Computermade Decisions

1
Computer-made Decisions
Chapter 3 4
2
Overview

• Variable Scope
• Conditionals
• Relational Operators
• AND, OR, NOT
• If Statements
• MsgBox function

3
Conditionals

• Sometimes it is necessary to alter ones
  behavior, depending on existing conditions
• Examples
• If the light is red, stop
• If it is Labor Day, dont come to class,
  otherwise attend lecture and lab
• If there is a quiz on Monday, then read the book
  Sunday

4
Conditionals

• True or False conditions are created
• when comparing variables with relational
  operators
• by evaluating a boolean variable
• Relational Operators
• gt Greater than lt
  Less than
• Equal to ltgt
  Not equal to
• gt Greater than or equal to lt Less
  than or equal to

5
Compound Conditions

• You may join separate conditions with the AND
  and the OR to create compound conditions
• The AND is data reducing it reduces choices that
  fit
• The OR is data expanding it increases the
  choices that fit

Blue Cars AND Four Doors is
????? Blue Cars OR Four Doors is
?????
Blue Cars
A B C
Four Doors
6
Compound Conditions
Union Both
Alternation Either
And
F
Or
T
T
F
T
T
T
T
F
T
F
F
F
F
T
F
7
Compound Conditions

• Evaluate Negations (NOTs) first
• Evaluate ANDs left to right
• Evaluate ORs after all ANDs left to right
• Parentheses can, as usual override this precedence

8
The If Test

• A conditional statement that permits portions of
  code to be executed only if and when some special
  condition applies
• I'd like root beer if they have it, otherwise
  I'll have a cola
• If it is raining, bring an umbrella

9
If Test

• If (Condition) Then
• ' (do true stuff)
• Else
• ' (do false stuff)
• End If

10
If Test (short form)

• If (Condition) Then
• ' (do true stuff)
• End If
• use only if there is nothing to do in the Else
  branch

11
IF Test Example

• Private Sub cmdDo_Click()
• If cmdDo.Caption "Talk" Then
• lblBox.Caption "ding ding"
• cmdDo.Caption "clear"
• Else 'in this case, cmdDo "clear"
• lblBox.Caption ""
• cmdDo.Caption "Talk"
• End If
• End Sub

12
IF Test Example
ltlt alternate on clicking gtgt
13
IF Test Example

• Private Sub cmdBlink_Click()
• If lblBox.Visible True then
• lblBox.Visible False
• Else
• lblBox.Visible True
• End If
• End Sub
• Since lblBox.Visible is boolean why not use
• If lblBox.Visible then

14
ElseIf Clause

• If condition1 Then
• statements to process when condition1 is true
• ElseIf condition2 Then
• statements to process when condition1 is false
  and condition2 is true
• Else
• statements to process when condition1 is false
  and condition2 is false
• End If 'ElseIf is much like select-case

15
Nested Selection Structure

• A nested selection structure is one in which
  either the true path or the false path includes
  yet another selection structure
• Any of the statements within either the true or
  false path of one selection structure may be
  another selection structure

16
Nested If Example

• Write an if statement that assigns a sales tax
  rate to the sngTax variable
• The tax rate is determined by the state code
  stored in the intCode variable
• Codes of 1 and 3 represent a 4 rate
• A code of 2 represents a 5 rate
• All other codes represent a 2 rate

17
Nested If Example

• If intCode 1 Or intCode 3 Then
• sngTax .04
• Else
• If intCode 2 Then
• sngTax .05
• Else
• sngTax .02
• End If
• End If

18
Nested If Example

• Write a selection structure that assigns a bonus
  to the sngBonus variable
• The bonus is determined by the salespersons code
  (intCode) and, in some cases, by the sales amount
  (sngSales)
• If the code is 1
• and the salesperson sold at least 10,000, then
  the bonus is 500
• otherwise these salespeople receive 200
• If the code is 2
• and the salesperson sold at least 20,000, then
  the bonus is 600
• otherwise these salespeople receive 550
• All others receive 150

19
Nested If Example

• If intCode 1 Then
• If sngSales gt 10000 Then
• sngBonus 500
• Else
• sngBonus 200
• End If
• ElseIf intCode 2 Then
• If sngSales gt 20000 Then
• sngBonus 600
• Else
• sngBonus 550
• End If
• Else
• sngBonus 150
• End If

20
Nested If Example

• If intCode 1 And sngSales gt 10000 Then
• sngBonus 500
• Else
• If intCode 1 And sngSales lt 10000 Then
• sngBonus 200
• Else
• If intCode 2 And sngSales gt 20000 Then
• sngBonus 600
• Else
• If intCode 2 And sngSales lt 20000 Then
• sngBonus 550
• Else
• sngBonus 150
• End If
• End If
• End If
• End If

21
ElseIf Example

• If intCode 1 And sngSales gt 10000 Then
• sngBonus 500
• ElseIf intCode 1 And sngSales lt 10000 Then
• sngBonus 200
• ElseIf intCode 2 And sngSales gt 20000 Then
• sngBonus 600
• ElseIf intCode 2 And sngSales lt 20000 Then
• sngBonus 550
• Else
• sngBonus 150
• End If

22
The Message Box

• VB allows the user to generate message boxes as a
  part of a user program. When invoked they look
  something like this

23
Format of MsgBox

• A simple example of the MsgBox function
• Dim intX As Integer
• intX MsgBox("Read the Password")

24
The Details

• Dim intX As Integer
• intX MsgBox("Do this now", _
• vbCritical, "Prompting Issue")

25
Response Choices
Dim intZ As Integer intZ MsgBox(Should I Do
This, _ vbYesNo ,Prompting Issue)

• The value of intZ above depends on which
  response is chosen.
• Yes - intZ 6
• No - intZ 7
• (the numerical values are defined by Visual
  Basic.)

26
Icon Choices

• vbExclamation
• vbQuestion
• vbCritical

27
Possible values for the 2nd argument