1. Programming Logic and Design Fourth Edition, Comprehensive
Chapter 6
Looping

2. Objectives Understand the advantages of looping
Control a while loop using a loop control variable
Increment a counter to control a loop
Loop with a variable sentinel value
Control a loop by decrementing a loop control variable
Programming Logic and Design, Introductory, Fourth Edition

3. Objectives (continued)
Avoid common loop mistakes
Use a for statement
Use do while and do until loops
Recognize the characteristics shared by all loops
Nest loops
Use a loop to accumulate totals
Programming Logic and Design, Introductory, Fourth Edition

4. Understanding the Advantages of Looping
Set of instructions that is executed repetitively based on a condition
Allows processing on large sets of data, such as complex payroll and benefits processing
Main loop:
Basic set of instructions that is repeated for every record
Programming Logic and Design, Introductory, Fourth Edition

5. Using a while Loop with a Loop Control Variable
Loops also occur within program modules
Three steps in every loop:
Initialize a control variable
Compare the control variable to a value to determine if the loop should continue
Alter the control variable within the loop
Programming Logic and Design, Introductory, Fourth Edition

6. Using a while Loop with a Loop Control Variable (continued)
Programming Logic and Design, Introductory, Fourth Edition

7. Using a while Loop with a Loop Control Variable (continued)
Loop control variable: determines whether a loop will continue to execute
Sentinel value: a limit or ending value to compare with the loop control variable
Loop body: statements inside the loop that are executed repetitively
Once the loop body is entered, the entire loop body must execute
Can exit from a structured loop only at the comparison test of the loop control variable
Programming Logic and Design, Introductory, Fourth Edition

8. Using a Counter to Control Looping
Developing the application:
The input file
Programming Logic and Design, Introductory, Fourth Edition

9. Using a Counter to Control Looping (continued)
Developing the application:
The main loop
Programming Logic and Design, Introductory, Fourth Edition

10. Using a Counter to Control Looping (continued)
Counter: numeric variable that counts how often an event occurs
Incrementing: adding to a variable, usually by 1
Loop continues executing until the condition is no longer met
Programming Logic and Design, Introductory, Fourth Edition

11. Using a Counter to Control Looping (continued)
Developing the application:
housekeeping() module
Programming Logic and Design, Introductory, Fourth Edition

12. Using a Counter to Control Looping (continued)
Developing the application:
createLabels() module
Programming Logic and Design, Introductory, Fourth Edition

13. Using a Counter to Control Looping (continued)
Three parts to the loop:
Initialize: Set labelCounter to 0
Compare: compare labelCounter to 100
Body: print labelLine and inFirstName, add 1 to labelCounter
When labelCounter has a value of 100, the loop ends
This loop is executed for each employee record
Programming Logic and Design, Introductory, Fourth Edition

14. Using a Counter to Control Looping (continued)
Developing the application:
finishUp() module
Programming Logic and Design, Introductory, Fourth Edition

15. Looping with a Variable Sentinel Value
Developing the application
Print labels based on employee’s production amount
The input file:
Programming Logic and Design, Introductory, Fourth Edition

16. Looping with a Variable Sentinel Value (continued)
Programming Logic and Design, Introductory, Fourth Edition

17. Looping by Decrementing
Decrementing: counting down
May be more convenient to control a loop by decrementing
May eliminate the need for a counter variable
Programming Logic and Design, Introductory, Fourth Edition

18. Avoiding Common Loop Mistakes
Neglecting to initialize the loop control variable
Neglecting to alter the loop control variable
Using the wrong comparison with the loop control variable
Including statements inside the loop that belong outside the loop
Initializing a variable that does not require initialization
Programming Logic and Design, Introductory, Fourth Edition

19. Neglecting to Initialize the Loop Control Variable
Uninitialized variables may contain unknown, unpredictable garbage
Makes the comparison for the loop test meaningless
May fail to enter the loop at all
Programming Logic and Design, Introductory, Fourth Edition

20. Neglecting to Alter the Loop Control Variable
May cause an infinite loop
Infinite loop: a loop that never stops executing
A structured loop must terminate on its own
Programming Logic and Design, Introductory, Fourth Edition

21. Using the Wrong Comparison with the Loop Control Variable
Using <= or >= when only < or > was required may cause an extra iteration through the loop
Programming Logic and Design, Introductory, Fourth Edition

22. Including Statements Inside the Loop that Belong Outside the Loop
A statement erroneously placed in a loop will execute as many times as the loop executes
Affects the performance and efficiency of the program
Carefully analyze what actions must be repeated, and place all other actions outside the loop
Programming Logic and Design, Introductory, Fourth Edition

23. Including Statements Inside the Loop that Belong Outside the Loop (continued)
Programming Logic and Design, Introductory, Fourth Edition

24. Initializing a Variable that Does Not Require Initialization
Consider whether the variable requires initialization:
Is an initial value required?
Will the variable be assigned a value within the loop?
Programming Logic and Design, Introductory, Fourth Edition

25. Using the for Statement
Indeterminate (or indefinite) loop: when the number of executions of the loop is not known in advance
Definite loop: when the number of executions of the loop is known in advance
while statement can be used with both definite and indefinite loops
for statement can be used with definite loops
Programming Logic and Design, Introductory, Fourth Edition

26. Using the for Statement (continued)
for statement performs three actions within a single statement:
Initializes the loop control variable
Evaluates the loop control variable
Alters the loop control variable (usually by incrementing)
for statement is a pretest loop
Programming Logic and Design, Introductory, Fourth Edition

27. Using the for Statement (continued)
This for statement accomplishes these tasks:
Initializes labelCounter to 0
Checks labelCounter to ensure it is less than or equal to the limit value 99
If the evaluation is true, the loop body is executed
After executing the loop body, labelCounter is incremented by 1 and compared to the limit value again
Programming Logic and Design, Introductory, Fourth Edition

28. Using the for Statement (continued)
Use a for statement when the loop will:
Start with a known starting value
End with a known ending value
Increase in equal increments
Starting, ending and increment values can be represented with variables
Size of the increment can be set
Programming Logic and Design, Introductory, Fourth Edition

29. Using the do while and do until Loops
With a pretest loop (for or while), the loop body may never execute
With a posttest loop, the loop body is always executed at least once
do while loop continues to execute as long as the condition remains true
do until loop continues to execute as long as the condition remains false
Programming Logic and Design, Introductory, Fourth Edition

30. Using the do while and do until Loops (continued)
Programming Logic and Design, Introductory, Fourth Edition

31. Using the do while and do until Loops (continued)
Programming Logic and Design, Introductory, Fourth Edition

32. Recognizing the Characteristics Shared by All Loops
All loops share these characteristics:
Loop-controlling question provides either an entry to or exit from the repeating structure
Loop-controlling question provides the only entry to or exit from the repeating structure
Structured loops do not allow premature exits
Programming Logic and Design, Introductory, Fourth Edition

33. Recognizing the Characteristics Shared by All Loops (continued)
Programming Logic and Design, Introductory, Fourth Edition

34. Nesting Loops Nesting loops: placing one loop inside another loop
Outer loop: a loop that contains another loop
Inner loop: a loop that is inside another loop
Developing the application:
The input file:
Programming Logic and Design, Introductory, Fourth Edition

35. Nesting Loops (continued)
Developing the application:
The desired output: ¼ of 1% raise in each pay period
Two pay periods per month
Programming Logic and Design, Introductory, Fourth Edition

36. Nesting Loops (continued)
Developing the application:
Use two counters:
One to track months
One to track the checks within the month
Use constants to self document the program:
Number of months in the year = 12
Number of checks in the month = 2
Rate of pay increase =
Programming Logic and Design, Introductory, Fourth Edition

37. Nesting Loops (continued)
Programming Logic and Design, Introductory, Fourth Edition

38. Nesting Loops (continued)
Programming Logic and Design, Introductory, Fourth Edition

39. Using a Loop to Accumulate Totals
Detail reports: show details, may also show totals or other overall statistics at end
Summary reports: show only totals or other overall statistics
Accumulator: a variable used to accumulate values
Accumulator variable must be initialized (usually to 0) to ensure it does not contain garbage
Programming Logic and Design, Introductory, Fourth Edition

40. Using a Loop to Accumulate Totals (continued)
Developing the application:
When finished processing the data file, the accumulator holds the grand total
Summary can then be printed at end of report
Programming Logic and Design, Introductory, Fourth Edition

41. Using a Loop to Accumulate Totals (continued)
Programming Logic and Design, Introductory, Fourth Edition

42. Summary
Loop: a set of statements that operates on multiple sets of data
Three steps must occur in a loop: initialize, compare, and alter loop control variable
Counter: variable used to count the number of times an event occurs
Sentinel value can be used to control a loop
Programming Logic and Design, Introductory, Fourth Edition

43. Summary (continued) Common loop mistakes:
Failing to initialize, or neglecting to alter the loop control variable
Using the wrong comparison operator
Including statements inside the loop that do not belong there
for statement: used with definite loops when you know the number of times the loop will execute
for statement automatically initializes, compares, and increments its loop control variable
Programming Logic and Design, Introductory, Fourth Edition

44. Summary (continued) do while and do until loops:
Test the condition at the end of the loop
Guarantee that the loop body executes at least once
All structured loops share these characteristics:
Loop controlling question provides either entry to or exit from the repeating structure
Loop controlling question provides the only entry to or exit from the repeating structure
Nesting loops: loops placed within other loops
Accumulator: variable used to accumulate values
Programming Logic and Design, Introductory, Fourth Edition