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ENGR 112

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

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  1. ENGR 112 Decision Structures

  2. Control Structures • Sequence 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. Is Condition Met? Statements in Else Clause Statements in Then Clause Selection Structure NO YES

  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” Else If A=2 Then value = “z” End If End If 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” Case is >= 80 YourGrade = “B” Case is >= 70 YourGrade = “C” Etc.

  16. Repetition Structures

  17. Determinant Loops

  18. 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

  19. 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

  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. 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

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

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

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

  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 entry = InputBox("Enter name") Loop Print "The total number of names is "; NameCount End Sub

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

  29. 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

  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

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