Chapter 4: The Selection Process in Visual Basic

Introduction to Programming with Visual Basic 6.0 by McKeown and Piercy

1Copyright © 2001 by Wiley. All rights reserved.

Chapter 4: The Selection Process in Visual Basic

Selection Process

Two Alternative Structure

If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Chapter 4: The Selection Process in

Visual Basic




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Learning Objectives •Understand the importance of the selection process in

programming.•Describe the various types of decisions that can be made in a

computer program.•Discuss the statements used in Visual Basic to make decisions.•Understand the various comparison operators used in implementing

decisions in Visual Basic.•Use the If-Then-Else, If-Then-ElseIf, and Case decision structures.•Use the list box control to select from a list of alternatives.•Work with complex comparison structures and nested decisions to

handle more sophisticated selection processes.•Use the scroll bar to input integer values.•Use the Form_Load event to execute a procedure when a form is

loaded.•Work with the debugging toolbar to find program errors.




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

The Selection Process

• One of the key operations of a computer is to select between two or more alternatives to make a decision.

• Every decision involves a comparison between a variable and a constant, variable, or expression using logical operators.

• Decisions can involve two-alternatives or multiple alternatives.




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

The If-Then-Else Decision Structure

• For two alternative decisions, the If-Then-Else decision structure should be used

• In pseudocode, this is:

If condition is true then

implement true alternative

Else

implement false alternative

End Decision




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Multiple Alternatives

• For multiple alternatives, the general form in pseudocode is:

Select one:

Condition 1 is true; implement alternative 1

Condition 2 is true: implement alternative 2

Condition 3 is true; implement alternative 3

End Selection.




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

The Two Alternative Decision Structure

• The If-Then-Else statement is:

If condition is true Then

statements for true alternative

Else

statements for false alternative

End if

• The If-Then conditiontest expression1 comparison operator test expression2

where comparison operator is one of these six operators:

Equal to: = Not equal to: <>Less then: < Greater than: >Less than or equal to: <= Greater than or equal to: >=




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

The If-Then-Else Decision Structure (cont.)




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Example of If-Then-Else Decision to Compute Payroll

sngPayRate = CCur(txtPayRate.text)intHours = CSng(txtHours.text)If intHours >= 40 Then curPay = sngPayRate * 40 + 1.5 * _ sngPayRate * (intHours - 40)Else curPay = intHours * sngPayRate EndiftxtPay.Text = Format(curPay, _ ”currency”)




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

One-Alternative Decision• If there is only a true alternative, then this is a special case of

the two-alternative decision structure

If condition is true Then

true alternative is implemented

End If• One-alternative decision can be combined with InputBox

used to validate user input, e.g., to test that Customer Name textbox has something in it:

If txtCustName.Text = “” then

txtCustName.Text = InputBox(“Enter _

Name and try again”)

Exit Sub

End If• Where the Exit Sub statement exits the event procedure




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

If-Then-ElseIf Decision Structure

• One way to implement a multiple alter-native decision structure is through the If-Then-ElseIf decision structure:If condition1 true Then

first set of statements

ElseIf condition2 true Then

second set of statements

ElseIf condition3 true Then

third set of statements

Else

last set of statements

End If




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

If-Then-ElseIf Decision Structure(Assume condition3 is True)

If condition1 true thenfirst set of statements

else if condition2 true thensecond set of statements

else if condition3 true thenthird set of statements

elsefourth set of statements

end if




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Example of If-Then-ElseIf for Letter Grade Determination

Dim intAverage as IntegerDim strLetterGrade as StringintAverage = CInt(txtAverage.Text)If intAverage >= 90 then strLetterGrade = “A”ElseIf Average >= 80 then strLetterGrade = “B”ElseIf Average >= 70 then strLetterGrade = “C” ElseIf Average >= 60 then strLetterGrade = “D”Else strLetterGrade = “F”End if.txtLetter.Text = strLetterGrade




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Using the Select Case Decision Structure for Multiple Alternatives

• General form:Select Case expressionCase Condition1 is true

First set of statementsCase Condition2 is true

Second set of statementsCase Condition3 is true

Third set of statementsCase Else

Last set of statementsEnd Select




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Case Conditions

• Conditions for Case statement can be in 3 forms:

Test Condition Example

Value or expression Case 91, 92, 93

Range of values Case 90 to 100

Comparison condition Case Is > 89




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Example of Select Case to Determine LetterGrade

Dim intAverage as IntegerDim strLetterGrade as StringintAverage = CInt(txtAverage.Text)Select Case intAverage Case Is >= 90

strLetterGrade = “A” Case Is >= 80

strLetterGrade = “B” Case Is >= 70

strLetterGrade = “C” Case Is >= 60

strLetterGrade = “D” Case Else

strLetterGrade = “F”End Select




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Using the List Box

• The List box enables the user to select from a list of items.• lst is the prefix for name and the List property of the list box can be set at design time or run time.• The Text property of the list box is equal to the selected item.• We can test the Text property to determine which item was selected.




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

More Complex Decisions

• Decisions within decisions are know as nested decisions

• Interior decision must be an alternative of outer decision

• Decisions that combine two or more test conditions using logical operators are known as compound decisions

• And, Or, Not, and Xor are logical operators• Both conditions must be able to stand alone




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Example of Nested Decisions• Need to check if employee is hourly before

checking for overtime:

If strPayType = “Hourly” then

If intHours > 40 Then

curPay = 40 * sngPayRate + 1.5 + _

(intHours-40) * sngPayRate

Else

curPay = intHours * sngPayRate

End If

Else

curPay = 40 * sngPayRate

End If




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Example of Compound Decisions

• Using compound condition to test for average AND number of absences

If sngAverage >= 90 AND intAbsences < 3 then

strLetterGrade = “A”

ElseIf sngAverage >= 80 AND intAbsences < 5 then

strLetterGrade = “B”

etc.

• In this case, if a student has an average of 93 and 4 absences, he/she will receive a grade of “B” because he/she has more than 3 absences.




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Example of Using List BoxDim strVideoType as StringDim curPrice as CurrencystrVideoType = lstTypes.Text Select Case strVideoType Case “Kids”

curPrice = 0.99 Case “Regular” curPrice = 1.99 Case “Classic” curPrice = 2.99 End Select txtVideoType.Text = Format(curPrice, _ ”currency”)End Sub




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Using the Form_Load Event

• The Form_Load event occurs when the project is started and the form is loaded.

• Code from a command button can be cut (or copied) and then pasted into the form_load event




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Using the Debug ToolBar• To view the Debug Toolbar, Select

View|Toolbars and click the Debug checkbox

• The Debug Toolbar Run Stop Step Into Step Out Immediate Window Quick Watch

Break Toggle Step Over Locals Watch Window Call Stack Breakpoint Window




Selection Process


If..Then..ElseIf

Select Case

Complex Decision

Form_Load Event

Debugging

Debugging With the Immediate Window

• By clicking the Immediate Window icon on the debug toolbar, you can then print the current value of a textbox or variable

• Use the Print variable or textbox command to display its current value

• This can enable you to find errors in the code

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Chapter 4: The Selection Process in Visual Basic

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