Programming Logic and Design Fourth Edition, Introductory

1
Programming Logic and Design Fourth Edition,
Introductory

• Chapter 6
• Looping

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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 employees 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

21
Using the Wrong Comparison with the Loop Control
Variable

• Using lt or gt when only lt or gt 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