Programming Logic and
Design
Fourth Edition, Comprehensive
Chapter 6
Looping
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
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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
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Understanding the Advantages of
Looping
• Loop:
– 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
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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
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Using a while Loop with a Loop
Control Variable (continued)
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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
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Using a Counter to Control Looping
• Developing the application:
– The input file
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Using a Counter to Control Looping
(continued)
• Developing the application:
– The main loop
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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
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Using a Counter to Control Looping
(continued)
• Developing the application:
– housekeeping() module
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Using a Counter to Control Looping
(continued)
• Developing the application:
– createLabels() module
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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
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Using a Counter to Control Looping
(continued)
• Developing the application:
– finishUp() module
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Looping with a Variable Sentinel Value
• Developing the application
– Print labels based on employee’s production amount
– The input file:
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Looping with a Variable Sentinel Value
(continued)
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Looping by Decrementing
• Decrementing: counting down
• May be more convenient to control a loop by
decrementing
• May eliminate the need for a counter variable
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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
• Initializing a variable that does not require
initialization
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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
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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
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Using the Wrong Comparison with the
Loop Control Variable
• Using <= or >= when only < or > was required may
cause an extra iteration through the loop
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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
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Including Statements Inside the Loop
that Belong Outside the Loop
(continued)
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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?
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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
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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
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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
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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
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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
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Using the do while and do until
Loops (continued)
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Using the do while and do until
Loops (continued)
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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
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Recognizing the Characteristics
Shared by All Loops (continued)
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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:
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Nesting Loops (continued)
• Developing the application:
– The desired output: ¼ of 1% raise in each pay period
– Two pay periods per month
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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 = 0.0025
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Nesting Loops (continued)
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Nesting Loops (continued)
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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
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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
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Using a Loop to Accumulate Totals
(continued)
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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
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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
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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
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