ASP.NET Conditions: Functional Conditions

	Functional Conditions

Alternatives to a Condition Being True/False?

The If...Then...ElseIf Condition

The If...Then...ElseIf statement acts like the If...Then...Else expression, except that it offers as many choices as necessary. The formula is:

If Condition1 Then
    Statement1
ElseIf Condition2 Then
    Statement2
ElseIf Conditionk Then
    Statementk
End If

The program will first examine Condition1. If Condition1 is true, the program will execute Statment1 and stop examining conditions. If Condition1 is false, the program will examine Condition2 and act accordingly. Whenever a condition is false, the program will continue examining the conditions until it finds one that is true. Once a true condition has been found and its statement executed, the program will terminate the conditional examination at End If. Here is an example:

<%@ Page Language="VB"  %>

<html>
<head>

<title>Exercise</title>

</head>
<body>

<%
        Dim MemberAge As Short

        MemberAge = 32

        If MemberAge <= 18 Then
            Response.Write("Membership: " & "Teen")
        ElseIf MemberAge < 55 Then
            Response.Write("Membership: " & "Adult")
        End If
%>

</body>
</html>

This would produce:

What If No Alternative is Valid?

There is still a possibility that none of the stated conditions be true. In this case, you should provide a "catch all" condition. This is done with a last Else section. The Else section must be the last in the list of conditions and would act if none of the primary conditions is true. The formula to use would be:

If Condition1 Then
    Statement1
ElseIf Condition2 Then
    Statement2
ElseIf Conditionk Then
    Statementk
Else
    CatchAllStatement
End If

Here is an example:

<%@ Page Language="VB"  %>

<html>
<head>

<title>Exercise</title>

</head>
<body>

<%
        Dim MemberAge As Short

        MemberAge = 65

        If MemberAge <= 18 Then
            Response.Write("Membership: " & "Teen")
        ElseIf MemberAge < 55 Then
            Response.Write("Membership: " & "Adult")
        Else
            Response.Write("Membership: " & "Senior")
        End If
%>

</body>
</html>

This would produce:

Conditional Statements and Functions

Introduction

As introduced in Lesson 5 and as seen in lessons thereafter, we know that a function is used to perform a specific assignment and produce a result. Here is an example:

<script language="vbscript" type="text/vbsscript" runat="server">

Private Function SetMembershipLevel$()
    Dim MemberAge%

    MemberAge% = 14

    Return ""
End Function

</script>

When performing its assignment, a function can encounter different situations, some of which would need to be checked for truthfulness or negation. This means that conditional statements can assist a procedure with its assignment.

Conditional Returns

A function is meant to return a value. Sometimes, it will perform some tasks whose results would lead to different results. A function can return only one value (we saw that, by passing arguments by reference, you can make a procedure return more than one value) but you can make it render a result depending on a particular behavior. If a function is requesting an answer from the user, since the user can provide different answers, you can treat each result differently. Consider the following function:

<html>
<head>

<script language="vbscript" type="text/vbsscript" runat="server">

Private Function SetMembershipLevel$()
        Dim MemberAge%

        MemberAge% = 38

        If MemberAge% < 18 Then
            Return "Teen"
        ElseIf MemberAge% < 55 Then
            Return "Adult"
        End If
End Function

</script>
<title>Exercise</title>

</head>
<body>

<%
        Dim Membership$

        Response.Write("Membership: " & Membership$)
%>

</body>
</html>

At first glance, this function looks fine. If given a number less than 18 (excluded), the function returns Teen. If given a number between 18 (included) and 55, the function returns the Adult.

What if there is an answer that does not fit those we are expecting? The values that we have returned in the function conform only to the conditional statements and not to the function. Remember that in If Condidion Statement, the Statement executes only if the Condition is true. Here is what will happen. If given a number higher than 55 (excluded), the function will not execute any of the returned statements. This means that the execution will reach the End Function line without encountering a return value. This also indicates to the compiler that you wrote a function that is supposed to return a value, but by the end of the method, it didn't return a value.

To solve this problem, you have various alternatives. If the function uses an If...Then condition, you can create an Else section that embraces any value other than those validated previously. Here is an example:

<html>
<head>

<script language="vbscript" type="text/vbsscript" runat="server">

Private Function SetMembershipLevel$()
        Dim MemberAge%

        MemberAge% = 62

        If MemberAge% < 18 Then
            Return "Teen"
        ElseIf MemberAge% < 55 Then
            Return "Adult"
        Else
            Return "Senior"
        End If

End Function

</script>
<title>Exercise</title>

</head>
<body>

<%
        Dim Membership$

        Membership$ = SetMembershipLevel$()
        Response.Write("Membership: " & Membership$)
%>

</body>
</html>

This time, the Else condition would execute if no value applies to the If or ElseIf conditions and the compiler would not produce a warning.

An alternative is to provide a last return value just before the End Function line. In this case, if the execution reaches the end of the function, it would still return something but you would know what it returns. This would be done as follows:

<script language="vbscript" type="text/vbsscript" runat="server">

Private Function SetMembershipLevel$()
    Dim MemberAge%

    MemberAge% = 42

    If MemberAge% < 18 Then
        Return "Teen"
    ElseIf MemberAge% < 55 Then
        Return "Adult"
    End If

    Return "Senior"
End Function

</script>

If the function uses an If condition, both implementations would produce the same result.

If-Condition Built-In Functions

Using the Immediate If Function

The IIf() function can also be used in place of an If...Then...ElseIf scenario. When the function is called, the Expression is checked. If the expression is true, the function returns the value of the TruePart argument and ignores the last argument. To use this function as an alternative to If...Then...ElseIf statement, if the expression is false, instead of immediately returning the value of the FalsePart argument, you can translate that part into a new IIf function. The pseudo-syntax would become:

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

In this case, if the expression is false, the function returns the TruePart and stops. If the expression is false, the compiler accesses the internal IIf function and applies the same scenario. Here is example:

<%@ Page Language="VB"  %>

<html>
<head>

<title>Exercise</title>

</head>
<body>

<%
        Dim MemberAge As Short
        Dim MembershipCategory As String

        MemberAge = 74

        MembershipCategory = _
	    IIf(MemberAge <= 18, "Teen", IIf(MemberAge < 55, "Adult", "Senior"))

        Response.Write("Membership: " & MembershipCategory)
%>

</body>
</html>

We saw that in an If...Then...ElseIf statement you can add as many ElseIf conditions as you want. In the same, you can call as many IIf functions in the subsequent FalsePart sections as you judge necessary:

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

Choose an Alternate Value

As we have seen so far, the Choose function takes a list of arguments. To use it as an alternative to the If...Then...ElseIf...ElseIf condition, you can pass as many values as you judge necessary for the second argument. The index of the first member of the second argument would be 1. The index of the second member of the second argument would be 2, and so on. When the function is called, it would first get the value of the first argument, then it would check the indexes of the available members of the second argument. The member whose index matches the first argument would be executed. Here is an example:

<%@ Page Language="VB"  %>

<html>
<head>

<title>Exercise</title>

</head>
<body>

<%
        Dim Status As UShort, EmploymentStatus As String

        Status = 3

        EmploymentStatus = Choose(Status, _
                                  "Full Time", _
                                  "Part Time", _
                                  "Contractor", _
                                  "Seasonal")

        Response.Write("Employment Status: " & EmploymentStatus)
%>

</body>
</html>

This would produce:

Choose

So far, we have used only strings for the values of the second argument of the Choose() function. In reality, the values of the second argument can be almost anything. One value can be a constant. Another value can be a string. Yet another value can come from calling a function. Here is an example:

<html>
<head>

<script language="vbscript" type="text/vbsscript" runat="server">

Private Function ShowContractors$()
        Return "=-= List of Contractors =-=" & "<br />" & _
               "Martin Samson" & "<br />" & _
               "Genevi�ve Lam" & "<br />" & _
               "Frank Viel" & "<br />" & _
               "Henry Rickson" & "<br />" & _
               "Samuel Lott"
End Function

</script>
<title>Exercise</title>

</head>
<body>

<%
        Dim Status As UShort, Result$

        Status = 3

        Result = Choose(Status, _
                        "Employment Status: Full Time", _
                        "Employment Status: Part Time", _
                        ShowContractors, _
                        "Seasonal Employment")
        Response.Write(Result)
%>

</body>
</html>

This would produce:

Choose

The values of the second argument can even be of different types.

Switching to an Alternate Value

The Switch() function is a prime alternative to the If...Then...ElseIf...ElseIf condition. The argument to this function is passed as a list of values. As seen previously, each value is passed as a combination of two values:

ConditionXToCheck, StatementX

As the function is accessed, the compiler checks each condition. If a condition X is true, its statement is executed. If a condition Y is false, the compiler skips it. You can provide as many of these combinations as you want. Here is an example:

<html>
<head>

<script language="vbscript" type="text/vbsscript" runat="server">

Private Enum EmploymentStatus
        FullTime
        PartTime
        Contractor
        Seasonal
End Enum

</script>
<title>Exercise</title>

</head>
<body>

<%
        Dim Status As EmploymentStatus
        Dim Result As String

        Status = EmploymentStatus.Contractor
        Result = "Unknown"

        Result = Microsoft.VisualBasic.Switch( _
                    Status = EmploymentStatus.FullTime, "Full Time", _
                    Status = EmploymentStatus.PartTime, "Part Time", _
                    Status = EmploymentStatus.Contractor, "Contractor", _
                    Status = EmploymentStatus.Seasonal, "Seasonal")

        Response.Write("Employment Status: " & Result)
%>

</body>
</html>

This would produce:

Switch

In a true If...Then...ElseIf...ElseIf condition, we saw that there is a possibility that none of the conditions would fit, in which case you can add a last Else statement. The Switch() function also supports this situation if you are using a number, a character, or a string. To provide this last alternative, instead of a ConditionXToCheck expressionk, enter True, and include the necessary statement. Here is an example:

<%@ Page Language="VB"  %>

<html>
<head>

<title>Exercise</title>

</head>
<body>

<%
        Dim Status As UShort
        Dim Result As String

        Status = 12

        Result = Microsoft.VisualBasic.Switch( _
                    Status = 1, "Full Time", _
                    Status = 2, "Part Time", _
                    Status = 3, "Contractor", _
                    Status = 4, "Seasonal", _
                    True, "Unknown")

        Response.Write("Employment Status: " & Result)
%>

</body>
</html>

This would produce:

Switch

Remember that you can also use True with a character. Here is an example:

<%@ Page Language="VB"  %>

<html>
<head>

<title>Exercise</title>

</head>
<body>

<%
        Dim Gender As Char
        Dim Result As String

        Gender = "H"

        Result = Microsoft.VisualBasic.Switch( _
                    Gender = "f", "Female", _
                    Gender = "F", "Female", _
                    Gender = "m", "Male", _
                    Gender = "M", "Male", _
                    True, "Unknown")

        Response.Write("Gender: " & Result)
%>

</body>
</html>

This would produce:

Switch