Programming Logic and Design Fifth Edition, Comprehensive

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

3
Objectives (continued)

• Use a for loop
• Use posttest loops
• Recognize the characteristics shared by all loops
• Learn about common loop applications

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

5
Understanding the Advantages of Looping
(continued)
Figure 5-1 The while loop
6
Controlling Loops with Counters and Sentinel
Values

• As long as a Boolean expression remains true,
  while loops body executes
• Must control number of repetitions to avoid an
  infinite loop
• Repetitions controlled by
• Counter
• Sentinel value

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

8
Using a Definite while Loop with a Counter
(continued)
Figure 5-2 A while loop that prints Hello four
times
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

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

11
Figure 5-3 Looping bank balance program
12
Using an Indefinite while Loop with a Sentinel
Value (continued)
Figure 5-4 Typical execution of the looping bank
balance program
13
Using an Indefinite while Loop with a Sentinel
Value (continued)
Figure 5-5 Crucial steps that must occur in
every loop
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

15
Figure 5-7 Flowchart and pseudocode for
AnswerSheet program
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
• Outer loop controlled by value of employee name
• Inner loop controlled by production level

17
Mixing Constant and Variable Sentinel Values
(continued)
Figure 5-8 Program that produces 100 labels for
every employee
18
Mixing Constant and Variable Sentinel Values
(continued)
Figure 5-8 Program that produces 100 labels for
every employee (continued)
19
Figure 5-9 Program that produces a variable
number of labels for every employee
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

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

22
Figure 5-10 Incorrect logic when loop control
variable initializations are removed from
label-making program
23
Avoiding Common Loop Mistakes (continued)
Figure 5-10 Incorrect logic when loop control
variable altering statements are removed from
label-making program (continued)
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
• Inner loop executes infinitely
• Always incorrect to create a loop that cannot
  terminate

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

26
Avoiding Common Loop Mistakes (continued)

• Example
• Correct loop performed 10 times
• counter 0
• while counter lt 10
• print Hello
• counter counter 1
• end while

27
Avoiding Common Loop Mistakes (continued)

• Example (continued)
• Error loop performed 11 times
• counter 0
• while counter lt 10
• print Hello
• counter counter 1
• end while

28
Avoiding Common Loop Mistakes (continued)

• Mistake including statements inside the loop
  that belong outside the loop
• Example calculating a users 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

29
Figure 5-12 Pay rate projection program
30
Figure 5-14 Improved pay rate projection program
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

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

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

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

35
Using Posttest Loops (continued)
Figure 5-15 Inner loop from label production
program in Figure 5-9
36
Using Posttest Loops (continued)
Figure 5-16 Label production program using a
do-until loop
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

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

39
Recognizing the Characteristics Shared by All
Loops (continued)
Figure 5-17 Examples of unstructured loops
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

41
Common Loop Applications (continued)

• Accumulate total real estate prices
• Declare numeric variable at beginning
• Initialize the accumulator to 0
• Read each transactions 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

42
Common Loop Applications (continued)
Figure 5-18 Month-end real estate sales report
43
Figure 5-19 Flowchart and pseudocode for real
estate sales report program
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

45
Common Loop Applications (continued)
Figure 5-20 Reprompting a user once after an
invalid month is entered
46
Common Loop Applications (continued)
Figure 5-21 Reprompting a user continuously
after an invalid month is entered
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

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

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

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