1. ENGR 112
Decision Structures

2. Control Structures Sequence structures Selection structures
Built into Visual Basic
Selection structures
If/Then  Single selection
If/Then/Else  Double selection
Select Case  Multiple selection

3. Control Structures
Repetition structures
For/Next
Do/While Loop

4. Selection Structures

5. Selection Structure Is Condition Met? NO YES Statements in Else Clause
Statements in Then Clause

6. 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

7. 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

8. 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

9. 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

10. 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

11. If/Then/Else Statement
Nested conditional statements
If A = 0 Then
value = “x”
Else
If A=1 Then
value = “y”
If A=2 Then
value = “z”
End If

12. 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

13. 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.

14. If/Then/Else Pseudocode Version
If Grade >= 90 Then
YourGrade = “A”
Else If Grade >= 80 Then
YourGrade = “B”
Else If Grade >= 70 Then
etc

15. Case Pseudocode Select Case Grade Case is >= 90 YourGrade = “A”
YourGrade = “B”
Case is >= 70
YourGrade = “C”
Etc.

16. Repetition Structures

17. Determinant Loops

18. 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
Optional
Increments counter

19. Determinate Structure
Set counter variable to starting value
Is counter value greater than ending value?
YES NO
Increment counter variable
Body of Loop
Move to statements after loop

20. 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

21. E E For/Next Statement Examples For X=1 To 10 Next X
Answ = 0
For X=1 To 10
Answ = Answ + X
Next X
For X=1 To 10 Step 2
If X > 10 Then Exit For E E

22. Indeterminate Loop

23. 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)

24. Indeterminate Structure 1
Test at End of Loop
Body of Loop
Is condition met?
YES
Move to statements after loop NO

25. Do Loop – Test at End Example
Dim password Do
password= inputBox$(“Password please?”)
Loop Until password = “Vanilla orange”

26. Indeterminate Structure 2
Test at Beginning of Loop
Is condition met?
YES
Move to statements after loop NO
Body of Loop

27. 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
Loop
Print "The total number of names is "; NameCount
End Sub

28. Do While Loop Do Loop Until variablename <> “”
Loop While variablename = “”
Loop Until variablename > 5
Loop While variablename <= 5
Equivalent
Equivalent

29. Do...While/Loop Statement
Example
X = 10
Do While X > 0
Ht = ( ) * X^4
Print Ht ‘Ht=Height of Rocket
X = X - 1
Loop
Condition
Condition being
affected

30. Problems with Loops What’s the problem with this loop?
Dim i As Integer
Do While i = 0
i = 0
Loop

31. 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

32. 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