ENGR 112 Decision Structures

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

Control Structures Repetition structures For/Next Do/While Loop

Selection Structures

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

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 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 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 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

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 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

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

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 Grade >= 90 Then YourGrade = “A” Else If Grade >= 80 Then YourGrade = “B” Else If Grade >= 70 Then etc

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

Repetition Structures

Determinant Loops

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

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

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

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

Indeterminate Loop

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 Test at End of Loop Body of Loop Is condition met? YES Move to statements after loop NO

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

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

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

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

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 Condition Condition being affected

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

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 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