Wyo VB Lecture Notes

Objective #1: Use While loops.

• A While loop can be used to execute a block of code many times over and over. The loop below causes a message box to appear 3 times.
  
  intNum = 0
  
  While (intNum <= 3)
     MessageBox.Show("Hello World")
     intNum = intNum + 1
  End While
  
  In the While loop above     (intNum <= 3)      is a control expression. The statements
  
  MessageBox.Show("Hello World")
  intNum = intNum + 1
  
  are considered the body of this loop since they are indented and placed between the While and the End While.
  
  Visual Basic evaluates the control expression to see if it is True or False. If the control expression is True then VB executes the indented statements in the body of the loop. If the control expression is False, VB skips the while loop and does not execute the statements inside the body of the loop. The body of the loop executes over and over again until the control expression becomes false. Each time a looIt is usually considered a logic error if the control expression never becomes false since the loop would execute forever. A loop that executes forever is called an infinite loop. Notice that the example above would be an infinite loop if the statement    intNum = intNum + 1   was removed.
  
  
• For good style, you should indent the statements inside the body of the loop and you should place parentheses around the control expression for readability. You should always surround your loops with blank lines in your code. This makes your program easier to read and debug. That is, above all While and For loops you should include a blank line and below all End While and Next statements, you should include a blank line. Blank lines used to make code easier to read are called white space. Tabs and blank spaces that are also used for the same purpose of making white space.
  
  
• A While loop should be used instead of a For loop when you do not know the exact number of iterations that the loop must make. If there is a situation in which some process must occur over and over again within a loop but you do not know when exactly it will end then you should use a While loop. For example, if you ask the user to input his test scores but your program does not necessarily know how many tests he has taken you could use a While loop and simply tell the user to enter the number -1 to end his list of input values. Of course, we know that -1 will not be confused with his actual exam scores since it is impossible to earn a -1 on an exam. In this case, -1 is called the sentinel value that warns the loop that the inputs have ended just like Paul Revere was a sentinel that warned the Americans that the British were coming.

• A While loop is called an indefinite loop (or indeterminate loop) because the programmer usually does not know beforehand exactly how many times it will iterate. Remember that a For loop is a definite loop (or determinate loop) because a programmer can count how many times it will iterate based on its initial value, limit value, and step value.
  
  
• Here is an example of a While loop that adds up the numbers from 1 to 5
  
  intNum = 1
  intSum = 0
  
  While (intNum <= 5)
     intSum += intNum
     intNum += 1
  End While
  
  however the loop can be modified to add up the values from 1 to 10 by changing the 5 in the control expression to 10
  
  intNum = 1
  intSum = 0
  
  While (intNum <= 10)
     intSum += intNum
     intNum += 1
  End While
  
  The loop can be modified to add the values from 5 to 10 by changing the initial value of intNum
  
  intNum = 5
  intSum = 0
  
  While (intNum <= 10)
     intSum += intNum
     intNum += 1
  End While
  
  or the loop could be used to add up the even numbers between 2 and 10 by setting the initial value of intNum to 2 and changing one of the body statements
  
  intNum = 2
  intSum = 0
  
  While (intNum <= 10)
     intSum += intNum
     intNum += 2
  End While
  
  
• Here is an example of a While loop that finds the sum of numbers inputed by the user until the user inputs -1. In this example, the value -1 is used as a sentinel value.                    

  intNum = 0
  intSum = 0
  
  While (intNum <> -1)
      intNum = InputBox("Enter an integer (-1 to quit)"
      intSum += intNum
  End While
  
  Note that the loop above could be rewritten using the Not operator
  
  intNum = 0
  intSum = 0
  
  While (Not (intNum = -1))
      intNum = InputBox("Enter an integer (-1 to quit)"
      intSum += intNum
  End While
  

• You can use an Exit While statement to exit or "break out of" a While loop. In this loop, the loop immediately stops iterating when the user inputs the sentinel value of -1 because the Exit While statement makes the loop immediately end.
  
  intNum = 0
  intSum = 0
  
  While (intNum <> -1)
     intNum = InputBox("Enter an integer (-1 to quit)")
     
     If (intNum = -1)
        Exit While
     Else
        intSum += intNum
     End If
  
  End While
  
  
• You must be careful not to add your sentinel value into your running total. The following While loop contains a logic error
  
  intNum = 0
  intSum = 0
  
  While (intNum <> -1)
     intNum = InputBox("Enter an integer (-1 to quit)")
     intSum += intNum
  End While
  
  In this case, the value -1 is added into the running total even though that is not the programmer's intent. So it could be fixed by using the Exit While statement that is used in an earlier example or by adding 1 to intSum after the loop is finished
  
  intNum = 0
  intSum = 0
  
  While (intNum <> -1)
     intNum = InputBox("Enter an integer (-1 to quit)")
     intSum += intNum
  End While
  
  intSum = intSum + 1   ' to make up for the extra input of -1
  
  
• Here is an example of a While loop that determines how many years it would take to make $100 turn into $150 at a 5% interest rate compounded yearly.
  
  dblBalance = 100
  intYears = 0
  
  While (dblBalance < 150)
     dblBalance = dblBalance + dblBalance * 0.05
     intYears += 1
  End While
  
  
• Here is an example of a While loop that counts how many teenagers there are in a group of people
  
  intTotal = 0
  intAge = 0
  
  While (intAge <> -1)
     intAge = InputBox("What is your age?")
  
     If (intAge >= 13 And intAge <= 19) Then
        intTotal += 1
     End If
  
  End While
  
  

Objective #2: Use flag variables to control While loops.

• A flag variable is often used to store the value 1 (meaning True) or 0 (meaning False). The programmer uses the flag variable as a way of controlling some process in an algorithm. Flag variables are often used in While loops to send a signal to the control expression. They are not used in For loops since a For loop executes a predetemined, definite number of times.
  
  For example, in the code segment below intGameIsOver is used as a flag variable. It sends a signal to the control expression of the loop if the user has scored enough points to end the game and exit the loop. intGameIsOver is initially set to the value 0 which indicates False (the game is NOT over). You should name flag variables appropriately. The variable should be named in a way that it's name indicates a True condition. (For example, in the segment below, you should not use a flag variable with the name, intGameIsNotOver, since it would probably make the logic of the algorithm harder for most fellow programmers to follow.)
  
  intGameIsOver = 0
  
  While (intGameIsOver = 0)
             ' a bunch of statements would be located here
             intUserPointsScored = intUserPointsScored + intLastRoundScore
  
             If (UserPointsScored > 1000) Then
                    intGameIsOver = 1
             End If
  End While
  
  
  Often programmers use variables with the Boolean data type as flag variables. A Boolean variable can be set to either True or False. Here is an example:
  
  Dim blnGameIsOver As Boolean = False
  
  While (blnGameIsOver = False)
             ' a bunch of statements would be located here
             intUserPointsScored = intUserPointsScored + intLastRoundScore
  
             If (intUserPointsScored > 1000) Then
                    blnGameIsOver = True
             End If
  End While
  
  By using a Boolean variable you can simplify the first line of the loop like this
  
  While (Not blnGameIsOver)
  
  Here's another example of a While loop that uses a flag variable. In this case, the flag variable has the Boolean data type.
  
  Dim blnOutOfBounds As Boolean = False
  
  While (blnOutOfBounds = False)
     picPlayer.Left = picPlayer.Left + 10
  
     If (picPlayer.Left > 300) Then
        blnOutOfBounds = True
     End If
  
  End While