The next conditional structure discussed in this lesson is the powerful Do…Loop structure. This section discusses each of the four variations of the Do…Loop that you can use.

Do While…Loop

The first loop structure you can use is the Do While…Loop structure. The basic syntax for this structure is

Do While condition
   ...code within the loop goes here
Loop

where the condition is either true or false. As long as the condition is true, the code within the loop gets executed. Once the condition becomes false, the loop stops and the code after the loop is executed. The only way for the program to break out of the loop is if the condition becomes false or if an Exit Do statement is encountered somewhere inside the loop. Consider the following example shown in Listing 6.7.

Listing 6.7. Using the Do While…Loop conditional structure.

Again = True
DoubleIt = 1

' Keep doubling the number as long as the user desires
Do While Again = True

       '  Show current results and prompt to see if we should continue

       If MsgBox("Current total is " & DoubleIt & ". Double it again ?",
             vbYesNo) = vbYes Then
          DoubleIt = DoubleIt * 2
       Else
          Again = False
       End If

Loop

In this example, the first line of code sets the variable that gets tested in the loop equal to true. The second line sets the variable that gets doubled in the code equal to one. Setting the loop variable equal to true allows the loop to get off and running because the third line says, "If Again is true, enter the loop." Because the variable has just been set to true, the loop begins. The first statement in the loop displays the result to the user. Because nothing has been doubled yet, the result is one-the initial value of the DoubleIt variable.

When the results are displayed, the user is asked if he wants to continue. If he chooses to go forward, the variable DoubleIt is multiplied by two. When the program hits the Loop instruction, it will return to the top of the loop and once again check to see if the condition is set to true. Because the condition has not changed, the loop executes again. It will continue to execute until the user chooses not to continue. Once that happens, the variable Again is set to False. Now, when Loop is reached, the code swings back up to the top of the loop and evaluates the condition once more. This time, the condition is false, so the loop does not execute again. The code moves on beyond the loop.

You can see an example of the Do While…Loop in action in a slightly revised golf average example. Here, the user once again enters golf scores to average them, but this time, things are handled a bit differently. Figure 6.5 shows the Web page, which is stored as golfdwlp.asp.

Figure 6.5 : A Web page that uses the Do While...Loop structure.

In this example, the user simply clicks the Get Golf Scores button, which proceeds to ask the user for a score using a special function called InputBox. We discuss this function in detail on Day 14, "Working with Documents and User Interface Functions." For now, just think of this function as a box where you put in a message for the user and out comes whatever he's entered in the box. If the user doesn't enter anything at all when the input box appears on the screen, the function returns an empty string, or "". The page shown in Figure 6.5 is asking the user for golf scores. Listing 6.8 shows the code that accomplishes this.

Listing 6.8. A Web page that uses the Do While…Loop structure.

<SCRIPT LANGUAGE="VBScript">
<!--  Option Explicit

Sub cmdGetScores_OnClick()

   Dim Count
   Dim Scores(10)
   Dim Score
   Dim Average
   Dim Sum
   Dim i

   Score = 0

   Do While Score <> "" And Count < 10
      Score = InputBox("Enter the next golf score
                        (Press Return to quit): ")
      If Score <> "" Then
         Count = Count + 1
         Scores(Count-1) = CSng(Score)
      End If
   Loop

   For i = 0 to Count-1
      Sum = Sum + Scores(i)
   Next

   Average = Sum / Count

   txtAverage.Value = Average

   Count = 0

End Sub

-->
</SCRIPT>

As you can see, the loop in this listing continues to ask the user for scores until the user doesn't enter anything or he's reached the ten score limit. At that point, the loop condition is no longer true and the average is calculated and displayed to the user. Notice that the score must be set to 0 before the loop begins, otherwise the expression

Score <> ""

would be false and the loop would never execute. In the next section, you will see a way to implement the program without that extra line of code.

The best way to exit from a Do…Loop is to make the condition false as you did in the previous listing. Sometimes, however, you might have to exit a function due to an error or some other factor that must pull you out of a loop. In that case, you can use the Exit Do statement, and VBScript will take you to the next statement below the loop.

Do…Loop While

In the Do While…Loop construct example shown previously, the condition that determined whether you stayed in the loop was a variable that had to be true. Notice that the code in Listing 6.5 first checks the conditional test before it enters the loop. You had better be sure to set the condition variable to true outside the loop before it starts. Otherwise, you have no way of making sure the variable Again was set to true before the loop began. If it had never been set or were set to False, the code inside the loop would never get executed.

Sometimes, it is more desirable to put the conditional test at the bottom of the loop rather than the top. You can accomplish this using the Do…Loop While structure. This control structure takes the form

Do
   ...code within the loop goes here
Loop While condition

As you can see, the only difference is that the condition is tested after the code inside the loop is executed. What difference does this make? Only this: The loop always executes at least one time because it doesn't test for the condition until after it's gone through the loop once. Then, at the end, it performs the test. If you want your loop to execute at least once regardless of the condition (the condition isn't even considered the first time through the loop), this is the structure to use. If you don't even want the loop to execute unless the condition is true first, use the Do While…Loop structure.

Listing 6.9 shows an example using the Do…Loop While structure.

Listing 6.9. Using the Do…Loop While conditional structure.

DoubleIt = 1

' Keep doubling the number as long as the user desires
Do
       '  Show current results and prompt to see if we should continue
       If MsgBox("Current total is " & DoubleIt & ". Double it again ?", _
             vbYesNo) = vbYes Then
   DoubleIt = DoubleIt * 2
          Again = True
Else
          Again = False
       End If

Loop While Again = True

Notice that you have to make sure that the test variable Again gets set one way or the other within the loop. In this case, you can be sure the user will move through the loop at least once. The code is a bit easier to read and understand. In some cases, you might have other conditions where you want to execute the loop at least once and then decide to execute it again. In other cases, you will want to do the test before running anything within the loop. It depends on the application, but both control structures are available for your use.

Consider once more the golf score Web page. In the last section, you used Do While…Loop to accomplish the task. Listing 6.10 points out that you can write the code a bit better using Do…Loop While. The revised Web page called golfdwlp.asp is on the CD-ROM that accompanies the guide.

Listing 6.10. A Web page that uses the Do…Loop While structure.

<SCRIPT LANGUAGE="VBScript">
<!--  Option Explicit

   Sub cmdGetScores_OnClick()

      Dim Scores(10)
      Dim Score
      Dim Average
      Dim Sum
      Dim i

      Do
         Score = InputBox("Enter the next golf score
                          (Press Return to quit): ")
         If Score <> "" Then
             Count = Count + 1
             Scores(Count-1) = CSng(Score)
         End If
      Loop While Score <> "" And Count < 10

      For i = 0 to Count-1
         Sum = Sum + Scores(i)
      Next

      Average = Sum / Count

      txtAverage.Value = Average

      Count = 0

   End Sub

-->
</SCRIPT>

Here, you guarantee the user will be asked for at least one score, and you don't have to worry about making sure the variable is equal to zero before you enter the loop. The Do…Loop While appears to be the better choice.

Do Until…Loop

The third type of Do…Loop is the Do Until…Loop structure. This is very similar to the Do While…Loop except that rather than perform the operations within the loop while the condition is true, it executes the operations inside the loop until the condition is true. If you think about this slight twist of logic for a moment, this will make some sense to you. To help, I'll use a familiar real-life analogy.

Suppose that your father wants you to clean out the garage until he cuts the grass. Using your logical, programming model of reality, you take this to mean that the only time you have to clean out the garage is until he gets outside and starts up the mower. Once he starts cutting the grass, you're outta there. You breathe a big sigh of relief when he walks out to the shed to start cutting the grass. Just before you escape out the back for the baseball diamond, however, your mother notices you. To your horror, she tells you that she wants you to clean out the garage while your father cuts the grass. Before, you only had to clean the garage until your dad started mowing the lawn. Now, you've got to slave away in the garage until he's done! What a life!

Besides the fact that you've got to do more work, can you see the difference? When you're constructing loops, you can set up the same kind of decision-making process. The Do Until…Loop structure takes the form

Do Until condition
   ...code within the loop goes here
Loop

where the code within the loop will only get executed until the condition becomes true. As long as the condition is false, the code in the loop will continue to be executed. Consider the result display implementation in Listing 6.8. What would that implementation look like using Do Until…Loop instead of Do While…Loop? Listing 6.11 provides the answer.

Listing 6.11. Using the Do Until…Loop conditional structure.

Again = True
DoubleIt = 1

' Keep doubling the number as long as the user desires
Do Until Again = False
       '  Show current results and prompt to see if we should continue
       If MsgBox("Current total is " & DoubleIt & ". Double it again ?", _
                 vbYesNo) = vbYes Then
   DoubleIt = DoubleIt * 2
Else
          Again = False
       End If

Loop

As you can see from the listing, rather than execute the loop while Again is true, this loop executes until Again is false. Because the logic has changed, you must change the test of Again from true to false to make the loop equivalent to the previous one.

You can also use the familiar golf score Web page to illustrate this loop structure. Listing 6.12 shows the revised Web page, which is also on the CD-ROM as golfdulp.asp.

Listing 6.12. A Web page that uses the Do Until…Loop structure.

<SCRIPT LANGUAGE="VBScript">
<!--  Option Explicit

   Sub cmdGetScores_OnClick()

      Dim Count
      Dim Scores(10)
      Dim Score
      Dim Average
      Dim Sum
      Dim i

      Score = 0

      Do Until Score = "" Or Count > 9
          Score = InputBox("Enter the next golf score
                           (Press Return to quit): ")
          If Score <> "" Then
              Count = Count + 1
              Scores(Count-1) = CSng(Score)
          End If
      Loop

      For i = 0 to Count-1
          Sum = Sum + Scores(i)
      Next

      Average = Sum / Count

      txtAverage.Value = Average

      Count = 0

   End Sub

-->
</SCRIPT>

Here, the logic of the loop changes a bit. Rather than loop while the scores are coming in, the program loops until the scores stop coming in. Also notice that you have to make sure Score equals zero before the loop starts because you check for its value before the code inside the loop begins.

Do…Loop Until

If you want to defer the decision in the loop to the end, you can use the Do…Loop Until structure. This takes on the form

Do
   ...code within the loop goes here
Loop Until condition

In this case, you're certain that VBScript will go through the loop at least one time, and the test takes place after that first iteration through the loop. From that point on, until the condition becomes true, the loop will continue to execute.

Take a look at the golf score Web page once again. As before, the code would read a bit better if you put the condition at the end of the loop rather than the beginning. Listing 6.13 shows how that is accomplished with the code named golfdulp.asp.

Listing 6.13. A Web page that uses the Do…Loop Until structure.

<SCRIPT LANGUAGE="VBScript">
<!--  Option Explicit

   Sub cmdGetScores_OnClick()

      Dim Count
      Dim Scores(10)
      Dim Score
      Dim Average
      Dim Sum
      Dim i

      Do
         Score = InputBox("Enter the next golf score (Press Return to quit): ")
         If Score <> "" Then
            Count = Count + 1
            Scores(Count-1) = CSng(Score)
         End If
      Loop Until Score = "" Or Count > 9

      For i = 0 to Count-1
         Sum = Sum + Scores(i)
      Next

      Average = Sum / Count

      txtAverage.Value = Average

      Count = 0

   End Sub

-->
</SCRIPT>

Of all the possible loop structures examined today, this one is probably the best for the golf score Web page. It accomplishes the task, yet it's easy to read and understand. As you see, any of the control structures in this section could work with this page. How do you know which is best to choose? The next section provides some good pointers.

You need to remember one more thing about Do…Loop conditional structures. In any of the Do…Loop structures, you can always force VBScript to exit the loop by using the statement Exit Do from within the loop itself. This is often useful for error conditions or special cases where the normal condition is insufficient for breaking out of the loop.

Putting It All Together

Now that you've seen all the decision structures at your command, you know that you have quite a wide variety of choices. You might be wondering at this point, "How do I know which control structure to use?" Oftentimes, you can structure your code where you can choose from one or more of the techniques explained today. I can't provide a specific set of rules to follow for every case. Often, it just boils down to using the structure that expresses what you want to do the most clearly. The If…Then…Else structure is fairly intuitive and straightforward. Still, you might want to keep the following points in mind:

• Do you want to take a single action based on a decision, or do you want to take more than one possible action based on a decision?
  If you only have one action to consider, the If…Then structure is best for you. If, however, you have several tests to do and actions to match each, you might want to use a series of If…Then…ElseIf statements. Often, it helps to write a flowchart or a graph of what you want the program to do under what circumstances.

• Sometimes, you might wonder whether you should use the For…Next control structure. Ask yourself the following question: Do you want the code in your loop to execute a certain number of times based on a counter that can be adjusted each time through the loop?
  If so, and this is often the case, use the For…Next loop. If, for instance, you have to set a series of elements within an array or perhaps perform some calculation a certain number of times, the For…Next loop is the structure to use. If you can't find a connection between a counter, start value, and stop value for repeating the code in your conditional, you might have to consider a Do…Loop.

You might hesitate when deciding what type of Do…Loop to use for the case in question.
You should keep the following tips in mind when making your decision:

• Do you want the code within your loop to always execute at least once?
  If so, you should use either the Do…Loop Until or Do…Loop While structures.
• Do you want to execute the code in the loop until something happens or while something is happening?
  If you want to loop through code until something happens, such as setting a variable or completing some procedure successfully, you should use the Do…Loop Until or Do Until…Loop structures. Any time you want to repeat the code in your loop while some condition is true or while something is happening, the Do…Loop While or Do While…Loop structures are probably best.

Take some time to think of the right looping approach. At the same time, realize that there is often no one best approach. Even experienced programmers might each choose different looping constructs to solve the same problem.

Summary

Today you have gotten a comprehensive overview of the various control structures you can use in a VBScript program. A control structure is a set of code statements you can use to enable blocks of code to repeat or to execute or not execute based on decisions made in those statements. You have seen many examples of each control structure available in VBScript. With your knowledge of variables, operators, and control structures, you are becoming an increasingly competent and knowledgeable VBScript programmer.

Control structures are at the heart of VBScript programming. Having read today's lesson, you should understand how they work and be quite comfortable using them. As you will see, the examples throughout this guide use them extensively. A program almost always has to make a lot of decisions. Today's lesson has provided you with the information and strategies you need to get the most out of directing and controlling code flow within a procedure.

Q&A

Do While Stop = False
   Name = InputBox("Enter a name:")
   If GetAddress(Name) = False Then
       Stop = True
   End If
Loop

In this case, it is possible that the user might not want to enter any more names. The way this loop is set up, it will continue until it finds a name whose address it cannot match. That's fine, but what if the user is done entering names? There's no way to break out of the loop. The solution is to set up another loop condition that takes this into account:

Do While Stop = False or NoName = True
   Name = InputBox("Enter a name:")
   If Name = "" Then
       NoName = True
   End If
   If GetAddress(Name) = False Then
       Stop = True
   End If
Loop

Now, the program can break out of the loop if the user hasn't supplied a name or if the address of the name the user entered cannot be found.

Workshop

Create a Web page that uses at least three of the control structures you've learned about today. Focus on the If…Then, For…Next, and Do While…Loop structures, but feel free to change or add new structures as needed.

Quiz

1. List three control structures you've seen today and a brief description of what each of them does. Provide a simple example of their use.
2. Write a simple program that asks the user for the year in which she was born. Use a control structure to inform the user if the year she's entered is invalid. Use the InputBox function to get the year from the user. InputBox hasn't been covered in detail yet, but you can use it in the same manner it was used in the example earlier today.
3. Now, include another control structure that keeps asking her for the year until she's entered the right one.