1. CIS 115 Lecture 7
Selection / Decisions

2. Control Structures
There are 3 control structures common to most computer languages that determine the flow, or path of execution, of the code:
Sequential
Selection / Decisions
Repetition / Looping

3. VB Decisions Visual Basic decision statements If…Then…ElseIf…Else
one-way selection structure
If…Then…Else
two-way selection structure
If…Then…ElseIf
multi-way selection structure
If…Then…ElseIf…Else
Select Case

4. If…Then Decision Structure
provides one choice
Evaluate the condition: True or False
Ex: Is it cold outside?
True – execute code
Ex: If yes, wear a coat
False – do not execute code
Ex: If no,
Condition
True
False
Conditional
Code

5. If…Then Statement Syntax
If condition Then
statement[s]
End If
Syntax explanation:
If , Then, and End – Keywords
Condition – True/False value, variable, function call or expression
Statement[s] – one or more code statements to be executed if condition is true

6. Conditions The execution of an If block is controlled by a condition
Must be (or evaluate to) either true or false
Can be a value, variable, or function call (Boolean DataType)
Can be formed by using the six Relational operators and the three Logical operators

7. Boolean Variables
A flag is a Boolean variable that signals when some condition exists in the program
Since a Boolean variable is either True or False, it can be used as the condition of an If
Note that an operator is not required (there is alternate syntax that does use operator)
If blnQuotaMet Then
lblMessage.Text = “Congratulations
you have met your sales quota"
End If

8. Boolean Functions
Boolean Functions return a single True or False Value
Since Boolean Functions return either True or False, a Boolean Function Call can be used as the condition of an If
Note that an operator is not required (there is alternate syntax that does use operator)
If isNumeric(strInput) Then
intNumber = Val(strInput)
End If

9. Relational Operators
Often a condition is formed using a relational operator
A relational operator determines if a specific relationship exists between two values
> Greater than
< Less than
= Equal to
<> Not equal to
>= Greater than or equal to
<= Less than or equal to

10. Relational Operators (cont.)
Relational operators are binary – meaning they use two operands
Either or both relational operator operands may be values, variables, expressions or function calls
length <= 10 (Is length less than or equal to 10)
len * wid > max + 1 (Is len * wid greater than max + 1)
Val(txtNum.Text) = 0 (Is Val result equal to 0 – not assignment)
Relational operators yield a True or False result

11. Relational Operators (cont.)
Either or both relational operator operands may be expressions
Math operators are evaluated before relational operators
x+y and a-b are evaluated first
Each result is then compared using the > operator
Either or both relational operator operands may be function calls
If x + y > a - b Then
lblMessage.Text = "It is true!"
End If
If Val(txtInput.Text) < getMinValue() Then
lblMessage.Text = "Invalid: Below Minimum"
End If

12. Logical Operators And Or Xor Not
These operators are used to evaluate boolean values and will yield a boolean result
And
Both operands must be true for the overall expression to be true, otherwise it is false Or
One or both operands must be true for the overall expression to be true, otherwise it is false
Xor
One operand (but not both) must be true for the overall expression to be true, otherwise it is false
Not
Reverses the logical value of an expression

13. The And Operator The truth table for the And Operator True False False
Expression 1 Expression 2 Expression 1 And Expression 2
True False False
False True False
False False False
True True True
If temperature < 20 And minutes > 12 Then
lblMessage.Text = “Temperature is in the danger zone."
End If
AndAlso operator works identically but does not test minutes>12 if temperature<20 is false

14. The Or Operator The truth table for the Or Operator True False True
Expression 1 Expression 2 Expression 1 Or Expression 2
True False True
False True True
True True True
False False False
If temperature < 20 Or temperature > 100 Then
lblMessage.Text = “Temperature is in the danger zone."
End If
OrElse operator works identically but does not test temperature>100 if temperature<20 is true

15. The Xor Operator The truth table for the Xor Operator True False True
Expression 1 Expression 2 Expression 1 Xor Expression 2
True False True
False True True
True True False
False False False
If total > 1000 Xor average > 120 Then
lblMessage.Text = “You may try again."
End If

16. The Not Operator The truth table for the Not Operator True False
Expression 1 Not Expression 1
True False
False True
If Not temperature > 100 Then
lblMessage.Text = "You are below the max temp."
End If

17. Example: Checking Numerical Ranges
Checking for a value inside a range uses And
Checking for a value outside a range uses Or
Must pay careful attention to differences in resulting range using: < vs <= or > vs >=
Check problem requirements for ranges carefully
If x >= 20 And x <= 40 Then
lblMessage.Text = “Value is in the acceptable range."
End If
If x < 20 Or x > 40 Then
lblMessage.Text = “Value is outside the acceptable range."
End If

18. Precedence of Logical Operators
Logical operators have an order of precedence just as arithmetic operators do
From highest to lowest precedence
Not
And Or
Xor
As with arithmetic operations, parentheses are often used to clarify order of operations

19. Relational & Logical Operators Combined
For example, in the statement
If x < 0 And y > 100 Or z = 50
x < 0 And y > 100 is evaluated first
If the And condition is true, we then evaluate
True Or z = 50
If the And condition is false, we then evaluate
False Or z = 50
If the Or condition is to be evaluated first parentheses must be used
If x < 0 And (y > 100 Or z = 50)

20. All Operators Precedence
Parenthesis
Arithmetic
Exponential
Multiplication / Division
Integer Division
MOD
Addition / Subtraction
String Concatenation
Relational Operators (< , > , >= , <= , <>)
Logical Operators
Not
And
Or, Xor)

21. Arithmetic, Relational, & Logical Operators Combined
Evaluate the following if a=5, b=7, x=100, y=30
If x > a * 10 And y < b + 20
Evaluating the math operators leaves us with
If x > 50 And y < 27
Evaluating the relational operators leaves
If True And False
Evaluating the logical operators leaves
False
Parentheses make order of operations clear
If (x > (a * 10)) And (y < (b + 20))

22. If…Then Examples
If (intSales > 50000) Then blnGetsBonus = True End If If ((blnGetsBonus)Or((intMissedDays < 2)And(intSales > 30000))) intDaysOff = intDaysOff + 1 intEmpRating += 1 If (Not(isNumeric(txtInput.text))) Then txtInput.text = “” MsgBox(“Please enter a number in the textbox”) If (intGrade >= 80)And(intGrade < 90) Then lblMessage.text = “B” If ((Val(txtGrade.text) < 0)Or(Val(txtGrade.text) > 100)) Then lblMessage.text = “Invalid Grade: Not in the range 0-100”

23. If…Then vs If…Then…Else
The If…Then construct will execute or ignore a group of statements (do something or do nothing)
The If…Then…Else construct will execute one group of statements or another group (do this or do that)
Condition
False
True
Statement(s)
If False
Statement(s)
If True
Condition
True
False
Statement(s)
If True

24. If…Then…Else Decision Structure
provides two choices
Evaluate condition:
True or False
True – execute code
in If…Then block
False – execute code
in Else Block
One of the two choices must be selected
They are mutually exclusive
Condition
False
True
Statement(s)
If False
Statement(s)
If True

25. If…Then…Else Syntax Syntax explanation: If condition Then
statement[s]1
Else
statement[s]2
End If
Syntax explanation:
If , Then, Else, and End – Keywords
Condition – True/False value, variable, function call or expression
Statement[s]1 – executed if condition is True
Statement[s]2 – executed if condition is False

26. If…Then…Else Examples
If (intSales > 50000) Then blnGetsDoubleBonus = True decBonus = Else decBonus = End If If (Not(isNumeric(txtInput.text))) Then MsgBox(“You did not enter a valid number – program will end”) End intNumber = Val(txtInput.text) If (intTemp >= 60)And(intTemp < 90)And(VisibRating() > 5) Then lblMessage.text = “Go - Weather conditions are ideal” lblMessage.text = “Wait - Weather conditions unacceptable”

27. If…Then…ElseIf Decision Structure
allows for multiple
mutually exclusive
choices
Each of the conditions
is tested in sequence
When a condition is
true, the corresponding
code is executed and the
remaining conditions are ignored C1
True
Statement(s)1
False C2
True
Statement(s)2
False C3
True
Statement(s)3
False

28. If…Then…ElseIf Conditions
If it is very cold Then
Wear a coat
Elseif it is chilly
Wear a light jacket
Elseif it is windy
Wear a windbreaker
Elseif it is hot
Wear no jacket
The order of the conditions is vital
Wrong order can result in wrong decision
What if it’s chilly and windy?
If windy is tested before chilly, you’d go out with a windbreaker when you need a jacket

29. If…Then…ElseIf Syntax
Syntax explanation:
If , Then, ElseIf, and End –
Keywords
Condition1 thru n – True/False
value, variable, function
call or expression
Statement[s]1 – executed if
condition1 is True
Statement[s]2 – executed if condition1 is False and if condition2 is True
Statement[s]n – executed if condition1 thru (n-1) is False and if conditionn is True
If condition1 Then
statement[s]1
ElseIf condition2
statement[s]2
• • •
ElseIf conditionn
statement[s]n
End If

30. If…Then…ElseIf Examples
If sngAvg < 59.5 Then
lblGrade.Text = "F"
ElseIf sngAvg < 69.5 Then
lblGrade.Text = "D"
ElseIf sngAvg < 79.5 Then
lblGrade.Text = "C"
ElseIf sngAvg < 89.5 Then
lblGrade.Text = "B"
ElseIf sngAvg <= 100 Then
lblGrade.Text = "A"
End If
In each example, does the order of the conditions matter?
What happens if the order is reversed in each example?
If radCredCrd.checked Then
CredCrdPayment(decSubTot)
ElseIf radDebCrd.checked Then
DebCrdPayment(decSubTot)
ElseIf radCheck.checked Then
CheckPayment(decSubTot)
End If

31. If…Then…ElseIf…Else (Trailing Else)
Else is simply an
If…Then…ElseIf
with an Else at the end
Called a Trailing Else
If the initial If and none
of the ElseIf conditions
are True, the trailing
Else statement(s) will
be executed C1
True
Statement(s)1
False C2
True
Statement(s)2
False C3
True
Statement(s)3
False
Statement(s)Else

32. If…Then…ElseIf …Else Syntax
Syntax explanation:
Same as If..Then…ElseIf
thru statement[s] n
Statement[s]Else –
executed if condition1 thru n
(all previous conditions) are False
If condition1 Then
statement[s]1
• • •
ElseIf conditionn
statement[s]n
Else
statement[s]Else
End If

33. If…Then…ElseIf…Else Examples
If sngAvg < 59.5 Then lblGrade.Text = "F" ElseIf sngAvg < 69.5 Then lblGrade.Text = "D" ElseIf sngAvg < 79.5 Then lblGrade.Text = "C" ElseIf sngAvg < 89.5 Then lblGrade.Text = "B" ElseIf sngAvg <= 100 Then lblGrade.Text = "A“ Else lblGrade.Text = "Invalid" End If
If intCredScr >= 700 Then
strLoanType = “Prime”
intLoanRate = 1
ElseIf intCredScr >= 600 Then
strLoanType = “Standard”
intLoanRate = 2
ElseIf intCredScr >= 500 Then
strLoanType = “Risk”
intLoanRate = 3
ElseIf intCredScr >= 400 Then
strLoanType = “HiRisk”
intLoanRate = 4
Else
MsgBox(“Not Qualified“)
End
End If

34. Nested If If Statements Within If Statements
Any type of statement may be used inside the statement(s) portion of any form of If
This includes other If statements
If statements within If statements create a more complex decision structure called a Nested If

35. Nested If Example A customer qualifies for a special rate loan if:
If credit score is higher than 650 and
Income is more than Or Debt is less than 1000
Or If credit score is higher than 700
If intCredScr > 650 Then
If decIncome > Then
lblMessage.Text = “qualified"
ElseIf DecDebt < 1000
lblMessage.Text = “qualified“
Else
lblMessage.Text = “not qualified“
End If
ElseIf intCredScr > 700 Then

36. Select Case Statement Similar to If…Then…ElseIf
Performs a series of tests
Conditionally executes the first true condition
Select Case is different in that:
A single test expression may be evaluated
The test expression is listed once
The possible values of the expression are then listed with their conditional statements
Case Else may be included and executed if none of the values match the expression

37. Select Case Statement Examples
Select Case Val(txtInput.Text)
Case 1
MsgBox("Day 1 is Monday.")
Case 2
MsgBox("Day 2 is Tuesday.")
Case 3
MsgBox("Day 3 is Wednesday.")
Case 4
MsgBox("Day 4 is Thursday.")
Case 5
MsgBox("Day 5 is Friday.")
Case 6
MsgBox("Day 6 is Saturday.")
Case 7
MsgBox("Day 7 is Sunday.")
Case Else
MsgBox("The value is invalid.")
End Select
Select Case strAnimal
Case "Dog“,"Cat"
MsgBox("House Pet")
Case "Cow“,"Pig“,"Goat"
MsgBox("Farm Animal")
Case "Lion“,"Tiger“,"Bear"
MsgBox("Oh My!")
End Select
Select Case intScore
Case Is >= 90
strGrade = “A”
Case 80 to 89
strGrade = “B”
Case 70 to 79
strGrade = “C”
Case 60 to 69
strGrade = “D”
Case 0 to 59
strGrade = “F”
End Select

38. Example Decision Problems
Write a program that will prompt the user to input a number. Check for valid input. If the input is invalid (non-numeric) – give an error message via MsgBox and end the Event Procedure. If valid – assign the number to a variable and output the number to the user Now experiment with validity checking for more restrictive input criteria with numbers (ex: only numbers from 1-100, only integers, only positive integers, etc.) and text (ex: only single characters, only the letters a-d, etc.)

39. Example Decision Problems
Write a program that inputs 2 values and displays their positive difference. For example, if the first input is 6 and the second input is 9, then the positive difference is 3 (note: 3 is still the answer if the first input is 9 and the second input is 6). Now add the code to handle invalid (non-numeric) input .

40. Example Decision Problems
Write a VB application to have the user input via textbox an integer from 1 t0 100,000 (inclusive). Determine if the input is a valid. If invalid, give an error message, clear the textbox, and end the event procedure (discuss). If valid, use a boolean function to determine if the integer is even or odd and use an integer function to determine if the integer is a perfect square (return the root if yes, return -1 if no). Report your results via label. Write a VB application to have the user input a 2 digit binary number via input box. Determine if the input is a valid 2 digit binary number. If not give a specific error message and terminate the app. If valid, convert the number to a decimal value (try using a function to do this) and report the results via message box. (try with 3 digits)

41. Homework Lab 6 and Homework 6 Visual Basic – Decisions
See handout for details and due date
Questions?