An Object-Oriented Approach to Programming Logic and Design

1
An Object-Oriented Approach to Programming Logic
and Design

• Chapter 6
• Looping

2
Objectives

• Understand the advantages of looping
• Control loops with variables, counters, and
  sentinel values
• Avoid common loop mistakes
• Use a for loop
• Use a do until loop

3
Objectives (continued)

• Recognize the characteristics shared by all loops
• Nest loops
• Use a loop to accumulate totals

4
Understanding the Advantages of Looping

• The power of computers is their ability to
  perform repetitive actions
• Loop a structure that repeats actions while some
  condition continues
• Loops allow code that is written only once to be
  used over and over

5
Controlling Loops with Variables, Counters and
Sentinel Values

• while loop asks a question, and performs the
  actions as long as the answer continues to be
  True
• To control the number of times a loop repeats,
  use one of the following
• Loop control variables
• Counters
• Sentinel values

6
Using a while Loop with a Loop Control Variable

• Main loop controls the overall program logic
  that is used for every data record to be
  processed
• Other loops may be contained within the main loop

7
Using a while Loop with a Loop Control Variable
(continued)
8
Using a while Loop with a Loop Control Variable
(continued)
9
Using a while Loop with a Loop Control Variable
(continued)

• To process records in a file, you must
• Open the file prepares the file to be read
• Read each record, one at a time
• Close the file makes the file no longer
  available for reading
• End of file a condition that allows the program
  to determine that all of the records have been
  read (or the file is empty)

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Using a while Loop with a Loop Control Variable
(continued)

• Three steps that must occur in every loop
• Provide a starting value to control the loop
• Make a comparison using the controlling value
• Alter the controlling value within the loop
• Loop control variable variable that determines
  whether the loop will continue

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Using a while Loop with a Loop Control Variable
(continued)

• Example without a loop

12
Using a while Loop with a Loop Control Variable
(continued)

• Example with a loop

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Using a while Loop with a Loop Control Variable
(continued)

• Indefinite (or indeterminate) loop a loop for
  which you cannot predict the number of
  repetitions
• Definite loop a loop for which you know the
  exact number of repetitions that will take place
• Loop control decision is always based on a
  Boolean comparison
• Loop body the statements that execute within the
  loop

14
Using a Counter to Control Looping

• Counter a numeric variable used to count
  repetitions
• A counter variable may start at any value
• Incrementing the counter adding a value
  (usually 1) to the counter variable

15
Using Constant and Variable Sentinel Values

• Constant sentinel value a hard-coded value
  that controls the number of loop repetitions
• Variable sentinel value a variable whose value
  at run-time will control the number of loop
  repetitions

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Using Constant and Variable Sentinel Values
(continued)
17
Using Constant and Variable Sentinel Values
(continued)
18
Looping by Decrementing

• Decrementing (counting down) a loop control
  variable is sometimes more convenient than
  incrementing

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Avoiding Common Loop Mistakes

• Most 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

20
Neglecting to Initialize the Loop Control Variable

• Uninitialized variables may contain unknown
  values in some languages

21
Neglecting to Alter the Loop Control Variable

• Infinite loop a loop that never stops executing
  usually caused by failure to alter the loop
  control variable

22
Using the Wrong Comparison with the Loop Control
Variable

• How many times will each of these loops execute?

counter 0 while counter lt 10 perform
someMethod() counter counter 1 endwhile
counter 0 while counter lt 10 perform
someMethod() counter counter 1 endwhile
23
Including Statements Inside the Loop that Belong
Outside the Loop

• Statements that do not need to be repeated should
  not be inside a loop

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Using a for Loop

• Most languages support a for loop
• for loop a definite loop
• Use the for loop when you already know how many
  repetitions are needed
• for statement handles three actions
  automatically
• Initialization of loop control variable
• Evaluation of loop condition
• Incrementing or decrementing of loop control
  variable

25
Using a for Loop (continued)

• Usual format of a for loop
• for initialValue to finalValue
• do something
• endfor
• Example
• for num 0 to 99
• print Made for you personally by ,
• aWorker.getFirstName()
• endfor

26
Using a do until Loop

• Unlike the for and while loops, a do until loop
  always executes at least once
• The loop condition is checked after the actions
  are taken

27
Recognizing the Characteristics Shared by All
Loops

• All structured loops share these characteristics
• The loop-controlling question provides either
  entry to or exit from the repeating structure
• The loop-controlling question provides the only
  entry to or exit from the repeating structure

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Recognizing the Characteristics Shared by All
Loops (continued)
29
Nesting Loops

• Nested loops one loop contained within another
  loop
• Outer loop the container loop
• Inner loop the loop inside the container loop
• Each loop must have a loop control variable that
  is initialized, tested, and altered

30
Nesting Loops (continued)
31
Nesting Loops (continued)

• Techniques to self-document the program code
• Choosing variable and constant names that
  describe their purpose
• Using variables or constants to hold frequently
  used values that will not change during run-time

32
Using a Loop to Accumulate Totals

• Summary report contains only counts and totals,
  not individual records processed
• Accumulator a variable used to accumulate values
  during repetitions a value is added to its
  current value during each repetition
• Accumulator variable must be initialized prior to
  entering the loop

33
Using a Loop to Accumulate Totals (continued)
34
Summary

• Loop allows repetition of a set of program
  statements
• Three steps must occur in every loop
• Initialize the loop control variable
• Compare the loop control variable to a value to
  determine when the loop stops
• Increment (or decrement) the loop control
  variable

35
Summary (continued)

• Loop control can be done with
• A counter
• A constant sentinel value
• A variable sentinel value
• Common loop mistakes
• Failing to initialize the loop control variable
• Failing to alter the loop control variable
• Using the wrong comparison with the loop control
  variable
• Placing non-repetitive statements inside a loop

36
Summary (continued)

• for loop incorporates the three loop steps in a
  single statement
• do until loop guarantees that the loop body will
  be executed at least once
• All structured loops share these characteristics
• Loop control question provides either entry to or
  exit from the repeating structure
• Loop control question provides the only entry to
  or exit from the repeating structure

37
Summary (continued)

• Nested loops are loops contained within other
  loops
• Summary reports contain only totals, no detail
  records
• Accumulator variables are used to accumulate
  totals