Selection Control Structures


Selection control structures are used when you want to make a decision based on a value.

The simplest form is the "If" statement.  An "If" statement test the truth of a condition.
If the condition is true, the code is executed.

	If x = 5 Then
		y = 10
	End If
	
The "End If" statement marks the end of the code to be executed.
You may have more than one line of code to execute when the statement is true.

Conditions must always evaluate to true or false.  A condition is a comparison between two variables, or a variable and a literal.  When comparing, use the conditional operators listed on VB245, Figure 4-7.

Special care must be taken when comparing Strings.  You don't know what the capitalization will be for a variable, it depends on how the user types it in.  You, the programmer, have control over the literal side of the comparison.  The comparison will always be case sensitive.

	If strName = "joe" Then
	
	strName could be "Joe", "jOe", "JOE", or any other variation
	
Use the UCase or LCase function to exercise control over both sides of the comparison
	
	If UCase(strName) = "JOE" Then
	
	If LCase(strName) = "joe" Then

The "If" executes code only when the condition is true.  To execute code when the condition
is false, use the "Else" clause:

	If x = 5 Then
		y = 10
	Else
		y = 2
	End If

You may have more than one line of code to execute when the statement is false.

Refer to page VB245, Figure 4-6, for the syntax of the If-Then-Else structure

"If" statements can get complicated in two ways:

	Compound conditions
	
		This occurs when you want to test more than one condition.
		It uses logical operators and familiarity with truth tables.
		
		And: All condition must be true
		Or : At least one condition must be true
		Not: Reverses the truth of the condition
		
		It's often helpful to enclose each condition in parentheses to make it easier to read
		
			If ( x = 5 ) And ( y = 2 ) Then
		
			If ( x = 5 ) Or ( y = 2 ) Then
		
			If ( x = 5 ) And Not ( y = 2 ) Then
		
		Avoid the following whenever possible:
		
			Use of the Not
			Mixing And with Or
	
	Nested If

		This occurs when you have an If inside of another If, or inside an Else:
		
			If x = 5 Then
				If y = 2 Then
					z = 7
				End If
			End If

		These are best avoided if at all possible.




"If" statements are good when you want to make a choice between two different paths.
But, what if you want to choose among more than two?

Use the "Select Case" statement.

An example of a "Select Case" statement that will assign a message based on a letter grade:

	Select Case strGrade
		Case "A"
			strMessage = "Excellent"
		Case "B"
			strMessage = "Good"
		Case "C"
			strMessage = "Fair"
		Case "D"
			strMessage = "Poor"
		Case "F"
			strMessage = "Failed"
		Case Else
			strMessage = "Unknown Value"
	End Select

The "End Select" statement marks the end of the "Select Case" structure.
	
Each path you want to take is a case based on the value of your variable.
The "Case Else" covers the case when the value does not match any of the other cases.

You may have more than one line of code to execute for each case.

Each Case above is an implied equality test.  You can use any of the other comparison operators.

Refer to page VB263, Figure 4-27 for the syntax of a "Select Case".


This is the "Select Case" I use in my database when computing letter grades:

    Select Case dblGrade

        Case Is >= 93
            strRetVal = "A"

        Case Is >= 90
            strRetVal = "A-"

        Case Is >= 87
            strRetVal = "B+"

        Case Is >= 83
            strRetVal = "B"

        Case Is >= 80
            strRetVal = "B-"

        Case Is >= 77
            strRetVal = "C+"

        Case Is >= 73
            strRetVal = "C"

        Case Is >= 70
            strRetVal = "C-"

        Case Is >= 67
            strRetVal = "D+"

        Case Is >= 63
            strRetVal = "D"

        Case Is >= 60
            strRetVal = "D-"

        Case Else
            strRetVal = "F"

    End Select


Classroom Exercises:

	On pages VB246-261, follow the directions in the beige boxes for Examples 1-5
	On pages VB264-270, follow the directions in the beige boxes for Examples 1-3