1. Programming Logic and Design Fifth Edition, Comprehensive
Chapter 5
Looping

2. Objectives Learn about the advantages of looping
Control loops with counters and sentinel values
Nest loops
Learn to avoid common loop mistakes
Programming Logic and Design, Fifth Edition, Comprehensive

3. Objectives (continued)
Use a for loop
Use posttest loops
Recognize the characteristics shared by all loops
Learn about common loop applications
Programming Logic and Design, Fifth Edition, Comprehensive

4. Understanding the Advantages of Looping
Looping makes computer programming efficient and worthwhile
Write one set of instructions to operate on multiple, separate sets of data
Loop: structure that repeats actions while some condition continues
Programming Logic and Design, Fifth Edition, Comprehensive

5. Understanding the Advantages of Looping (continued)
Figure 5-1 The while loop
Programming Logic and Design, Fifth Edition, Comprehensive

6. Controlling Loops with Counters and Sentinel Values
As long as a Boolean expression remains true, while loop’s body executes
Must control number of repetitions to avoid an infinite loop
Repetitions controlled by
Counter
Sentinel value
Programming Logic and Design, Fifth Edition, Comprehensive

7. Using a Definite while Loop with a Counter
Three actions make a while loop end correctly:
Loop control variable is initialized
Prior to entering the loop
Loop control variable is tested
If result is true, loop body entered
Loop control variable must be altered in loop body
while expression eventually evaluates to false
Loop control variables altered by:
Incrementing
Decrementing
Programming Logic and Design, Fifth Edition, Comprehensive

8. Using a Definite while Loop with a Counter (continued)
Figure 5-2 A while loop that prints “Hello” four times
Programming Logic and Design, Fifth Edition, Comprehensive

9. Using a Definite while Loop with a Counter (continued)
Definite loop: number of iterations predetermined
Also called counted loop
Counter: numeric variable used to count number of times an event occurs
Loop control variable may be altered by user input
Indefinite loop: loop iterates until some condition is true
Number of iterations may vary
Programming Logic and Design, Fifth Edition, Comprehensive

10. Using an Indefinite while Loop with a Sentinel Value
Indefinite loop: loop performed a different number of times each time the program executes
Three crucial steps:
Starting value to control the loop must be provided
Comparison must be made using the value that controls the loop
Within the loop, value that controls the loop must be altered
Loop control variable: any variable that determines whether the loop will continue
Programming Logic and Design, Fifth Edition, Comprehensive

11. Figure 5-3 Looping bank balance program
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12. Using an Indefinite while Loop with a Sentinel Value (continued)
Figure 5-4 Typical execution of the looping bank balance program
Programming Logic and Design, Fifth Edition, Comprehensive

13. Using an Indefinite while Loop with a Sentinel Value (continued)
Figure 5-5 Crucial steps that must occur in every loop
Programming Logic and Design, Fifth Edition, Comprehensive

14. Nested Loops Nested loops: loops within loops
Outer loop: loop that contains the other loop
Inner loop: loop that is contained
Needed when values of two (or more) variables repeat to produce every combination of values
Programming Logic and Design, Fifth Edition, Comprehensive

15. Figure 5-7 Flowchart and pseudocode for AnswerSheet program
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16. Mixing Constant and Variable Sentinel Values
Number of times a loop executes can depend on a constant or a value that varies
May not want to repeat every pass through a loop the same number of times
Example:
Print 100 labels for every employee
Outer loop controlled by value of employee name
Inner loop executes 100 times
Print variable number of labels for every employee
Inner loop controlled by production level
Programming Logic and Design, Fifth Edition, Comprehensive

17. Mixing Constant and Variable Sentinel Values (continued)
Figure 5-8 Program that produces 100 labels for every employee
Programming Logic and Design, Fifth Edition, Comprehensive

18. Mixing Constant and Variable Sentinel Values (continued)
Figure 5-8 Program that produces 100 labels for every employee (continued)
Programming Logic and Design, Fifth Edition, Comprehensive

19. Figure 5-9 Program that produces a variable number of labels for every employee
Programming Logic and Design, Fifth Edition, Comprehensive

20. 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
Programming Logic and Design, Fifth Edition, Comprehensive

21. Avoiding Common Loop Mistakes (continued)
Mistake: neglecting to initialize the loop control variable
Example: get name statement removed
Value of name unknown or garbage
Program may end before any labels printed
100 labels printed with an invalid name
labelCounter = 0 statement removed
Value of labelCounter unpredictable
Loop might not execute
If automatically initialized to zero, only first labels printed
Programming Logic and Design, Fifth Edition, Comprehensive

22. Figure Incorrect logic when loop control variable initializations are removed from label-making program
Programming Logic and Design, Fifth Edition, Comprehensive

23. Avoiding Common Loop Mistakes (continued)
Figure Incorrect logic when loop control variable altering statements are removed from label-making program (continued)
Programming Logic and Design, Fifth Edition, Comprehensive

24. Avoiding Common Loop Mistakes (continued)
Mistake: neglecting to alter the loop control variable
Remove get name instruction from outer loop
User never enters a name after the first one
Inner loop executes infinitely
Remove the statement that increments labelCounter
Always incorrect to create a loop that cannot terminate
Programming Logic and Design, Fifth Edition, Comprehensive

25. Avoiding Common Loop Mistakes (continued)
Mistake: using the wrong comparison with the loop control variable
Programmers must use correct comparison
Off-by-one error
Seriousness depends on actions performed within a loop
Overcharge insurance customer by one month
Overbook a flight on airline application
Dispense extra medication to patients in pharmacy
Programming Logic and Design, Fifth Edition, Comprehensive

26. Avoiding Common Loop Mistakes (continued)
Example:
Correct: loop performed 10 times
counter = 0
while counter < 10
print “Hello”
counter = counter + 1
end while
Programming Logic and Design, Fifth Edition, Comprehensive

27. Avoiding Common Loop Mistakes (continued)
Example (continued):
Error: loop performed 11 times
counter = 0
while counter <= 10
print “Hello”
counter = counter + 1
end while
Programming Logic and Design, Fifth Edition, Comprehensive

28. Avoiding Common Loop Mistakes (continued)
Mistake: including statements inside the loop that belong outside the loop
Example: calculating a user’s projected weekly pay raise
Value of weeklyPay recalculated on every pass through the loop, although it does not change
Inefficient, especially for complicated calculations or large numbers of calculations
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29. Figure 5-12 Pay rate projection program
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30. Figure 5-14 Improved pay rate projection program
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31. Using a for Loop for statement or for loop is a definite loop
Provides three actions in one structure
Initializes
Evaluates
Increments
Takes the form:
for initialValue to finalValue
do something
endfor
Programming Logic and Design, Fifth Edition, Comprehensive

32. Using a for Loop (continued)
Example:
for count = 0 to 99
print LABEL_TEXT, name
endfor
Initializes count to 0
Checks count against the limit value 99
If evaluation is true, for statement body prints the label
Increases count by 1
Programming Logic and Design, Fifth Edition, Comprehensive

33. Using a for Loop (continued)
while statement could be used in place of for statement
Step value: number used to increase a loop control variable on each pass through a loop
Programming languages can:
Require a statement that indicates the step value
Have a step value default of 1
Specify step value when each pass through the loop changes the loop control variable by value other than 1
Programming Logic and Design, Fifth Edition, Comprehensive

34. Using Posttest Loops
Loop body may never execute in while loop and for loop
Use posttest loop when loop body must execute at least once
do-until loop
do-while loop
do-until loop executes until condition is true
do-while loop executes until condition is false
Programming Logic and Design, Fifth Edition, Comprehensive

35. Using Posttest Loops (continued)
Figure Inner loop from label production program in Figure 5-9
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36. Using Posttest Loops (continued)
Figure Label production program using a do-until loop
Programming Logic and Design, Fifth Edition, Comprehensive

37. Recognizing the Characteristics Shared by All Loops
Every logical problem could be solved using only the while loop
Other forms are conveniences
while loop
Loop-controlling question placed at beginning of steps that repeat
do-until loop
Loop-controlling question placed at end of steps that repeat
Programming Logic and Design, Fifth Edition, Comprehensive

38. Recognizing the Characteristics Shared by All Loops (continued)
Characteristics of all structured loops:
Loop-controlling question must provide entry or exit from repeating structure
Loop-controlling question provides the only entry to or exit from the repeating structure
Exactly one loop-controlling value
Provides only entry to or exit from the loop
Programming Logic and Design, Fifth Edition, Comprehensive

39. Recognizing the Characteristics Shared by All Loops (continued)
Figure Examples of unstructured loops
Programming Logic and Design, Fifth Edition, Comprehensive

40. Common Loop Applications
Using a loop to accumulate totals
Examples:
Business reports often include totals
Telephone bill provides a total
List of real estate sold and total value
Accumulator: variable that gathers values
Similar to a counter
Counter increments by one
Accumulator increments by some value
Programming Logic and Design, Fifth Edition, Comprehensive

41. Common Loop Applications (continued)
Accumulate total real estate prices
Declare numeric variable at beginning
Initialize the accumulator to 0
Read each transaction’s data record
Add its value to accumulator variable
Read the next record until eof
Variables exist only for the life of the application
Run the application a second time, variables occupy different memory location
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42. Common Loop Applications (continued)
Figure Month-end real estate sales report
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43. Figure 5-19 Flowchart and pseudocode for real estate sales report program
Programming Logic and Design, Fifth Edition, Comprehensive

44. Common Loop Applications (continued)
Using a loop to validate data
When prompting a user for data, no guarantee that data is valid
Validate data: make sure data falls in acceptable ranges
Example: user enters birth month
If number less than 1 or greater than 12
Display error message and stop the program
Assign default value for the month
Reprompt the user for valid input
Programming Logic and Design, Fifth Edition, Comprehensive

45. Common Loop Applications (continued)
Figure Reprompting a user once after an invalid month is entered
Programming Logic and Design, Fifth Edition, Comprehensive

46. Common Loop Applications (continued)
Figure Reprompting a user continuously after an invalid month is entered
Programming Logic and Design, Fifth Edition, Comprehensive

47. Summary
When using a loop, write one set of instructions that operates on multiple, separate data
Three steps must occur in every loop:
Initialize loop control variable
Compare variable to some value
Alter the variable that controls the loop
Nested loops: loops within loops
Nested loops maintain two individual loop control variables
Alter each at the appropriate time
Programming Logic and Design, Fifth Edition, Comprehensive

48. Summary (continued) Common mistakes made by programmers:
Neglecting to initialize loop control variable
Neglecting to alter loop control variable
Using wrong comparison with loop control variable
Including statements inside the loop that belong outside the loop
Most computer languages support a for statement
for loop used with definite loops
When number of iterations is known
Programming Logic and Design, Fifth Edition, Comprehensive

49. Summary (continued) for loop automatically:
Initializes
Evaluates
Increments
Use posttest loop when loop body must execute at least one time
Control variable evaluated after loop body executes
Programming Logic and Design, Fifth Edition, Comprehensive

50. Summary (continued) Characteristics of all structured loops:
Loop-controlling question provides entry or exit from repeating structure
Loop-controlling question provides the only entry or exit from repeating structure
Accumulator: variable that gathers values
Loops used to ensure user data is valid by reprompting the user
Programming Logic and Design, Fifth Edition, Comprehensive