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Introduction to Conditional Statements

If a Condition is True

Introduction

A conditional statement is an expression that produces a True or False result. To create the expression, you use the Boolean operators we studied in the previous lesson.

Creating a Boolean Condition

To check whether an expression is true or not in order to use its Boolean result, you can create an If...Then statement. The formula to follow is:

If condition Then statement

The condition can be the type of Boolean operation we studied in the previous lesson. That is, it can have the following formula:

operand1 Boolean-operator operand2

If the condition produces a True result, then the statement executes. This can be illustrated as follows:

If Condition

If the conditional statement is short, you can write the condition and the statement on the same line. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim pricePerCCF As Double
    Dim monthlyCharges As Double
    Dim consumption As Double

    pricePerCCF = 50.0
    monthlyCharges = 0.0
    consumption = CDbl(txtConsumption.Text)

    If consumption >= 0.50 Then pricePerCCF = 35.00

    txtPricePerCCF.Text = pricePerCCF

    pricePerCCF = CDbl(txtPricePerCCF.Text)

    monthlyCharges = consumption * pricePerCCF

    txtMonthlyCharges.Text = monthlyCharges
End Sub
</script>
<title>Gas Utility Company</title>
</head>
<body>
<div align="center">

<form id="frmBillPreparation" runat="server">
<h3>Gas Utility Company</h3>

<table border=0>
  <tr>
    <td>Consumption:</td>
    <td><asp:TextBox id="txtConsumption"
                     runat="server" /></td>
  </tr>
  <tr>
    <td>&nbsp</td>
    <td style="text-align: center">
        <asp:Button id="btnCalculate"
                    text="Calculate"
                    OnClick="btnCalculateClick"
                    runat="server"></asp:Button></td>
  </tr>
  <tr>
    <td>Price Per CCF:</td>
    <td><asp:TextBox id="txtPricePerCCF" runat="server" /></td>
  </tr>
    <tr>
    <td>Monthly Charges:</td>
    <td><asp:TextBox id="txtMonthlyCharges" runat="server" /></td>
  </tr>
</table>
</form>
</div>
</body>
</html>

Here is an example of using the webpage:

Creating a Boolean Condition Creating a Boolean Condition
Creating a Boolean Condition Creating a Boolean Condition

If the conditional statement is long, you can write the If...Then expression one line and the statement on the next line but then you must indicate the end of the conditional statement with End If. It is also a good idea (to make your code easy to read) to indent the statement. The formula to follow is:

If condition Then
    statement
End If

Here is an example:

<script runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim pricePerCCF As Double
    Dim monthlyCharges As Double
    Dim consumption As Double

    pricePerCCF = 50.0
    monthlyCharges = 0.0
    consumption = CDbl(txtConsumption.Text)

    If consumption >= 0.50 Then
        pricePerCCF = 35.00
    End If

    txtPricePerCCF.Text = pricePerCCF

    pricePerCCF = CDbl(txtPricePerCCF.Text)

    monthlyCharges = consumption * pricePerCCF

    txtMonthlyCharges.Text = monthlyCharges
End Sub
</script>

You can also write the statement on its own line even if the statement is short enough to fit on the same line with the If...Then expression. If the statement contains, or must use, more that one line of code, you must create the statements in the line next to the If...Then expression. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script language="VB" runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim weeklySalary As Double
    Dim hourlySalary As Double
    Dim timeWorked As Double
    Dim overtime As Double
    Dim overtimePay As Double

    weeklySalary = 0.0
    hourlySalary = CDbl(txtHourlySalary.Text)
    timeWorked = CDbl(txtTimeWorked.Text)

    weeklySalary = hourlySalary * timeWorked

    If timeWorked > 40.0 Then
        overtime = timeWorked - 40.0
        overtimePay = hourlySalary * 1.5 * overtime

        weeklySalary = (hourlySalary * 40.0) + overtimePay
    End If

    txtWeeklySalary.Text = weeklySalary
End Sub
</script>
<title>Payroll Preparation</title>
</head>
<body>
<div align="center">

<form id="frmPayroll" runat="server">
<h3>Payroll Preparation</h3>

<table border=0>
  <tr>
    <td style="text-align: left">
        <asp:Label ID="lblHourlySalary" runat="server" Text="Hourly Salary:"></asp:Label>
      </td>
    <td style="text-align: left">
        <asp:TextBox id="txtHourlySalary" style="Width:65px"
                                text="0.00" runat="server" /></td>
  </tr>
  <tr>
    <td style="text-align: left">
        <asp:Label ID="lblTimeWorked" runat="server" Text="Time Worked:"></asp:Label></td>
    <td style="text-align: left">
        <asp:TextBox id="txtTimeWorked" text="0.00" runat="server" style="Width:65px" />
        <asp:Button id="btnCalculate"
        text = "Calculate"
        OnClick = "btnCalculateClick"
                              runat="server"></asp:Button></td>
  </tr>
    <tr>
    <td style="text-align: left">
        <asp:Label ID="lblWeeklySalary" runat="server" Text="Weekly Salary:"></asp:Label></td>
    <td style="text-align: left">
        <asp:TextBox id="txtWeeklySalary" text="0.00" style="Width:65px" runat="server" /></td>
  </tr>
</table>
</form>
</div>
</body>
</html>

If a Condition is False

Something Else

To address an alternative to an If...Then expression, you can use the Else condition. The formula to follow is:

If condition Then)
    statement1
Else
    statement2
End If

If the condition produces a True result, then statement1 would execute. If the condition is false, statement2 would execute. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head runat="server">
<script runat="server">
    Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side As Double
    Dim perimeter As Double
    Dim area As Double

        If txtSide.Text = "" Then
            side = 0.00
        Else
            side = txtSide.Text
        End If

        perimeter = side * 4
        area = side * side

        txtPerimeter.Text = perimeter
        txtArea.Text = area
    End Sub
</script>
<title>Geometry: Square</title>
</head>
<body>
<div align="center">
<h2>Geometry: Square</h2>

<form id="frmSquare" method="get" runat="server">
  <table>
    <tr>
      <td><asp:Label id="lblSide" Text="Side:" runat="server"></asp:Label></td>
      <td>
	<asp:TextBox Runat="server" id="txtSide"></asp:TextBox>
	<asp:Button Runat="server" id="btnCalculate"
                    Text="Calculate"
        OnClick="btnCalculateClick"></asp:Button></td>
    </tr>
    <tr>
      <td><asp:Label id="lblPerimeter" Text="Perimeter:"
                     runat="server"></asp:Label></td>
      <td><asp:TextBox Runat="server"
                       id="txtPerimeter"></asp:TextBox></td>
    </tr>
    <tr>
      <td><asp:Label id="lblArea" Text="Area:" runat="server"></asp:Label></td>
      <td><asp:TextBox Runat="server"
                       id="txtArea"></asp:TextBox></td>
    </tr>
  </table>
</form>
</div>
</body>
</html>

Here is an example of running the program:

If a Condition is False If a Condition is False
If a Condition is False If a Condition is False

Like the If statement, the Else statement can cover more than one line of code, in which case the lines should be indented. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script language="VB" runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim weeklySalary As Double
    Dim hourlySalary As Double
    Dim timeWorked As Double
    Dim overtime As Double
    Dim overtimePay As Double

    weeklySalary = 0.0
    hourlySalary = CDbl(txtHourlySalary.Text)
    timeWorked = CDbl(txtTimeWorked.Text)

    If timeWorked <= 40.0 Then
        weeklySalary = hourlySalary * timeWorked
    Else
        overtime = timeWorked - 40.0
        overtimePay = hourlySalary * 1.50 * overtime

        weeklySalary = (hourlySalary * 40.0) + overtimePay
    End If

    txtWeeklySalary.Text = FormatNumber(weeklySalary)
End Sub
</script>
<title>Payroll Preparation</title>
</head>
<body>
<div align="center">

<form id="frmPayroll" runat="server">
<h3>Payroll Preparation</h3>

<table border=0>
  <tr>
    <td style="text-align: left">
        <asp:Label ID="lblHourlySalary" runat="server" Text="Hourly Salary:"></asp:Label>
      </td>
    <td style="text-align: left">
        <asp:TextBox id="txtHourlySalary" style="Width:65px"
                                text="0.00" runat="server" /></td>
  </tr>
  <tr>
    <td style="text-align: left">
        <asp:Label ID="lblTimeWorked" runat="server" Text="Time Worked:"></asp:Label></td>
    <td style="text-align: left">
        <asp:TextBox id="txtTimeWorked" text="0.00" runat="server" style="Width:65px" />
        <asp:Button id="btnCalculate"
                    text = "Calculate"
                    OnClick = "btnCalculateClick"
                    runat="server"></asp:Button></td>
  </tr>
    <tr>
    <td style="text-align: left">
        <asp:Label ID="lblWeeklySalary" runat="server" Text="Weekly Salary:"></asp:Label></td>
    <td style="text-align: left">
        <asp:TextBox id="txtWeeklySalary" text="0.00" style="Width:65px" runat="server" /></td>
  </tr>
</table>
</form>
</div>
</body>
</html>

Here is an example of using the webpage:

Creating a Boolean Condition Creating a Boolean Condition

Here is another example of using the webpage:

Creating a Boolean Condition Creating a Boolean Condition

The If Operator

To provide a simple way to create a conditional statement, the Visual Basic language provides the If operator. The formula to use it is:

If(boolean-condition, true-result, false-result)

The.boolean-condition is first evaluated. If it produces a true result, the second argument is produced. If not, the third argument is produced. Here is an example:

<script runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim side As Double
    Dim perimeter As Double
    Dim area As Double

    side = If(txtSide.Text = "", 0.00, txtSide.Text)

    perimeter = side * 4
    area = side * side

    txtPerimeter.Text = perimeter
    txtArea.Text = area
End Sub
</script>

What Else If a Condition is True?

An If...Then...Else conditional statement can process only two statements. If you need to process more conditions, you can add an ElseIf condition. The formula to follow is:

If Condition1 Then
    statement1
ElseIf condition2 Then
    statement2
End If

The first condition, condition1, would be checked. If condition1 is true, then statement1 would execute. If condition1 is false, then condition2 would be checked. If condition2 is true, then statement2 would execute. Any other result would be ignored.

If you need to process more than two conditions, you can add more ElseIf sections. The formula to follow would be:

If condition1 Then
    statement1
ElseIf condition2 Then
    statement2
ElseIf condition3 Then
    Statement3
. . .
ElseIf condition-n Then
    Statement-n
End If

Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script runat="server">
    Sub btnFindClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim chemicalSymbol As String
    Dim atomicNumber As String
    Dim elementName As String
    Dim atomicMass As Single

        If txtChemicalSymbol.Text = "" Then
            chemicalSymbol = "H"
        Else
            chemicalSymbol = txtChemicalSymbol.Text
        End If

        If chemicalSymbol = "H" Then
            atomicNumber = 1
            elementName = "Hydrogen"
            atomicMass = 1.0079
        ElseIf chemicalSymbol = "He" Then
            atomicNumber = 2
            elementName = "Helium"
            atomicMass = 4.002682
        ElseIf chemicalSymbol = "Li" Then
            atomicNumber = 3
            elementName = "Lithium"
            atomicMass = 6.941
        ElseIf chemicalSymbol = "Be" Then
            atomicNumber = 4
            elementName = "Berylium"
            atomicMass = 9.0122
        ElseIf chemicalSymbol = "B" Then
            atomicNumber = 5
            elementName = "Boron"
            atomicMass = 10.811
        End If

        lblAtomicNumber.Text = atomicNumber
        lblElementName.Text = elementName
        lblAtomicMass.Text = CStr(atomicMass)
    End Sub
</script>
<title>Chemistry: Compound Properties</title>
</head>
<body>
<div align="center">

<form id="frmChemistry" runat="server">
<h3>Chemistry: Compound Properties</h3>

<table border=0>
  <tr>
    <td>Chemical Symbol:</td>
    <td><asp:TextBox id="txtChemicalSymbol" style="width: 40px" runat="server" />
        <asp:Button id="btnFind" text="Find"
                    OnClick="btnFindClick"
                    runat="server"></asp:Button></td>
  </tr>
  <tr>
    <td>Atomic Number</td>
    <td><asp:Label id="lblAtomicNumber" runat="server"></asp:Label></td>
  </tr>
    <tr>
    <td>Element Name:</td>
    <td><asp:Label id="lblElementName" runat="server" /></td>
  </tr>
  <tr>
    <td>Atomic Mass:</td>
    <td><asp:Label id="lblAtomicMass" runat="server" /></td>
  </tr>
</table>
</form>
</div>
</body>
</html>

Here is an example of using the webpage:

What ElseIf a Condition is True? What ElseIf a Condition is True?
   
What ElseIf a Condition is True? What ElseIf a Condition is True?

Whatever Else?

If none of the conditions matches, you can add a last Else condition. The formulas to follow are:

If condition1 Then
    statement1
ElseIf condition2 Then
    statement2
Else
    else-statement
End If

or

If condition1 Then
    statement1
ElseIf condition2 Then
    statement2
ElseIf condition3 Then
    Statement3
. . .
ElseIf condition-n Then
    Statement-n
Else
    else-statement
End If

Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script runat="server">
Sub btnFindClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim chemicalSymbol As String
    Dim atomicNumber As String
    Dim elementName As String
    Dim atomicMass As Single

        If txtChemicalSymbol.Text <> "" Then
            chemicalSymbol = txtChemicalSymbol.Text
        End If

        If chemicalSymbol = "H" Then
            atomicNumber = 1
            elementName = "Hydrogen"
            atomicMass = 1.0079
        ElseIf chemicalSymbol = "He" Then
            atomicNumber = 2
            elementName = "Helium"
            atomicMass = 4.002682
        ElseIf chemicalSymbol = "Li" Then
            atomicNumber = 3
            elementName = "Lithium"
            atomicMass = 6.941
        ElseIf chemicalSymbol = "Be" Then
            atomicNumber = 4
            elementName = "Berylium"
            atomicMass = 9.0122
        ElseIf chemicalSymbol = "B" Then
            atomicNumber = 5
            elementName = "Boron"
            atomicMass = 10.811
        Else
            atomicNumber = 0
            elementName = "Unknown"
            atomicMass = 0.0000
        End If

        lblAtomicNumber.Text = atomicNumber
        lblElementName.Text = elementName
        lblAtomicMass.Text = CStr(atomicMass)
End Sub
</script>
<title>Chemistry: Compound Properties</title>
</head>
<body>
<div align="center">

<form id="frmChemistry" runat="server">
<h3>Chemistry: Compound Properties</h3>

<table border=0>
  <tr>
    <td>Chemical Symbol:</td>
    <td><asp:TextBox id="txtChemicalSymbol" style="width: 40px" runat="server" />
        <asp:Button id="btnFind" text="Find"
                    OnClick="btnFindClick"
                    runat="server"></asp:Button></td>
  </tr>
  <tr>
    <td>Atomic Number</td>
    <td><asp:Label id="lblAtomicNumber" runat="server"></asp:Label></td>
  </tr>
    <tr>
    <td>Element Name:</td>
    <td><asp:Label id="lblElementName" runat="server" /></td>
  </tr>
  <tr>
    <td>Atomic Mass:</td>
    <td><asp:Label id="lblAtomicMass" runat="server" /></td>
  </tr>
</table>
</form>
</div>
</body>
</html>

Here is an example of using the webpage:

What ElseIf a Condition is True? What ElseIf a Condition is True?
   
What Else If a Condition is True? What Else If a Condition is True?
 
 
 

Accessories for Conditional Statements

Something Is

The Visual Basic language provides a comparison operator used to compared two objects of classes. The operator is named Is. The formula to use it is:

comparison-result = object1 Is object2

The Is operator compares the references of two objects, not the values of the objects. If the references point to the same object, the comparison produces a True result. Otherwise, the comparison is False.

Something Is Not

To let you find out if two objects are not the same, the Visual Basic language provides the IsNot comparison operator. The formula to use it is:

comparison-result = object1 IsNot object2

The IsNot operator compares the references of two objects. If the objects are the same, the comparison produces True result; otherwise, it produces False.

If Something Is Nothing

Remember that when an object is created, if the New keyword is not used to allocate memory for it, the object is said to be Nothing. Also remember that, if an object was already difined, if for some or any reason you want to undefine it, you can assign Nothing to it. Here is an example we saw in Lesson 07:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script Language="VB" runat="server">
Class Employee

End Class
</script>

<title>Department Store - Employee Record</title>
</head>
<body>
<h4>Department Store - Employee Record</h4>

<%
Dim staff As New Employee

staff = Nothing
%>

</body>
</html>

At any time, to find out whether an object is currently defined or it is not holding a clear value, use the Is operator to compare it to the Nothing object. Here are examples:

<%@ Page Language="VB" %>

<!DOCTYPE html>

<html>
<head>
<script runat="server">
Class Employee

End Class
</script>

<title>Department Store - Employee Record</title>
</head>
<body>
<h4>Department Store - Employee Record</h4>

<%
    Dim staff As New Employee

    Response.Write("Dim staff As New Employee")

    If staff Is Nothing Then
        Response.Write("<br>The staff member is currently not defined.")
    Else
        Response.Write("<br>The staff member has a defined object.")
    End If

    staff = Nothing

    Response.Write("<br><br>staff = Nothing")

    If staff Is Nothing Then
        Response.Write("<br>The staff member is currently not defined.")
    Else
        Response.Write("<br>The staff member has a defined object.")
    End If
%>

</body>
</html>

This would produce:

If Something Is Nothing

The If Operator for Nothing

To provide a simple way to find out if a value or an object is Nothing, the If operator provides the following formula:

If(object/value is nothing, object/value-result)

If the first argument is nothing, the second argument is produced.

Going To a Statement

In the flow of your code, you can jump from one statement or line to another. This is made possible with the GoTo keyword. Before using it, first create or insert a name on a particular section of your procedure or function. The name, also called a label, is made of one word and it can be anything. That name is followed by a colon ":". Here is an example:

<script language="VB" runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)

    ' Do some thing(s) here

RegularTime:
    ' Do some other thing(s) here
End Sub

After creating the label, if a condition calls to jump to the label, type GoTo followed by the label. In the same way, you can create as many labels as you want. The name of each label must be unique in the same procedure. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script language="VB" runat="server">
Sub btnCalculateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim weeklySalary As Double
    Dim hourlySalary As Double
    Dim timeWorked As Double
    Dim overtime As Double
    Dim overtimePay As Double

    weeklySalary = 0.0
    hourlySalary = CDbl(txtHourlySalary.Text)
    timeWorked = CDbl(txtTimeWorked.Text)

    If timeWorked > 40.0 Then
        GoTo GetOvertimeApproval
    Else
        GoTo RegularTime
    End If

RegularTime:
    weeklySalary = hourlySalary * timeWorked
    GoTo DisplayWeeklySalary

GetOvertimeApproval:
    overtime = timeWorked - 40.0
    overtimePay = hourlySalary * 1.50 * overtime

    weeklySalary = (hourlySalary * 40.0) + overtimePay
    lblApproval.Text = "Make sure you have received approval for paid overtime."
    GoTo DisplayWeeklySalary

DisplayWeeklySalary:
    txtWeeklySalary.Text = FormatNumber(weeklySalary)
End Sub
</script>
<title>Payroll Preparation</title>
</head>
<body>
<div align="center">

<form id="frmPayroll" runat="server">
<h3>Payroll Preparation</h3>

<table border=0>
  <tr>
    <td style="text-align: left">
        <asp:Label id="lblHourlySalary" runat="server" Text="Hourly Salary:"></asp:Label>
      </td>
    <td style="text-align: left">
        <asp:TextBox id="txtHourlySalary" style="Width:65px"
                                text="0.00" runat="server" /></td>
  </tr>
  <tr>
    <td style="text-align: left">
        <asp:Label id="lblTimeWorked" runat="server" Text="Time Worked:"></asp:Label></td>
    <td style="text-align: left">
        <asp:TextBox id="txtTimeWorked" text="0.00" runat="server" style="Width:65px" />
        <asp:Button id="btnCalculate"
        text = "Calculate"
        OnClick = "btnCalculateClick"
                              runat="server"></asp:Button></td>
  </tr>
    <tr>
    <td style="text-align: left">
        <asp:Label id="lblWeeklySalary" runat="server" Text="Weekly Salary:"></asp:Label></td>
    <td style="text-align: left">
        <asp:TextBox id="txtWeeklySalary" text="0.00" style="Width:65px" runat="server" /></td>
  </tr>
</table>

<asp:Label ID="lblApproval" runat="server" Text=""></asp:Label>
</form>
</div>
</body>
</html>

Here is an example of using the webpage:

Going To a Statement

Going To a Statement

Going to a Statement

Negating a Conditional Statement

As you should be aware by now, Boolean algebra stands by two values, True and False, that are opposite each other. In fact, Visual Basic provides an operator that can be used to reverse a Boolean value or to negate a Boolean expression. This is the role of the Not operator. The formula to use it is:

Not Expression

To use the operator, type Not followed by a logical expression. The expression can be a simple Boolean value. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script language="VB" runat="server">

</script>
<title>Payroll Preparation</title>
</head>
<body>
<%
Dim IsMarried As Boolean

Response.Write("Employee is married: " & IsMarried)
Response.Write("<br>Employee is married: " & Not IsMarried) 
%>
</body>
</html>

This would produce:

Not

In this case, the Not operator is used to change the logical value of the variable. When a Boolean variable has been "notted", its logical value has changed. If the logical value was True, it would be changed to False and vice versa. Therefore, you can inverse the logical value of a Boolean variable by "notting" or not "notting" it. Here is an example:

<script runat="server">
Sub btnFindClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim chemicalSymbol As String

    If Not txtChemicalSymbol.Text = "" Then
        chemicalSymbol = txtChemicalSymbol.Text
    End If
End Sub
</script>

To make the code easier to read, it is a good idea to pu the negative expression in parentheses

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script runat="server">
Sub btnFindClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim chemicalSymbol As String
    Dim atomicNumber As String
    Dim elementName As String
    Dim atomicMass As Single

    If Not (txtChemicalSymbol.Text = "") Then
        chemicalSymbol = txtChemicalSymbol.Text
    End If

    If chemicalSymbol = "H" Then
        atomicNumber = 1
        elementName = "Hydrogen"
        atomicMass = 1.0079
    ElseIf chemicalSymbol = "He" Then
        atomicNumber = 2
        elementName = "Helium"
        atomicMass = 4.002682
    ElseIf chemicalSymbol = "Li" Then
        atomicNumber = 3
        elementName = "Lithium"
        atomicMass = 6.941
    ElseIf chemicalSymbol = "Be" Then
        atomicNumber = 4
        elementName = "Berylium"
        atomicMass = 9.0122
    ElseIf chemicalSymbol = "B" Then
        atomicNumber = 5
        elementName = "Boron"
        atomicMass = 10.811
    Else
        atomicNumber = 0
        elementName = "Unknown"
        atomicMass = 0.0000
    End If

    lblAtomicNumber.Text = atomicNumber
    lblElementName.Text = elementName
    lblAtomicMass.Text = CStr(atomicMass)
End Sub
</script>
<title>Chemistry: Compound Properties</title>
</head>
<body>
<div align="center">

<form id="frmChemistry" runat="server">
<h3>Chemistry: Compound Properties</h3>

<table border=0>
  <tr>
    <td>Chemical Symbol:</td>
    <td><asp:TextBox id="txtChemicalSymbol" style="width: 40px" runat="server" />
        <asp:Button id="btnFind" text="Find"
                    OnClick="btnFindClick"
                    runat="server"></asp:Button></td>
  </tr>
  <tr>
    <td>Atomic Number</td>
    <td><asp:Label id="lblAtomicNumber" runat="server"></asp:Label></td>
  </tr>
    <tr>
    <td>Element Name:</td>
    <td><asp:Label id="lblElementName" runat="server" /></td>
  </tr>
  <tr>
    <td>Atomic Mass:</td>
    <td><asp:Label id="lblAtomicMass" runat="server" /></td>
  </tr>
</table>
</form>
</div>
</body>
</html>

Here is an example of using the webpage:

What ElseIf a Condition is True? What ElseIf a Condition is True?
   
What Else If a Condition is True? What Else If a Condition is True?

Conditional Statements and Functions

Introduction

When performing its assignment, a function can encounter different situations, some of which would need to be checked for a true value, a false value or a negation. Therefore, a conditional statement can assist a procedure or function to decide what action to take or what value to produce.

Conditional Returns

A function can return only one value but you can make it produce a result that depends on some condition. Here are two examples:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script language="VB" runat="server">
Function GetDiscountRate(ByVal price As Double, ByVal days As Integer) As Integer
    Dim discountRate As Double

    If days > 70 Then
        discountRate = 70
    ElseIf days > 50 Then
        discountRate = 50
    ElseIf days > 30 Then
        discountRate = 35
    ElseIf days > 15 Then
        discountRate = 15
    End If

    Return discountRate
End Function

Function CalculateMarketPrice(ByVal price As Double, ByVal Optional rate As Integer = 0.00) As Integer
    Dim discountAmt As Double
    Dim markedPrice As Double

    If rate = 0.00 Then
        markedPrice = 0.00
    Else
        markedPrice = price * rate / 100.0
    End If

    Return markedPrice
End Function

Sub btnCreateRecordClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim unitPrice As Double
    Dim markedPrice As Double
    Dim discountRate As Double
    Dim daysInStore As Integer
    Dim discountedAmount As Double

    markedPrice = 0.00

    daysInStore = CInt(txtDaysInStore.Text)
    unitPrice = CDbl(txtUnitPrice.Text)
    discountRate = GetDiscountRate(unitPrice, daysInStore)

    If discountRate = 0.00 Then
        discountedAmount = CalculateMarketPrice(unitPrice)
    Else
        discountedAmount = CalculateMarketPrice(unitPrice, discountRate)
    End If

    markedPrice = unitPrice - discountedAmount

    lblItemNumber.Text = txtItemNumber.Text
    lblItemName.Text = txtItemName.Text
    LblDiscountAmount.Text = FormatCurrency(discountedAmount)
    lblMarkedPrice.Text = FormatCurrency(markedPrice)
End Sub
</script>
<title>Department Store - Inventory Creation</title>
</head>
<body>
<div align="center">

<form id="frmStoreItem" runat="server">
<h3>Department Store - Inventory Creation</h3>

<table>
  <tr>
    <td>Item #:</td>
    <td><asp:TextBox id="txtItemNumber" runat="server" /></td>
  </tr>
  <tr>
    <td>Item Name:</td>
    <td><asp:TextBox id="txtItemName" style="width: 250px;" runat="server" /></td>
  </tr>

  <tr>
    <td>Unit Price:</td>
    <td><asp:TextBox id="txtUnitPrice" runat="server" /></td>
  </tr>
    <tr>
    <td>Days in Store:</td>
    <td><asp:TextBox id="txtDaysInStore" runat="server" /></td>
  </tr>
    <tr>
    <td>&nbsp;</td>
    <td><asp:Button id="btnCreateSaleRecord"
                              text="Create Sale Record"
                              OnClick="btnCreateRecordClick"
                              runat="server"></asp:Button> </td>
  </tr>
</table>

<h3>Department Store - Store Inventory</h3>

<table>
  <tr>
    <td>Item #:</td>
    <td><asp:Label id="lblItemNumber" runat="server" /></td>
  </tr>
  <tr>
    <td>Item Name:</td>
    <td><asp:Label id="lblItemName" runat="server" /></td>
  </tr>
  <tr>
    <td>Discount Amouont:</td>
    <td><asp:Label id="LblDiscountAmount" runat="server" /></td>
  </tr>
  <tr>
    <td>Marked Price:</td>
    <td><asp:Label id="lblMarkedPrice" runat="server" /></td>
  </tr>
</table>

</form>
</div>
</body>
</html>

Here is an example of using the webpage:

Conditional Statements and Functions

Conditional Statements and Functions

Conditional Statements and Functions

In our examples, we first declared a variable that was going to hold the value to return. This is not always necessary. If you already have the value or the expression to return, you can precede it with the Return. You can repeat this for each section where the value or expression can/must be returned. Here are examples:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script language="VB" runat="server">
Function GetDiscountRate(ByVal price As Double, ByVal days As Integer) As Integer
        If days > 70 Then
            Return 70
        ElseIf days > 50 Then
            Return 50
        ElseIf days > 30 Then
            Return 35
        Else
            Return 15
        End If
End Function

Function CalculateMarketPrice(ByVal price As Double, ByVal Optional rate As Integer = 0.00) As Integer
    Dim discountAmt As Double
    Dim markedPrice As Double

        If rate = 0.00 Then
            Return 0.00
        Else
            Return price * rate / 100.0
        End If
End Function

Sub btnCreateRecordClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim unitPrice As Double
    Dim markedPrice As Double
    Dim discountRate As Double
    Dim daysInStore As Integer
    Dim discountedAmount As Double

        markedPrice = 0.00

        daysInStore = CInt(txtDaysInStore.Text)
        unitPrice = CDbl(txtUnitPrice.Text)
        discountRate = GetDiscountRate(unitPrice, daysInStore)

        If discountRate = 0.00 Then
            discountedAmount = CalculateMarketPrice(unitPrice)
        Else
            discountedAmount = CalculateMarketPrice(unitPrice, discountRate)
        End If

        markedPrice = unitPrice - discountedAmount

        lblItemNumber.Text = txtItemNumber.Text
        lblItemName.Text = txtItemName.Text
        LblDiscountAmount.Text = FormatCurrency(discountedAmount)
        lblMarkedPrice.Text = FormatCurrency(markedPrice)
End Sub
</script>
<title>Department Store - Inventory Creation</title>
</head>
<body>
<div align="center">

<form id="frmStoreItem" runat="server">
<h3>Department Store - Inventory Creation</h3>

<table>
  <tr>
    <td>Item #:</td>
    <td><asp:TextBox id="txtItemNumber" runat="server" /></td>
  </tr>
  <tr>
    <td>Item Name:</td>
    <td><asp:TextBox id="txtItemName" runat="server" /></td>
  </tr>

  <tr>
    <td>Unit Price:</td>
    <td><asp:TextBox id="txtUnitPrice" runat="server" /></td>
  </tr>
    <tr>
    <td>Days in Store:</td>
    <td><asp:TextBox id="txtDaysInStore" runat="server" /></td>
  </tr>
    <tr>
    <td>&nbsp;</td>
    <td><asp:Button id="btnCreateSaleRecord"
                    text="Create Sale Record"
                    OnClick="btnCreateRecordClick"
                    runat="server"></asp:Button> </td>
  </tr>
</table>

<h3>Department Store - Store Inventory</h3>

<table>
  <tr>
    <td>Item #:</td>
    <td><asp:Label id="lblItemNumber" runat="server" /></td>
  </tr>
  <tr>
    <td>Item Name:</td>
    <td><asp:Label id="lblItemName" style="width: 250px;" runat="server" /></td>
  </tr>
  <tr>
    <td>Discount Amouont:</td>
    <td><asp:Label id="LblDiscountAmount" runat="server" /></td>
  </tr>
  <tr>
    <td>Marked Price:</td>
    <td><asp:Label id="lblMarkedPrice" runat="server" /></td>
  </tr>
</table>

</form>
</div>
</body>
</html>

Here is an example of using the webpage

Conditional Statements and Functions

Conditional Statements and Functions

Conditional Statements and Functions

Conditional Statements and Functions

Introduction to Built-In Boolean Functions

Introduction

The Visual Basic language contains many functions that can assist you in dealing with conditional statements. Also many classes have built-in methods that produce a Boolean value. For example, the class of every object used in Visual Basic is equipped with a method named Equals. It can be used to find out if one object or value is equal to another.

If a Value is Numeric

Remember that any value that a visitor enters in a text box is primarily considered as text. Before using or converting a value that is supposed to be numeric, to let you checke whether it is a number, Visual Basic provides a function named IsNumeric. This function takes one argument as the value to check. If the argument is an integer or a floating-point number, the function returns True. If not, it returns False. Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script runat="server">
Sub btnEvaluateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim creditScore As Integer
    Dim hasGoodCredit As Boolean

    If IsNumeric(txtCreditScore.Text) Then
        creditScore = CInt(txtCreditScore.Text)
    Else
        creditScore = 500
        txtCreditScore.Text = "500"
    End If

    hasGoodCredit = (creditScore >= 680)

    lblMessage.Text = "The applicant has good credit: " & CStr(hasGoodCredit) & "."
End Sub
</script>
<title>Loan Evaluation</title>
</head>
<body>
<form id="frmGrade" runat="server">
<h3>Loan Evaluation</h3>

<p>Enter Customer Credit Score: 
   <asp:TextBox id="txtCreditScore" style="width: 60px"
                runat="server" />
   <asp:Button id="btnEvaluate"
               text = "Evaluate"
               runat="server" OnClick="btnEvaluateClick"></asp:Button></p>
<p><asp:Label id="lblMessage" runat="server" /></p>
</form>
</body>
</html>

Here is an example of using the webpage:

If a Value is Numeric

If a Value is Numeric

If a Value is Numeric

If an Object Is Nothing

When you declare a variable, especially when you create an object (declaring a variable from a class), you should initialize, but you can declare a variable without immediately initializing. This technique creates the risk of using a variable that has not yet been initializing, which can cause errors sometimes. To let you find out if a variable has been previously initialized or whether the variable currently holds a significant value, the Visual Basic library provides a function named IsNothing. This function takes one argument as the object or the expression to check. The IsNothing() function produces a True result if the argument holds a value. Otherwise, it returns False.

If a Variable is a Reference Type

As you probably know already, when you declare a variable from a class, the variable is stored in an area of computer referred to as the heap. Such a variable is referred to as a reference type. At any time, to let you check whether a variable is a reference type, Visual Basic provides a function named IsReference. This function takes one argument that is the object or an expression to check.

The Condition-Based Function

To assist you with checking a condition and its alternative, the Visual Basic language provides a function named IIf. This function operates like an If...Then...Else conditional statement. It takes three required arguments and returns a result of a type of your choice (actually an Object). It can be presented as follows:

Public Function IIf(ByVal Expression As Boolean,
                    ByVal TruePart As Object, 
                    ByVal FalsePart As Object) As Object

The condition to check, a Boolean expression, is passed as the first argument:

  • If that Expression is true, the function returns the value of the second argument, the TruePart. The third argument or last argument is ignored
  • If the Expression is false, the first argument is ignored and the function returns the value of the second argument

Here is an example:

<%@ Page Language="VB" %>

<!DOCTYPE html>
<html>
<head>
<script runat="server">
Sub btnEvaluateClick(ByVal sender As Object, ByVal e As EventArgs)
    Dim decision As String
    Dim creditScore As Integer
    Dim hasGoodCredit As Boolean

    creditScore = CInt(txtCreditScore.Text)
    hasGoodCredit = (creditScore >= 680)

    decision = IIf(hasGoodCredit = True, "is approved", "has been denied")

    lblMessage.Text = "Decision: The loan " & decision & "."
End Sub
</script>
<title>Loan Evaluation</title>
</head>
<body>
<form id="frmGrade" runat="server">
<h3>Loan Evaluation</h3>

<p>Enter Customer Credit Score: 
   <asp:TextBox id="txtCreditScore" style="width: 60px"
                runat="server" />
   <asp:Button id="btnEvaluate"
               text = "Evaluate"
               runat="server" OnClick="btnEvaluateClick"></asp:Button></p>
<p>
   <asp:Label id="lblMessage" runat="server" /></p>
</form>
</body>
</html>

Here is an example of using the webpage:

The Text of a Control

The Text of a Control

The Text of a Control

 
 
   
 

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