Functional Conditions
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Functional Conditions |
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: |
Module Exercise
Public Function Main() As Integer
Dim MemberAge As Short
MemberAge = 32
If MemberAge <= 18 Then
MsgBox("Membership: " & "Teen")
ElseIf MemberAge < 55 Then
MsgBox("Membership: " & "Adult")
End If
Return 0
End Function
End Module
This would produce:
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: Module Exercise
Public Function Main() As Integer
Dim MemberAge As Short
MemberAge = 65
If MemberAge <= 18 Then
MsgBox("Membership: " & "Teen")
ElseIf MemberAge < 55 Then
MsgBox("Membership: " & "Adult")
Else
MsgBox("Membership: " & "Senior")
End If
Return 0
End Function
End Module
This would produce:
A function can be made to return a Boolean value. In most cases, you may have to declare a local Boolean variable in the function, manipulate its value, and return it. Here is an example: Module Exercise
Private Function AreEqual(ByVal x As Integer, _
ByVal y As Integer) As Boolean
If x = y Then
Return True
Else
Return False
End If
End Function
Public Function Main() As Integer
Dim a As Integer = 26
Dim b As Integer = 26
MsgBox("a = b: " & AreEqual(a, b))
b = 28
MsgBox("a = b: " & AreEqual(a, b))
End Function
End Module
This would produce:
Instead of writing the If...Then...Else conditional statement as done above, you can write Return followed by the appropriate Boolean expression. Here is an example: Private Function AreEqual(ByVal x As Integer, _
ByVal y As Integer) As Boolean
Return x = y
End Function
This would have the same effect as previously.
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: Private Function SetMembershipLevel$()
Dim MemberAge%
MemberAge% = InputBox("Enter the Member's Age")
Return ""
End Function
When performing its assignment, a function can encounter different situations, some of which would need to be checked for truthfulness or negation. This means that conditional statements can assist a procedure with its assignment.
A function is meant to return a value. Sometimes, it will perform some tasks whose results would lead to different results. A function can return only one value (we saw that, by passing arguments by reference, you can make a procedure return more than one value) but you can make it render a result depending on a particular behavior. If a function is requesting an answer from the user, since the user can provide different answers, you can treat each result differently. Consider the following function: Module Exercise
Private Function SetMembershipLevel$()
Dim MemberAge%
MemberAge% = InputBox("Enter the Member's Age")
If MemberAge% < 18 Then
Return "Teen"
ElseIf MemberAge% < 55 Then
Return "Adult"
End If
End Function
Public Function Main() As Integer
Dim Membership$
MsgBox("Membership: " & Membership$)
Return 0
End Function
End Module
At first glance, this function looks fine. The user is asked to provide a number. If the user enters a number less than 18 (excluded), the function returns Teen. Here is an example of running the program:
If the user provides a number between 18 (included) and 55, the function returns the Adult. Here is another example of running the program:
What if there is an answer that does not fit those we are expecting? The values that we have returned in the function conform only to the conditional statements and not to the function. Remember that in If Condidion Statement, the Statement executes only if the Condition is true. Here is what will happen. If the user enters a number higher than 55 (excluded), the function will not execute any of the returned statements. This means that the execution will reach the End Function line without encountering a return value. This also indicates to the compiler that you wrote a function that is supposed to return a value, but by the end of the method, it didn't return a value. Here is another example of running the program:
The compiler would produce a warning: Warning 1 Function 'SetMembershipLevel' doesn't return a value on all code paths. To solve this problem, you have various alternatives. If the function uses an If...Then condition, you can create an Else section that embraces any value other than those validated previously. Here is an example: Module Exercise
Private Function SetMembershipLevel$()
Dim MemberAge%
MemberAge% = InputBox("Enter the Member's Age")
If MemberAge% < 18 Then
Return "Teen"
ElseIf MemberAge% < 55 Then
Return "Adult"
Else
Return "Senior"
End If
End Function
Public Function Main() As Integer
Dim Membership$
Membership$ = SetMembershipLevel$()
MsgBox("Membership: " & Membership$)
Return 0
End Function
End Module
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. Here is another example of running the program:
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: Private Function SetMembershipLevel$()
Dim MemberAge%
MemberAge% = InputBox("Enter the Member's Age")
If MemberAge% < 18 Then
Return "Teen"
ElseIf MemberAge% < 55 Then
Return "Adult"
End If
Return "Senior"
End Function
If the function uses an If condition, both implementations would produce the same result.
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The IIf() function can also be used in place of an If...Then...ElseIf scenario. When the function is called, the Expression is checked. As we saw already, 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 axample: Module Exercise
Public Function Main() As Integer
Dim MemberAge As Short
Dim MembershipCategory As String
MemberAge = 74
MembershipCategory = _
IIf(MemberAge <= 18, "Teen", IIf(MemberAge < 55, "Adult", "Senior"))
MsgBox("Membership: " & MembershipCategory)
Return 0
End Function
End Module
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
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: Module Exercise
Public Function Main() As Integer
Dim Status As UShort, EmploymentStatus As String
Status = 3
EmploymentStatus = Choose(Status, _
"Full Time", _
"Part Time", _
"Contractor", _
"Seasonal")
MsgBox("Employment Status: " & EmploymentStatus)
Return 0
End Function
End Module
This would produce:
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: Module Exercise
Private Function ShowContractors$()
Return "=-= List of Contractors =-=" & vbCrLf & _
"Martin Samson" & vbCrLf & _
"Geneviève Lam" & vbCrLf & _
"Frank Viel" & vbCrLf & _
"Henry Rickson" & vbCrLf & _
"Samuel Lott"
End Function
Public Function Main() As Integer
Dim Status As UShort, Result$
Status = 3
Result = Choose(Status, _
"Employment Status: Full Time", _
"Employment Status: Part Time", _
ShowContractors, _
"Seasonal Employment")
MsgBox(Result)
Return 0
End Function
End Module
This would produce:
The values of the second argument can even be of different types.
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: Module Exercise
Private Enum EmploymentStatus
FullTime
PartTime
Contractor
Seasonal
End Enum
Public Function Main() As Integer
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")
MsgBox("Employment Status: " & Result)
Return 0
End Function
End Module
This would produce:
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: Module Exercise
Public Function Main() As Integer
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")
MsgBox("Employment Status: " & Result)
Return 0
End Function
End Module
This would produce:
Remember that you can also use True with a character. Here is an example: Module Exercise
Public Function Main() As Integer
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")
MsgBox("Gender: " & Result)
Return 0
End Function
End Module
This would produce:
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Practical
Learning: Using If...Then...ElseIf
