, date2 ,
  firstdayofweek ,firstweekofyear )

16
Performing Calculations on Dates (2)

• Interval argument is an enumeration
• Day, DayOfYear, Hour, Minute, Month, Quarter,
  Second, Weekday, WeekOfYear, and Year
• date1 and date2 arguments contain the two dates
  used in the calculation
• If date1 is greater than date2, then the result
  is negative
• Optional firstdayofweek argument specifies which
  day of the week that VB .NET considers the first
  day of the week
• Optional firstweekofyear argument defines which
  week is the first week of the year

17
DateDiff Function (Example)

• Calculate the number of elapsed months and years
  between two dates. Result is 22 months and 1
  year
• Private mdatStart, mdatEnd As DateTime
• Private mdblElapsed As Double
• mdatStart System.DateTime.Today
• mdatEnd System.DateTime.Today.AddMonths(22)
• mdblElapsed DateDiff(DateInterval.Month,
  mdatStart, _
• mdatEnd,FirstDayOfWeek.Monday,FirstWeekOfYear.
  Jan1)
• mdblElapsed DateDiff(DateInterval.Year,
  mdatStart, _
• mdatEnd,FirstDayOfWeek.Monday,FirstWeekOfYear.
  Jan1)

18
The DateTimePicker Control

• Allows the user to select date from calendar

DateTimePicker control
19
The DateTimePicker Control (Properties)

• Font - controls the font appearing in the
  control's list portion
• CalendarFont - controls the font appearing in the
  drop-down calendar
• CalendarForeColor - controls the foreground color
  of the drop-down calendar
• Format - stores an enumeration defining the
  appearance of the date
• MaxDate, MinDate - defines the minimum and
  maximum user-selectable dates
• Value - stores current date and time

20
DateTimePicker Control (Events)

• CloseUp event fires when user closes drop-down
  calendar
• ValueChanged event fires when the user changes
  the Value property at run time

21
Storing Values in a DateTimePicker

• Store the current date value and the maximum date
  value in two DateTimePicker control instances
• dtpStartDate.Value Today.AddDays(-1)
• dtpStartDate.MaxDate Today
• dtpEndDate.Value Today
• dtpEndDate.MaxDate Today

22
The CheckBox Control

• Allows the user to select a yes / no response
• Also allows the user to select from a third
  choice called indeterminate
• Indeterminate usually indicates that no choice
  has been made
• CheckBox displays textual description along with
  a box that can be checked or unchecked (or
  indeterminate)

23
CheckBox Control (Properties)

• CheckAlign - controls position of box within the
  visible region of the control instance
• CheckState - defines whether the box is checked,
  not checked, or indeterminate
• Text - defines the text the appearing in visible
  region
• TextAlign - defines the alignment of text within
  the visible region
• ThreeState - allows the CheckBox to store three
  possible values instead of just two

24
CheckBox Control (Events)

• CheckedChanged event fires whenever value of
  Checked property changes
• Example Setting the CheckState property
• chkInsurance.CheckState CheckState.Checked
• chkInsurance.CheckState CheckState.Unchecked
• chkInsurance.CheckState CheckState.Indeterminate

25
ScrollBar Controls

• Used to select a value from a range of values
• Often used to represent a percentage
• Values are always integers
• Two controls work the same way
• HScrollBar appears horizontally
• Standard prefix is "hsb"
• VScrollBar appears vertically
• Standard prefix is "vsb"
• Value grows as scroll bar is moved downward

26
ScrollBar Controls (Properties and Events)

• Properties
• Maximum and Minimum - defines the valid range
• Default values are 100 and 0 respectively
• Value - contains current value between the
  defined range
• Value must be between Minimum and Maximum
• SmallChange and LargeChange - controls amount of
  change to Value property as user clicks control
  instance region
• Events
• ValueChanged event fires when user changes value
  property.

27
ScrollBar Behavior
Click here to change the Value property by
SmallChange
Click here to change the Value property by
LargeChange
Relative indicator of Value Property
28
ScrollBar (ValueChanged Example)

• Store the scroll bar's value into the Label named
  lblFuel when the value changes
• Private Sub hsbFuel_ValueChanged(. . .)
• lblFuel.Text hsbFuel.Value.ToString
• End Sub

29
The NumericUpDown Control

• Similar to scroll bars
• Displays a value within a range of values
• Displays an adjacent label containing the current
  value
• Supports same fundamental properties as scroll
  bars
• Maximum, Minimum, Value properties
• ValueChanged event
• Standard prefix is "nud"

30
The GroupBox Control

• Acts as a container for other control instances
  such as RadioButton control instances
• Other control instances can be grouped in a
  GroupBox too
• Properties
• BackColor and ForeColor define colors
• Text property defines text appearing at the top
  of the visible region
• Replaces the Frame control found in VB 6

31
The RadioButton Control

• User selects one radio button from a group of
  buttons
• called a button group
• Button group contained by a GroupBox control
  instance
• To create multiple button groups, create multiple
  GroupBox control instances

32
The RadioButton Control (Caveats)

• Be careful when creating a button group
• Create RadioButtons inside of a group box
• Test by moving group box. Contained radio buttons
  should move with the group box
• If copying and pasting radio buttons, select
  GroupBox before pasting radio button

33
RadioButton Control (Properties and Events)

• Properties
• Text - contains text appearing next to radio
  button
• Checked - indicates whether radio button is
  checked or not
• Events
• CheckedChanged event fires when user clicks radio
  button
• Standard prefix is "rad"

34
RadioButtons (Example)
RadioButtons in a ButtonGroup
35
An Introduction to Boolean Data

• Boolean data operates similar to an on/off switch
• Standard prefix is "bln"
• Stores the values True and False
• Note that True and False are reserved words
• Most frequently used in decision making
  statements
• Declare a Boolean variable named mblnValid and
  store the values True and False in the variable
• Private mblnValid As Boolean
• mblnValid True
• mblnValid False

36
Decision Making

• Group of statements called decision making
  statements execute different blocks of code in
  different circumstances
• If some condition is True then
• Execute one group of statements.
• Otherwise
• Execute a different group of statements
• End of If statement.

37
Comparison Operators (1)

• Conditional statements execute one group of
  statements when a condition is True and another
  group of statements when the condition is False
• Conditional statements apply the following
  comparison operators to test conditions
• Equal to ()
• Not equal to (ltgt)
• Less than (lt)
• Greater than (gt)
• Less than or equal to (lt)
• Greater than or equal to (gt)

38
Comparison Operators (2)

• Always produce a Boolean value as the result
• Comparison operators all have the same precedence
• Note that arithmetic operations are performed
  before comparison operators are evaluated
• Comparison operators are evaluated from left to
  right
• Comparison operators are evaluated after any
  arithmetic operations are performed
• Use parenthesis to override the default precedence

39
Comparison Operators (Example)
mintResult ( mintValue1 2 ) lt ( mintValue2
1 )
mintResult ( 3 2 ) lt ( 5 1 )
mintResult ( 5 ) lt ( 5 1 )
Evaluate arithmetic expressions from left to right
mintResult ( 5 ) lt ( 4 )
Evaluate condition
mintResult False
5 is not less than 4 so the result is False
40
The If Statement

• Conditional operators seldom used alone
• Conditions typically appear in a decision-making
  statement (If statement)
• Execute one group of statements only when
  condition is True
• Called a one-way If statement
• Statements executed in an If statement are called
  a statement block

41
One-Way If Statement (Syntax)

• If condition Then
• statements
• End If
• If statement tests value of condition
• statements execute only if condition is true

42
One-Way If Statement (Example)

• Execute statements when the variable
  mblnCondition is True
• Note that the ( True ) is optional
• Private mblnCondition As Boolean
• mblnCondition True
• If mblnCondition True Then
• ' Statements
• End If

43
One-Way If Statement (Logic Flow)
If condition Then Statements End
If Statements following the End If
condition
True
False
44
Two-Way If Statement

• Executes one statement block when a condition is
  True and another statement block when condition
  is False
• Also called an If Then Else statement

45
Two-Way If Statement (Syntax)

• If condition Then
• statements (true)
• Else
• statements (false)
• End If
• The If statement tests the value of some
  condition
• statements (true) execute only if the condition
  is true
• statements (false) execute otherwise
• Execution continues at the statement following
  the End If statement

46
Two-Way If Statement (Example)

• Based on the contents of the check box named
  chkInsurance, store values in a Label control
  instance
• If chkInsurance.CheckState CheckState.Checked
  Then
• lblInsuranceRate.Text cdblInsuranceRate.ToSt
  ring
• Else
• lblInsuranceRate.Text ""
• End If

47
Two-Way If Statement (Logic Flow)
If condition Then Statements (true) Else
Statement (false) End If Statements following
the End If
condition
True
False
48
Multi-Way If Statement

• Provides decision-making based on three or more
  outcomes

49
Multi-Way If Statement (Syntax)

• If condition1 Then
• statements
• ElseIf condition2 Then
• statements
• Else
• statements
• End If

50
Multi-Way If Statement (Syntax Dissection)

• The If statement tests the value of some
  condition
• If condition1 is True the statements between the
  If and the first ElseIf statements execute
• If condition1 is False, then condition2 is tested
• If condition2 is True the statements following
  the Else If execute
• If no condition is True, the statements following
  the Else execute
• Execution continues at the statement following
  the End If statement

51
Multi-Way If Statement (Example)
If mintGrade gt 90 Then mstrLetterGrade
"A" ElseIf mintGrade gt 80 Then
mstrLetterGrade "B" ElseIf mintGrade gt 70 Then
mstrLetterGrade "C" ElseIf mintGrade gt 60
Then mstrLetterGrade "D" Else
mstrLetterGrade "F" End If
52
Multi-Way If Statement (Logic Flow)
If condition1 Then statements ElseIf
condition2 Then elseifstatements
Else elsestatements End If Statements
following the End If
True
condition1
False
True
condition2
False
53
Nested If Statements

• If statements can be nested inside each other as
  shown in the following

If mintGrade gt 90 Then mstrLetterGrade "A"
If mintGrade gt 95 Then mstrNote
"Excellent" End If ElseIf mintGrade gt 80
Then mstrLetterGrade "B" ElseIf mintGrade gt
70 Then mstrLetterGrade "C" ElseIf
mintGrade gt 60 Then mstrLetterGrade
"D" Else mstrLetterGrade "F" End If
54
The Select Case Statement (1)

• Limited form of If statement
• Use the Select Case statement when multiple cases
  test against the same expression
• Select Case testexpression
• Case expressionlist-1
• statement-block1
• Case expressionlist-2
• statement-block2
• Case expressionlist-n
• statement-blockn
• Case Else
• statements
• End Select

55
The Select Case Statement (2)

• The testexpression is evaluated once when the
  Select Case statement first starts
• Testexpression is compared with expressionlist-1.
  If the condition is true then statement-block1
  executes
• If the condition is not true expressionlist-2 is
  tested
• The process continues until no more conditions
  remain to be tested
• The statements after the optional Case Else
  execute only if no conditions match the test
  expression

56
The Select Case Statement (Example)

• Assign a letter grade based on a numeric grade
• Select Case mintGrade
• Case 90 To 100
• mstrLetterGrade "A"
• Case 80 To 89
• mstrLetterGrade "B"
• Case 70 To 79
• mstrLetterGrade "C"
• Case 60 To 69
• mstrLetterGrade "D"
• Case Else
• mstrLetterGrade "F"
• End Select

57
The Select Case Statement (Variations)

• Select Case statements can be written in
  different ways using other comparison operators
• Case Is comparisonoperator value
• Case Is gt 90
• A Case may contain a comma separated list of
  values
• Case 1, 3, 4
• A Case may contain a range of values
• Case 5 To 10

58
The Select Case Statement (Example)

• The following Select Case statement illustrates a
  comma separated list and a range of values
• Select Case mintTest
• Case 1, 3, 4
• ' The value is 1, 3 or 4
• Case 5 To 10
• ' The value is 5, 6, 7, 8, 9
• ' or 10
• End Select

59
Logical Operators (1)

• Purpose is to perform conjunction (and) and
  disjunction (or) in an If statement
• Meaning similar to meaning in English
• Logical operators have precedence
• Arithmetic operators are evaluated first
• Comparison operators are evaluated next
• Logical operators are evaluated last from left to
  right in the following order
• Not, And, Or, Xor

60
Logical Operators (List)

• Not Unary operator to perform negation
• And Conjunction. Both conditions must be true
  for the expression to be true
• Or Disjunction. One or both conditions can be
  true fro the expression to be true
• Xor One condition must be true for the
  expression to be true. If both conditions are
  true then the condition is false