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ENGR 112. Decision Structures. Control Structures. Sequence structures Built into Visual Basic Selection structures If/Then  Single selection If/Then/Else  Double selection Select Case  Multiple selection. Control Structures. Repetition structures For/Next Do/While Loop.

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Engr 112

ENGR 112

Decision Structures


Control structures
Control Structures

  • Sequence structures

    • Built into Visual Basic

  • Selection structures

    • If/Then  Single selection

    • If/Then/Else  Double selection

    • Select Case  Multiple selection


Control structures1
Control Structures

  • Repetition structures

    • For/Next

    • Do/While Loop



Selection structure

Is Condition Met?

Statements in Else Clause

Statements in Then Clause

Selection Structure

NO

YES


If then else statement
If/Then/Else Statement

  • Allows us to execute one statement if TRUE and another if FALSE

    If A=0 Then

    msg = “ERROR...Division by ZERO”

    MsgBox(msg)

    Exit Sub

    Else

    C = B/A

    End If


If then else for checking
If/Then/Else for checking

  • To determine if user has entered a string in the from of a date

    If IsDate(variable) Then

    ‘do what you want with the date

    Else

    ‘provide a message to the user

    End If


Example
Example

Dim DT

Private Sub Command1_Click()

DT = Text1.Text

If IsDate(DT) Then

MsgBox "Good Job"

Else

MsgBox "please enter the text in the form of a date"

End If

End Sub


If then else for checking1
If/Then/Else for checking

  • To determine if variable can be converted to a number

    If IsNumeric(variable) Then

    ‘do what you want with the number

    Else

    ‘provide a message to the user

    End If


Example1
Example

Dim NT

Private Sub Command2_Click()

NT = Text2.Text

If IsNumeric(NT) Then

MsgBox "Good Job"

Else

MsgBox "please enter a number"

End If

End Sub


If then else statement1
If/Then/Else Statement

  • Nested conditional statements

    If A = 0 Then

    value = “x”

    Else

    If A=1 Then

    value = “y”

    Else

    If A=2 Then

    value = “z”

    End If

    End If

    End If


Select case structure
Select Case structure

  • Practical when more than three levels of if/Then/Else are necessary

    Select Case inputNumber

    Case 0

    value = “x”

    Case 1

    value = “y”

    Case 2

    value = “z”

    Case 3

    value = “Null”

    End Select


Example2
Example

  • Design a program to compute grades based on the average of 4 exams

  • If the average is 90 or higher, the student gets an “A”

  • If the average is 80 – 89, the student gets a “B”

  • Etc.


If then else pseudocode version
If/Then/Else Pseudocode Version

If Grade >= 90 Then

YourGrade = “A”

Else If Grade >= 80 Then

YourGrade = “B”

Else If Grade >= 70 Then

etc


Case pseudocode
Case Pseudocode

Select Case Grade

Case is >= 90

YourGrade = “A”

Case is >= 80

YourGrade = “B”

Case is >= 70

YourGrade = “C”

Etc.




For next statement

Optional

Increments counter

For/Next Statement

  • Also known as a For … Next loop

  • Allows repetition of statements

  • Syntax

    For CounterVar = StartNum to EndNum [Step StepNum]

    VB Statements

    Next CounterVar


Determinate structure

Set counter variable to starting value

Increment counter variable

Body of Loop

Move to statements after loop

Is counter value greater than ending value?

Determinate Structure

YES

NO


For next statement1
For/Next Statement

  • Counter increments by 1 unless otherwise specified

  • VB keyword Step is used to increment by a number other than 1 (including negative numbers)

  • Can be terminated prematurely with keywords Exit For


For next statement2
For/Next Statement

  • Examples

    Answ = 0

    For X=1 To 10

    Answ = Answ + X

    Next X

    Answ = 0

    For X=1 To 10 Step 2

    Answ = Answ + X

    If X > 10 Then Exit For

    Next X

E

E



Do loop statements
Do…Loop Statements

  • Conditional loops are executed as long or until a condition exists

  • Their key feature is the condition

  • The condition can be

    • A Boolean variable (True or False)

    • Value of a property

    • Expression (NumVal < 15)


Indeterminate structure 1

Move to statements after loop

Body of Loop

Is conditionmet?

Indeterminate Structure 1

Test at End of Loop

YES

NO


Do loop test at end example
Do Loop – Test at End Example

Dim password

Do

password= inputBox$(“Password please?”)

Loop Until password = “Vanilla orange”


Indeterminate structure 2

Move to statements after loop

Body of Loop

Is conditionmet?

Indeterminate Structure 2

Test at Beginning of Loop

YES

NO


Do loop test at beginning example
Do Loop –Test at beginning example

Dim entry

Dim NameCount As Integer

Private Sub Command1_Click()

NameCount = 0

entry = InputBox("Enter name")

Do Until entry = "zzz"

NameCount = NameCount + 1

entry = InputBox("Enter name")

Loop

Print "The total number of names is "; NameCount

End Sub


Do while loop

Equivalent

Equivalent

Do While Loop

Do

Loop Until variablename <> “”

Do

Loop While variablename = “”

Do

Loop Until variablename > 5

Do

Loop While variablename <= 5


Do while loop statement

Condition

Condition being

affected

Do...While/Loop Statement

  • Example

    X = 10

    Do While X > 0

    Ht = (60 + 2.13) * X^4

    Print Ht ‘Ht=Height of Rocket

    X = X - 1

    Loop


Problems with loops
Problems with Loops

  • What’s the problem with this loop?

    Dim i As Integer

    Do While i = 0

    i = 0

    Loop


Problems with loops1
Problems with Loops

  • What’s the problem with this loop?

    Private Sub CmdDisplay_Click()

    Dim x As Single

    x = 1

    Do While x > 0

    x = x + 1

    PicOutput.Print x

    Loop

    End Sub


Problems with loops2
Problems with Loops

  • What’s the problem with this loop?

    Private Sub CmdDisplay_Click()

    Dim m As Single

    For m = 1 To 20.5 Step – 1

    PicOutput.Print m

    Next m

    End Sub


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