1. Chapter 12: How Long Can This Go On?
(Pretest Loops)

2. Chapter Objectives After studying Chapter 12, you should be able to:
Write a looping condition and its opposing loop exit condition
Show a pretest loop in both pseudocode and a flowchart
Write a pretest loop using the Do…Loop statement
Utilize counter and accumulator variables
Refresh the screen
Delay program execution
Display a dialog box using the InputBox function
Abbreviate assignment statements using the arithmetic assignment operators
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3. Over and Over Again Repetition structure (loop)
Used to repeatedly process one or more program instructions
The loop’s condition determines if and for how long the instructions are repeated
The requirement for repeating the instructions is referred to as the looping condition because it indicates when the computer should continue “looping” through the instructions
The requirement for not repeating the instructions is referred to as the loop exit condition because it tells the computer when to exit (or stop) the loop
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4. Over and Over Again
A repetition structure can be either a pretest loop or a posttest loop
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5. Over and Over Again (Cont.)
Figure 12-1 Examples of looping and loop exit conditions
Pretest loop
Loop condition is evaluated before the instructions within the loop are processed
Posttest loop
Evaluation occurs after the instructions within the loop are processed
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6. Over and Over Again
Depending on the result of the evaluation, the instructions in a pretest loop may never be processed
The instructions in a posttest loop, however, will always be processed at least once
Of the two types of loops the pretest loop is the most commonly used
You will learn about pretest loops in this chapter and in chapter 14. Postttest loops are covered in chapter 13
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7. Over and Over Again (Cont.)
Loop Body is the statements between the repeat and end-repeat clauses
Figure 12-2 A problem that requires the sequence and selection structures
Figure 12-3 A problem that requires the sequence and repetition structures
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8. Over and Over Again (Cont.)
Figure 12-4 Another problem that requires the sequence and selection structures
Figure 12-5 A problem that requires the sequence, selection, and repetition structures
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9. Over and Over Again (Cont.)
Figure 12-6 Two versions of the modified algorithm from Figure 12-5
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10. The Do…Loop Statement
Use the Do…Loop statement to code both pretest and posttest loops
Statements begin with the Do clause and end with the Loop clause
Between both clauses, instructions the computer is to repeat are entered
Referred to as the loop body
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11. The Do…Loop Statement (Cont.)
Figure 12-7 Syntax and examples of the Do…Loop statement for a pretest loop
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12. Counter Variables Counter Used to count something
Always numeric variables
Initializing counter variables
Typically assigned a beginning value of either 0 or 1, depending on the value required by the application
Updating
Adding a number to the value stored in the counter variable is called incrementing
Subtracting a number from the value stored in the counter variable is called decrementing
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13. Counter Variables (Cont.)
Figure 12-8 Code and processing steps for Example 1 in Figure 12-7
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14. Counter Variables (Cont.)
Repetition structures use counters to count such things as the number of employees paid in a week or the number of positive numbers entered by the user, for the purpose of counting we use integer variables to count
In page 268 the integer variable intNum is referred to as a counter
(work the next exercise)
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15. The Do…Loop Statement Exercise1
Write a statement or a set of statements to accomplish each of the following
Sum the odd integers between 1 and 15 using a Do While…Loop repetition statement. Use integer variables sum and count
Sum the squares of the even integers between 1 and 15 using a Do While…Loop repetition statement. Use integer variables sum and count and initialize them to 0 and 2, respectively
Display the numbers from 5 to 1 in txtResult using a Do Until and Integer counter variable counterIndex. Initialize counterIndex to 5
Repeat exercise in part c using a Do While…Loop
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16. The Do…Loop Statement Exercise2
Write a Visual Basic statement to accomplish each of the following tasks
Declare variables sum and number to be of type integer
Assign 1 to variable number
Assign 0 to the variable sum
Total variables number and sum and assign the result to variable sum
Display “The sum is: “ followed by the value of the variable sum in the lblResult
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17. The Do…Loop Statement Exercise 3
Combine the statements that you wrote in the previous exercise into an application that calculates and displays the sum o f integers from 1 to 10. Use a Do While… loop to loop through the calculations and increment statements. The loop should terminate when the value of control variable number becomes 11.
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18. The Do…Loop Statement Sum += value Loop Exercise 4
Identify and correct the error(s) in each of the following (you may need to add code):
The following loop should total the values from 1 to 50. Assume that value is 50
Do While value >= 0
Sum += value
Loop
The following code should display the squares of 1 to 10 in listBoxResult
Dim number As Integer = 1
Do While number < 10
listBoxResult.Items.Add(number ^ 2)
End While
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19. The Do…Loop Statement
c) This segment should display the integers from 888 to 1000 in ListBoxResult. Initialize variable value to 888
Dim value As Integer = 888
Do While value < = 1000
Value -= 1
loop
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20. Counter Variables (Cont.)
Figure 12-9 Example of a partial game program that uses a counter
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21. Counter Variables (Cont.)
The interface for the Cheerleader application uses a repetition structure to make the “Go Team!” message blink several times when the user clicks the Click Here button
Figure Interface for the Cheerleader application
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22. Counter Variables (Cont.)
Refresh method
Ensures computer processes previous lines of code that affect the form’s appearance
Syntax: Me.Refresh()
Sleep method
Used to delay program execution
Syntax: System.Threading.Thread.Sleep(milliseconds)
Figure Algorithm (pseudocode and flowchart) for the Click Here button (Continues)
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23. Counter Variables (Cont.)
Figure btnClickHere_Click procedure in the Cheerleader application
Figure Algorithm (pseudocode and flowchart) for the Click Here button
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24. My Dream Car Application
The interface for the My Dream Car application uses a repetition structure to drag the car image from the left edge of the form to the center of the form
The car image is contained in the picCar control, whose Visible property is set to False in the Properties window
Figure Interface for the My Dream Car application
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25. My Dream Car Application (Cont.)
Figure Algorithm for the Show the Car button’s Click event procedure
Figure btnShow_Click procedure in the My Dream Car application
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26. The Sales Express Application
Accumulator variable
Numeric variable used for accumulating something
Assigned a value outside the loop
Updated within the loop
Incremented by an amount that varies
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27. The Sales Express Application (Cont.)
Priming read
Used to prepare (prime) the loop
Initializes the loop condition by providing its first value
Update read
Allows user to update the value of the input item
Infinite (endless) loop
Loop that has no way to end
To force an infinite loop to end
Click Debug on the menu bar, then Stop Debugging
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28. The Sales Express Application (Cont.)
Figure Interface for the Sales Express application
Figure Problem specification and algorithm for the Sales Express application
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29. The Sales Express Application (Cont.)
Rather than using a text box, the application will use an input dialog box
An InputBox function returns a value depending on the button the user chooses
If the user clicks the OK button, the function returns the value contained in the input area of the dialog box; the return value is always treated as a string
If the user clicks either the Cancel button in the dialog box or the Close button on the dialog box’s title bar, the function returns an empty string
The empty string is represented by the String.Empty constant
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30. The Sales Express Application (Cont.)
Figure Example of an input dialog box created by the InputBox function
Figure Syntax and examples of the InputBox function
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31. The Sales Express Application (Cont.)
Figure btnCalc_Click procedure in the Sales Express application
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32. The Sales Express Application (Cont.)
Figure Test data chart for the Sales Express application
Figure Sample run of the Sales Express application
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33. Can I Abbreviate That Assignment Statement?
Use the arithmetic assignment operators to abbreviate an assignment statement that contains an arithmetic operator
Figure Syntax and examples of the arithmetic assignment operators
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34. Summary
Use a repetition structure (loop) to repeatedly process one or more program instructions
Repetition structures can be either a pretest loop or a posttest loop - Pretest loops are more commonly used
Use the Do…Loop statement to code a pretest loop
The While keyword in the statement’s condition indicates that the loop instructions should be processed while (as long as) the condition is true
The Until keyword in the statement’s condition indicates that the loop instructions should be processed until the condition becomes true
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35. Summary (Cont.)
Counters and accumulators must be initialized and updated
The initialization is done outside of the loop that uses the counter or accumulator, and the updating is done within the loop
Counters are updated by a constant amount, whereas accumulators are usually updated by an amount that varies
In a flowchart, the loop condition is represented by the decision symbol, which is a diamond
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36. Summary (Cont.) Use the Refresh method to refresh (redraw) the form
The method’s syntax is Me.Refresh()
Use the Sleep method to delay program execution
The method’s syntax is System.Threading.Thread.Sleep(milliseconds)
The priming read appears above the loop that it controls
The InputBox function displays an input dialog box that contains a message, an OK button, a Cancel button, and an input area
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37. Summary (Cont.)
A run time error occurs, you can stop the application by clicking DEBUG on the menu bar and then clicking Stop Debugging
Before using a variable as the divisor in an expression, verify that the variable does not contain the number 0
Dividing by 0 is mathematically impossible and will cause a run time error to occur
Use an arithmetic assignment operator to abbreviate an assignment statement that has the following format, in which variableName is the name of the same variable:
variableName = variableName arithmeticOperator value
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