If-Then-Else and Select Case structure

If-Then is VBA’s most important control structure. You’ll probably use this command on a daily basis.

As in many other aspects of life, effective decision-making is the key to success in writing CAD or any other macros.

The If-Then structure has this basic syntax:

If condition Then statements [Else elsestatements]

Use the If-Then structure when you want to execute one or more statements conditionally.

The optional Else clause, if included, lets you execute one or more statements if the condition you’re testing is not true.

Sound confusing? Don’t worry; a few examples make this crystal clear.

If-Then examples

The following routine demonstrates the If-Then structure without the optional Else clause:

Sub GoodMorning()
  If Time < 0.5 Then MsgBox Good Morning.
End Sub

The GoodMorning procedure uses VBA’s Time function to get the system time.

If the current system time is less than .5 (in other words, before noon), the routine displays a friendly greeting.

If Time is greater than or equal to .5, the routine ends and nothing happens.

To display a different greeting if Time is greater than or equal to .5, add another If-Then statement after the first one:

Sub GoodMorning2()
  If Time < 0.5 Then MsgBox Good Morning.
  If Time >= 0.5 Then MsgBox Good Afternoon.
End Sub

Notice that I used >= (greater than or equal to) for the second If-Then statement.

This ensures that the entire day is covered. Had I used > (greater than), then no message would appear if this procedure were executed at precisely 12:00 noon.

If-Then-Else examples

Another approach to the preceding problem uses the Else clause.

Here’s the same routine recoded to use the If-Then-Else structure:

Sub GoodMorning3()
  If Time < 0.5 Then MsgBox Good Morning. Else _
  MsgBox Good Afternoon.
End Sub

Notice that I use the line continuation character (underscore) in the preceding example.

The If-Then-Else statement is actually a single statement.

VBA provides a slightly different way of coding If-Then-Else constructs that use an End-If statement.

Therefore, the GoodMorning procedure can be rewritten as:

Sub GoodMorning4()
  If Time < 0.5 Then
    MsgBox Good Morning.
    MsgBox Good Afternoon.
  End If
End Sub

In fact, you can insert any number of statements under the If part, and any number of statements under the Else part.

I prefer to use this syntax because it’s easier to read and makes the statements shorter.

What if you need to expand the GoodMorning routine to handle three conditions: morning, afternoon, and evening?

You have two options: Use three If-Then statements or use a nested If-Then-Else structure.

Nesting means placing an If-Then-Else structure within another If-Then-Else structure.

The first approach, the three statements, is simplest:

Sub GoodMorning5()
  If Time < 0.5 Then Msg = Morning.
  If Time >= 0.5 And Time < 0.75 Then Msg = Afternoon.
  If Time >= 0.75 Then Msg = Evening.
  MsgBox Good & Msg
End Sub

The Msg variable gets a different text value, depending on the time of day.

The final MsgBox statement displays the greeting: Good Morning, Good Afternoon, or Good Evening.

The following routine performs the same action but uses an If-Then-End If structure:

Sub GoodMorning6()
  Dim Msg As String
  If Time < 0.5 Then
    Msg = “Morning.”
  If Time >= 0.5 And Time < 0.75 Then
    Msg = “Afternoon.”
  If Time >= 0.75 Then
    Msg = “Evening.”
  End If
  MsgBox “Good” & Msg
End Sub

If-ElseIf-Else examples

In the previous examples, every statement in the routine is executed — even in the morning.

A more efficient structure would exit the routine as soon as a condition is found to be true.

In the morning, for example, the procedure should display the Good Morning message and then exit — without evaluating the other superfluous conditions.

With a tiny routine like this, you don’t have to worry about execution speed.

But for larger applications in which speed is important, you should know about another syntax for the If-Then structure.

The ElseIf syntax follows:

If condition Then
[Else condition-n Then

Here’s how you can rewrite the GreetMe routine by using this syntax:

Sub GoodMorning7()
  Dim Msg As String
  If Time < 0.5 Then
    Msg = Morning.
  ElseIf Time >= 0.5 And Time < 0.75 Then
    Msg = Afternoon.
    Msg = Evening.
  End If
  MsgBox Good & Msg
End Sub

When a condition is true, VBA executes the conditional statements and the If structure ends.

In other words, VBA doesn’t waste time evaluating the extraneous conditions, which makes this procedure a bit more efficient than the previous examples.

The trade-off (there are always trade-offs) is that the code is more difficult to understand. (Of course, you already knew that.)

Select Case structure

The Select Case structure is useful for decisions involving three or more options (although it also works with two options, but using If-Then-Else structure is more efficient for that).

The syntax for the Select Case structure follows:

Select Case testexpression
[Case expressionlist-n 
[Case Else
End Select

Don’t be scared off by this official syntax. Using the Select Case structure is quite easy.

Select Case example

The following example shows how to use the Select Case structure.

This also shows another way to code the examples presented in the previous section:

Sub SelectPartLength()
  Dim PartNumber As Integer
  Dim PartLength As Integer
  PartNumber = InputBox(Please Enter part number:)
  Select Case PartNumber
    Case Part001
      PartLength = 1
    Case Part002
      PartLength = 2
    Case Part003
      PartLength = 3
  End Select
  MsgBox Part Length for this & PartNumber & is & PartLength
End Sub

In this example, the PartNumber variable is being evaluated.

The routine is checking for three different cases.

Next post will be about VBA Looping.