There is a logical type in VBThere is a logical type in VB

Data Type Prefix

Boolean bln

Dim blnIsStudent As Boolean

blnIsStudent = True

….blnIsStudent = False

Boolean

A Boolean variable can only have one of two values, True or False

Note that “True” is not the same as True

Dim intMarysAge As Integer

intMarysAge = 22

intMarysAge = 10 + 12

intMarysAge = 10 + intA

Dim blnIsDrunk As Boolean

blnIsDrunk = False

blnIsDrunk = (1 < 2)

blnIsDrunk = (intNumBeers > 2)

We have seen this stuff for numbers…

Here are some analogous ideas for the Boolean type

blnIsDrunk = (1 < 2)

This is a relational operator,Here is the full set of relational operators…

Symbol Name

=  Equality < >  Inequality ()<  Less Than>  Greater Than<=  Less Than or Equal To () >=  Greater Than or Equal To ()

Dim blnB As Boolean

blnB = False

blnB = (1 < 2)

blnB = (2 = 2)

blnB = (1 = 2)

blnB = (1 <> 2)

blnB = (5 <> 5)

blnB = (5 < 5)

blnB = (5 <= 5)

The relational operators work as you might expect for numbers (with one important exception, which we will see later)

Dim blnB As Boolean

blnB = False

blnB = (“eamonn” = “joe”)

blnB = (“cat” = “cat”)

blnB = (“cat” < “can”)

blnB = (“Dog” < “dog”)

blnB = (“5” < “Five”)

blnB = ("5" = (1 + 4).ToString)

blnB = ("eamonn" > "eamon")

The relational operators are also defined for strings

“1”< “2”< ... <“A” < “B” <…< “a” < “b”…

True: Any character is alphabetically after none.

So (“E” > “”) is True

“cat”

“can”

Suppose we want a Boolean variable blnIsAdult to be True if some is aged 18 or older. Further suppose we have the persons age in shtAge…

Dim blnIsAdult As Boolean

blnIsAdult = (shtAge >= 18)

Suppose we want a Boolean variable blnNoSeniorDiscount to be True if some is aged 64 or less.

Dim blnNoSeniorDiscount As Boolean

blnNoSeniorDiscount = (shtAge <= 64)

Dim blnPaysFullPrice As Boolean

blnPaysFullPrice = (intAge >= 18) And (intAge <= 64)

Now suppose we want a Boolean variable blnPaysFullPrice to be True if some is aged 18 or older and 64 or younger…

Here the word And is a Boolean operator…

We know that the arithmetic operators take two numbers, and produce another number 5 + 2

7The logical operator And takes two logical values, and produce a logical value

True And False

False

a b a And bTrue True TrueTrue False FalseFalse True FalseFalse False False

a b a Or bTrue True TrueTrue False TrueFalse True TrueFalse False False

a Not a

True False

False True

a b a And bTrue True TrueTrue False FalseFalse True FalseFalse False False

Let a be true if a man is rich Let b be true if a man is tall

Would Patty marry...

Shaquille O'NealBill Gates Eamonn (dirt poor, 6’3)

Joe (dirt poor, 5’1)

I would marry a man if he was rich and tall

I would marry a man if he was rich and tall

a b a Or bTrue True TrueTrue False TrueFalse True TrueFalse False False

Let a be true if a woman is an astronaut Let b be true if a woman is a foreigner

Would Carl marry...•Valentina Tereshkova (Vostok 7)

•Sally Ride (STS-7 83) •Michelle Yeoh•Hillary Clinton

I would marry a woman if she was an astronaut or was a foreigner

I would marry a woman if she was an astronaut or was a foreigner

Let a be true if a woman is a divorcee

Would Ned marry...

• Elizabeth Taylor• Britney Spears • Mary Kate Olsen

a Not a

True False

False TrueI would marry a

woman if she was not a divorcee

I would marry a woman if she was not a divorcee

We know it is legal to combine multiple arithmetic operators...

2 * 3 + 5

6 + 5

11

The same is true for logic operators...

False Or False Or True

False Or True

TrueBut just like arithmetic there is a problem of ambiguity … (next slide)

A * B + C

X Or Y Or Z

I would marry a guy if he were

good looking and tall or very rich

I would marry a guy if he were

good looking and tall or very rich

This is ambiguous!

False And False Or True

False Or True

True

False And False Or True

False And True

False

Working left to right Working right to left

The ambiguity is solved with precedence

work from left to right Parentheses ()

Negation NotConjunction AndDisjunction Or

Precedence can be overwritten

with parentheses

False AND ( False OR True)

True

Never test for equality with real numbersNever test for equality with real numbers

Dim blnIsAdult As Boolean

Dim intAge As Integer

intAge = ….

blnIsAdult = (intAge = 18)

Dim blnIsPi As Boolean

Dim decAnswer As Decimal

decAnswer = ….

blnIsPi = (decAnswer = 3.1415)

This is fine

This is not

Never test for equality with real numbersNever test for equality with real numbers

Dim blnIsPi As Boolean

Dim decA As Decimal

decA = ….

blnIsPi = (decA > 3.1415 – 0.1) And (decA < 3.1415 + 0.1)

True Pi

2 3 4

0.1

Or…Or…

Dim blnIsPi As Boolean

Dim decA As Decimal

Const sngLoPi As Single = 3.1, sngHiPi As Single = 3.2

blnIsPi = (decA > sngLoPi) And (decA < sngHiPi )

True Pi

2 3 4 0.1

Note, I wrote sngLoPi to save space, sngLowestAcceptableValueForPi would be better

Or…Or…Dim blnIsPi As Boolean

Dim decA As Decimal

Const sngtolerance As Single = 0.1, sngPi As Single = 3.1415

blnIsPi = (decA > sngPi – sngtolerance ) And (decA < sngPi + sngtolerance )

True Pi

2 3 4

0.1

Common mistake: Unreachable codeCommon mistake: Unreachable code

(A > B) And (B = A) (A > B) Or (B >= A)

Can never be true Can never be false

Public Class Form1

Inherits System.Windows.Forms.Form

Dim intA As Integer

Private Sub bntBlueDemo_Click(…

intA = 0

bntRedDemo.Text = (intA).ToString

End Sub

Private Sub bntRedDemo_Click(….

intA = intA + 1

bntRedDemo.Text = intA.ToString

If intA = 10 Then

bntRedDemo.Text = "You have reached ten!"

End If

End Sub

End Class

Windows From Designer generated code

…

Conditional BranchingConditional Branching

If (Logical Expression) Then

Program statements to execute if expression evaluates to True

End If

If (Logical Expression) Then

Program statements to execute if expression evaluates to True

End If

We use If statement whenever we want to execute code, only under some circumstances

It is your birthday todayThe student has paid feesThe password is correct

Dim shtFavNumber As Short

shtFavNumber = …. ‘Assume we get the number from user

If shtFavNumber = 7 Then bntRedDemo.Text = “That’s my favorite number too!”End If

bntBlueDemo.Text = “Goodbye”

That’s my favorite number too!

Goodbye

Goodbye

Dim sngExamScore As Single ‘I am assuming the range [0-100]

sngExamScore = ….

If sngExamScore >= 90 Then bntRedDemo.Text = “Well Done!”End If

Well Done!

Dim sngExamScore As Single ‘I am assuming the range [0-100]

sngExamScore = ….

If sngExamScore >= 90 Then bntRedDemo.Text = “Well Done!”End If

If sngExamScore < 90 Then bntBlueDemo.Text = “Work Harder!”End If

Well Done!

Work Harder!

Legal, but bad idea

Legal, but bad idea

Dim sngExamScore As Single ‘I am assuming the range [0-100]

sngExamScore = ….

If sngExamScore >= 90 Then bntRedDemo.Text = “Well Done!”End If

If sngExamScore <= 90 Then bntRedDemo.Text = “Work Harder!”End If

Suppose we got the range wrong…

Suppose we got the range wrong…

Well Done!

Work Harder!

Well Done!

Work Harder!

45

97

90

If (Logical Expression) Then

Program statements to execute if expression evaluates to True

Else

Program statements to execute if expression evaluates to False

End If

If (Logical Expression) Then

Program statements to execute if expression evaluates to True

Else

Program statements to execute if expression evaluates to False

End If

Dim sngExamScore As Single ‘I am assuming the range [0-100]

sngExamScore = ….

If sngExamScore >= 90 Then

bntRedDemo.Text = “Well Done!”

Else

bntBlueDemo.Text = “Work Harder!”End If

Well Done!

Work Harder!

Much better than previous example

Much better than previous example

97

45

We use If Then Else statement whenever we want to divide the world into two, mutually exclusive possibilities…

Student/ nonStudentMarried/ notMarriedLiving/ Dead18 or older/ Under 18

Sometimes there are 3 or more mutually exclusive possibilities…

Student/ nonStudentMarried/ notMarriedLiving/ Dead18 or older/ Under 18

Undergrad/Grad/ Professor/ nonStudentMarried/ Widowed/ DivorcedLiving/ Dead / Never livedChild/ Adult / Senior

We can use the If Then Else If statement …

If sngExamScore >= 90 Then bntRedDemo.Text = “Well Done!”Else If sngExamScore >= 80 bntRedDemo.Text = “Not Bad!”Else bntRedDemo.Text = “Work Harder!”End If

Well Done!

Not Bad!

Work Harder!

If sngExamScore >= 80 Then bntRedDemo.Text = “Not Bad!”Else If sngExamScore >= 90 bntRedDemo.Text = “Well Done!”Else bntRedDemo.Text = “Work Harder!”End If

Not Bad

Not Bad!

Work Harder!

Unreachable code

Unreachable code

0 10 20 30 40 50 60 70 80 90 100

1st 2nd

0 10 20 30 40 50 60 70 80

1st

0 10 20 30 40 50 60 70

2nd

If (Condition) Then Do SomethingElseIf (Condition 2) Then Do Something ElseElseIf (Condition 3) Then Do Something Else...Else Do Something ElseEnd If

If (Condition) Then Do SomethingElseIf (Condition 2) Then Do Something ElseElseIf (Condition 3) Then Do Something Else...Else Do Something ElseEnd If

We can have as many Else If’s as we want …

We use If Then Else If statement whenever we want to divide the world into three or more, mutually exclusive possibilities…

Select Case Expression Case Possible Value or Range of Values Statement(s) Case Another Possible Value or Range of Values Statement(s) . . .

Case Else Statement(s)End Select

Select Case Expression Case Possible Value or Range of Values Statement(s) Case Another Possible Value or Range of Values Statement(s) . . .

Case Else Statement(s)End Select

The expression in a Select Case statement may be• a numeric variable

• a string variable

• a simple expression composed of operators and variables

The possible values in a Case statement may be• a numeric constant

• a string constant

• a numeric variable

• a string variable

• a range of values

• a combination of the above

Dim shtFavNumber As Short

shtFavNumber = …. ‘Assume we ask for a number between 0 and 10

Select Case shtFavNumber

Case 7

bntRedDemo.Text = “Seven is my favorite too!”

Case Else

bntRedDemo.Text = “That’s different to my favorite”

End Select

Dim shtFavNumber As Short

shtFavNumber = …. ‘Assume we ask for a number between 0 and 10

Select Case shtFavNumber

Case 7

bntRedDemo.Text = “Seven is my favorite too!”

Case Else

bntRedDemo.Text = “That’s different to my favorite”

End Select

Select Case shtFavNumber

Case 1 , 3 , 5 , 9

bntRedDemo.Text = “Odd number!”

Case 7

bntRedDemo.Text = “Odd number, and my favorite too”

Case 0, 2 , 4 , 6 , 8

bntRedDemo.Text = “Even number”

Case Else

bntRedDemo.Text = “Error!”

End Select

Select Case shtFavNumber

Case 1 , 3 , 5 , 9

bntRedDemo.Text = “Odd number!”

Case 7

bntRedDemo.Text = “Odd number, and my favorite too”

Case 0, 2 , 4 , 6 , 8

bntRedDemo.Text = “Even number”

Case Else

bntRedDemo.Text = “Error!”

End Select

Dim strUserInput As String

strUserInput = “Lenny”

Select Case strUserInput

Case “Homer”, “Marge”, “Maggie”, “Lisa”, “Bart”

bntRedDemo.Text = “Simpson Clan”

Case “Monty”, “Waylon”, “Carl”, “Lenny”

bntRedDemo.Text = “PowerPlant Worker”

Case Else

bntRedDemo.Text = “Other”

End Select

Dim strUserInput As String

strUserInput = “Lenny”

Select Case strUserInput

Case “Homer”, “Marge”, “Maggie”, “Lisa”, “Bart”

bntRedDemo.Text = “Simpson Clan”

Case “Monty”, “Waylon”, “Carl”, “Lenny”

bntRedDemo.Text = “PowerPlant Worker”

Case Else

bntRedDemo.Text = “Other”

End Select