Do While LoopsFlavor One

Do While Loops ”Flavor One

Several flavors of Do - While loops are available to you. The major difference between them is where the relational test is performed. The first version we will explore has the syntax form:

  Do While   Expression1   DoWhileStatementBlock   Loop  

Expression1 is a Boolean expression that evaluates to either logic True or False. If Expression1 is logic True, the statements in the Do - While statement block (that is, DoWhileStatementBlock ) are executed. When the Loop keyword is reached, control is directed back to the Do - While test on Expression1 . As long as Expression1 is logic True, the loop statements continue to be executed. When Expression1 becomes logic False, control is sent to the first statement following Loop .

For example, let's write a program that simulates tossing a coin. Let's further assume that we want to see how many coin tosses it takes before we get a certain number of heads in a row. Start a new project named CoinTosses and add the objects shown in Figure 13.2.

Figure 13.2. The placement of objects for the CoinTosses project.

The first text box, named txtNumber , is the number of heads we want in a row. Text box txtTossCounter shows how many tosses were made before we got the desired number of heads. Consider Listing 13.4 for the btnStart Click event.

Listing 13.4 Code for the btnStart Click Event

 Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As _               System.EventArgs) Handles btnStart.Click  Dim Tosses, Heads, ThisToss, Target As Integer  Heads = 0  Tosses = 0  If txtNumber.Text <> "" Then   ' Make sure they entered a number   Target = CInt(txtNumber.Text)  Else   MessageBox.Show("You need to enter a number.")   Exit Sub  End If  Do While Heads <> Target   ThisToss = TossACoin()  ' Function returns 1 for head   If ThisToss = 1 Then    Heads += 1       ' It was a Heads, increment the count   Else    Heads = 0        ' Tails...start over   End If   Tosses += 1        ' The toss counter  Loop  txtTossCounter.Text = CStr(Tosses) End Sub 

Notice how we check to make sure that the user entered a number into the txtNumber text box. If the user didn't enter a number, an error message is displayed and we leave the sub-routine.

We've written a function named TossACoin() that returns a 1 if a heads was tossed, or if a tails was the result. When we enter the Do - While loop, Heads is , which means the relational test that Heads is not equal to Target is logic True. This causes control to enter the Do - While statement block. The code for the TossACoin() is presented in Listing 13.5.

Listing 13.5 Function to Simulate Tossing a Coin

 Private Function TossACoin() As Integer  ' Purpose: To simulate tossing a coin. The function  '      return 1 for a Head, and 0 for a tail.  '  ' Parameters  '  ' Return value  '  integer    1 for Head, 0 for tails  '  TossACoin = Int(2 * Rnd()) End Function 

A call to the Randomize() function seeds the random number generator so that a nonrepeatable series of pseudo-random numbers is produced. The function needs to be called only once before Rnd() is used, so we placed the call in the Form_Load event. (The Randomize() function uses the system timer for this purpose.) The call to Rnd() returns a value between 0 and 1 as a Single data type. When you want a value to fall between two values, you use the formula:

 Int((upperlimit - lowerlimit + 1) * Rnd() + lowerlimit) 

Because we want either a 1 or a 0, our formula becomes

 Int((1 - 0 + 1) * Rnd() + 0)  Int(2 * Rnd()) 

which is the statement we used in Listing 13.4.

Programmer's Tip

If you call Rnd() with a negative number as an argument, such as Rnd(-1) , the function returns a repeatable sequence of random numbers. That is, you'll get the same series of random numbers each time. This can be useful when debugging a program. Obviously, if you're seeding Rnd() this way, there's no reason to execute a call to Randomize() .

I'll assume that by now you're comfortable adding the code for the Exit button. Figure 13.3 shows a sample run.

Figure 13.3. Sample run of the CoinTosses project.

As you can see from Figure 13.3, it took 47,152 tosses before we got 12 heads in a row. If you care to repeat the experiment, I've seen as few as 14 tosses give 12 heads in a row! You might want to put a breakpoint on the Do - While statement and single-step the program so that you can see how the loop works.