Programming Logic and
Design
Fifth Edition, Comprehensive
Chapter 5
Looping
Objectives
•
•
•
•
Learn about the advantages of looping
Control loops with counters and sentinel values
Nest loops
Learn to avoid common loop mistakes
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Objectives (continued)
• Use a for loop
• Use posttest loops
• Recognize the characteristics shared by all loops
• Learn about common loop applications
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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
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Understanding the Advantages of
Looping (continued)
Figure 5-1 The while loop
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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
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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
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Using a Definite while Loop
with a Counter (continued)
Figure 5-2 A while loop that prints “Hello” four times
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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
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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
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Figure 5-3 Looping bank balance program
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Using an Indefinite while Loop
with a Sentinel Value (continued)
Figure 5-4 Typical execution of the looping bank balance program
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Using an Indefinite while Loop
with a Sentinel Value (continued)
Figure 5-5 Crucial steps that must occur in every loop
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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
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Figure 5-7 Flowchart and pseudocode for AnswerSheet program
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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
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Mixing Constant and Variable Sentinel
Values (continued)
Figure 5-8 Program that produces 100 labels for every employee
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Mixing Constant and Variable Sentinel
Values (continued)
Figure 5-8 Program that produces 100 labels for every employee (continued)
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Figure 5-9 Program that produces a variable number of labels for every employee
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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
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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
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Figure 5-10 Incorrect logic when loop control variable initializations are
removed from label-making program
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Avoiding Common Loop Mistakes
(continued)
Figure 5-10 Incorrect logic when loop control variable altering statements are
removed from label-making program (continued)
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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
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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
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Avoiding Common Loop Mistakes
(continued)
• Example:
– Correct: loop performed 10 times
counter = 0
while counter < 10
print “Hello”
counter = counter + 1
end while
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Avoiding Common Loop Mistakes
(continued)
• Example (continued):
– Error: loop performed 11 times
counter = 0
while counter <= 10
print “Hello”
counter = counter + 1
end while
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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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Figure 5-12 Pay rate projection program
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Figure 5-14 Improved pay rate projection program
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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
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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
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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
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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
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Using Posttest Loops (continued)
Figure 5-15 Inner loop from label production program in Figure 5-9
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Using Posttest Loops (continued)
Figure 5-16 Label production program using a do-until loop
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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
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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
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Recognizing the Characteristics
Shared by All Loops (continued)
Figure 5-17 Examples of unstructured loops
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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
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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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Common Loop Applications
(continued)
Figure 5-18 Month-end real estate sales report
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Figure 5-19 Flowchart and pseudocode for real estate sales report program
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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
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Common Loop Applications
(continued)
Figure 5-20 Reprompting a user once after an invalid month is entered
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Common Loop Applications
(continued)
Figure 5-21 Reprompting a user continuously after an invalid month is entered
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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
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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
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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
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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
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