Selection / Decisions

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Selection / Decisions

• CIS 115 Lecture 7

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
• IfThen
• one-way selection structure
• IfThenElse
• two-way selection structure
• IfThenElseIf
• multi-way selection structure
• IfThenElseIfElse
• multi-way selection structure
• Select Case
• multi-way selection structure

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IfThen Decision Structure

• IfThen 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
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IfThen Statement Syntax

• If condition Then
• statements
• End If
• Syntax explanation
• If , Then, and End Keywords
• Condition True/False value, variable, function
  call or expression
• Statements one or more code statements to be
  executed if condition is true

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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

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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
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Relational Operators

• Often a condition is formed using a relational
  operator
• A relational operator determines if a specific
  relationship exists between two values
• gt Greater than
• lt Less than
• Equal to
• ltgt Not equal to
• gt Greater than or equal to
• lt Less than or equal to

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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 lt 10 (Is length less than or equal to
  10)
• len wid gt 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

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Relational Operators (cont.)

• Either or both relational operator operands may
  be expressions
• Math operators are evaluated before relational
  operators
• xy and a-b are evaluated first
• Each result is then compared using the gt operator
• Either or both relational operator operands may
  be function calls

If x y gt a - b Then lblMessage.Text "It is
true!" End If
If Val(txtInput.Text) lt getMinValue()
Then lblMessage.Text "Invalid Below
Minimum" End If
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Logical Operators

• 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

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The And Operator
The truth table for the And Operator Expression
1 Expression 2 Expression 1 And Expression
2 True False False False True False False F
alse False True True True
If temperature lt 20 And minutes gt 12
Then lblMessage.Text Temperature is in the
danger zone." End If

• AndAlso operator works identically but does not
  test minutesgt12 if temperaturelt20 is false

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The Or Operator
The truth table for the Or Operator Expression 1
Expression 2 Expression 1 Or Expression
2 True False True False True True True True
True False False False
If temperature lt 20 Or temperature gt 100
Then lblMessage.Text Temperature is in the
danger zone." End If

• OrElse operator works identically but does not
  test temperaturegt100 if temperaturelt20 is true

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The Xor Operator
The truth table for the Xor Operator Expression
1 Expression 2 Expression 1 Xor Expression
2 True False True False True True True True
False False False False
If total gt 1000 Xor average gt 120
Then lblMessage.Text You may try again." End
If
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The Not Operator
The truth table for the Not Operator Expression
1 Not Expression 1 True False False True
If Not temperature gt 100 Then lblMessage.Text
"You are below the max temp." End If
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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 lt vs lt or gt vs gt
• Check problem requirements for ranges carefully

If x gt 20 And x lt 40 Then lblMessage.Text
Value is in the acceptable range." End If
If x lt 20 Or x gt 40 Then lblMessage.Text
Value is outside the acceptable range." End If
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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

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Relational Logical Operators Combined

• For example, in the statement
• If x lt 0 And y gt 100 Or z 50
• x lt 0 And y gt 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 lt 0 And (y gt 100 Or z 50)

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All Operators Precedence

• Parenthesis
• Arithmetic
• Exponential
• Multiplication / Division
• Integer Division
• MOD
• Addition / Subtraction
• String Concatenation
• Relational Operators (lt , gt , gt , lt , ltgt)
• Logical Operators
• Not
• And
• Or, Xor)

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Arithmetic, Relational, Logical Operators
Combined

• Evaluate the following if a5, b7, x100, y30
• If x gt a 10 And y lt b 20
• Evaluating the math operators leaves us with
• If x gt 50 And y lt 27
• Evaluating the relational operators leaves
• If True And False
• Evaluating the logical operators leaves
• False
• Parentheses make order of operations clear
• If (x gt (a 10)) And (y lt (b 20))

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IfThen Examples

• If (intSales gt 50000) Then
• blnGetsBonus True
• End If
• --------------------------------------------------
  ---------------
• If ((blnGetsBonus)Or((intMissedDays lt
  2)And(intSales gt 30000)))
• intDaysOff intDaysOff 1
• intEmpRating 1
• End If
• --------------------------------------------------
  ---------------
• If (Not(isNumeric(txtInput.text))) Then
• txtInput.text
• MsgBox(Please enter a number in the textbox)
• End If
• --------------------------------------------------
  ---------------
• If (intGrade gt 80)And(intGrade lt 90) Then
  lblMessage.text B
• --------------------------------------------------
  ---------------
• If ((Val(txtGrade.text) lt 0)Or(Val(txtGrade.text)
  gt 100)) Then
• lblMessage.text Invalid Grade Not in the
  range 0-100
• End If

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IfThen vs IfThenElse

• The IfThen construct will execute or ignore a
  group of statements (do something or do nothing)

• The IfThenElse construct will execute one group
  of statements or another group (do this or do
  that)

Condition
True
False
Statement(s) If True
Statement(s) If False
Condition
True
False
Statement(s) If True
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IfThenElse Decision Structure

• IfThenElse
• provides two choices
• Evaluate condition
• True or False
• True execute code
• in IfThen block
• False execute code
• in Else Block
• One of the two choices must be selected
• They are mutually exclusive

Condition
True
False
Statement(s) If True
Statement(s) If False
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IfThenElse Syntax

• If condition Then
• statements1
• Else
• statements2
• End If
• Syntax explanation
• If , Then, Else, and End Keywords
• Condition True/False value, variable, function
  call or expression
• Statements1 executed if condition is True
• Statements2 executed if condition is False

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IfThenElse Examples

• If (intSales gt 50000) Then
• blnGetsDoubleBonus True
• decBonus 4000.00
• Else
• decBonus 2000.00
• End If
• --------------------------------------------------
  ---------------
• If (Not(isNumeric(txtInput.text))) Then
• MsgBox(You did not enter a valid number
  program will end)
• End
• Else
• intNumber Val(txtInput.text)
• End If
• --------------------------------------------------
  ---------------
• If (intTemp gt 60)And(intTemp lt
  90)And(VisibRating() gt 5) Then
• lblMessage.text Go - Weather conditions are
  ideal
• Else
• lblMessage.text Wait - Weather conditions
  unacceptable
• End If

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IfThenElseIf Decision Structure

• IfThenElseIf
• 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
True
C2
Statement(s)2
False
True
C3
Statement(s)3
False
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IfThenElseIf 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 its chilly and windy?
• If windy is tested before chilly, youd go out
  with a windbreaker when you need a jacket

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IfThenElseIf Syntax

• Syntax explanation
• If , Then, ElseIf, and End
• Keywords
• Condition1 thru n True/False
• value, variable, function
• call or expression
• Statements1 executed if
• condition1 is True
• Statements2 executed if condition1 is False
  and if condition2 is True
• Statementsn executed if condition1 thru
  (n-1) is False and if conditionn is True

• If condition1 Then
• statements1
• ElseIf condition2
• statements2
• ElseIf conditionn
• statementsn
• End If

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IfThenElseIf Examples

• If sngAvg lt 59.5 Then
• lblGrade.Text "F"
• ElseIf sngAvg lt 69.5 Then
• lblGrade.Text "D"
• ElseIf sngAvg lt 79.5 Then
• lblGrade.Text "C"
• ElseIf sngAvg lt 89.5 Then
• lblGrade.Text "B"
• ElseIf sngAvg lt 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

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IfThenElseIfElse (Trailing Else)

• IfThenElseIf
• Else is simply an
• IfThenElseIf
• 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
True
C2
Statement(s)2
False
True
C3
Statement(s)3
False
Statement(s)Else
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IfThenElseIf Else Syntax

• Syntax explanation
• Same as If..ThenElseIf
• thru statements n
• StatementsElse
• executed if condition1 thru n
• (all previous conditions) are False

• If condition1 Then
• statements1
• ElseIf conditionn
• statementsn
• Else
• statementsElse
• End If

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IfThenElseIfElse Examples

• If sngAvg lt 59.5 Then
• lblGrade.Text "F"
• ElseIf sngAvg lt 69.5 Then
• lblGrade.Text "D"
• ElseIf sngAvg lt 79.5 Then
• lblGrade.Text "C"
• ElseIf sngAvg lt 89.5 Then
• lblGrade.Text "B"
• ElseIf sngAvg lt 100 Then
• lblGrade.Text "A
• Else
• lblGrade.Text "Invalid"
• End If

• If intCredScr gt 700 Then
• strLoanType Prime
• intLoanRate 1
• ElseIf intCredScr gt 600 Then
• strLoanType Standard
• intLoanRate 2
• ElseIf intCredScr gt 500 Then
• strLoanType Risk
• intLoanRate 3
• ElseIf intCredScr gt 400 Then
• strLoanType HiRisk
• intLoanRate 4
• Else
• MsgBox(Not Qualified)
• End
• End If

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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

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Nested If Example

• A customer qualifies for a special rate loan if
• If credit score is higher than 650 and
• Income is more than 30000 Or Debt is less than
  1000
• Or If credit score is higher than 700
• If intCredScr gt 650 Then
• If decIncome gt 30000 Then
• lblMessage.Text qualified"
• ElseIf DecDebt lt 1000
• lblMessage.Text qualified
• Else
• lblMessage.Text not qualified
• End If
• ElseIf intCredScr gt 700 Then
• lblMessage.Text qualified"
• Else
• lblMessage.Text not qualified
• End If

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Select Case Statement

• Similar to IfThenElseIf
• 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

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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("Far
m Animal") Case "Lion,"Tiger,"Bear" MsgBox("O
h My!") End Select
Select Case intScore Case Is gt 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
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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.)

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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 .

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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)

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Homework

• Lab 6 and Homework 6
• Visual Basic Decisions
• See handout for details and due date
• Questions?