An Object-Oriented Approach to Programming Logic and Design

An Object-Oriented Approach to An Object-Oriented Approach to

Programming Logic and DesignProgramming Logic and Design

Chapter 6Looping

An Object-Oriented Approach to Programming Logic and Design 2

ObjectivesObjectives

• 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


Objectives (continued)Objectives (continued)

• Recognize the characteristics shared by all loops

• Nest loops• Use a loop to accumulate totals


Understanding the Advantages of Understanding the Advantages of LoopingLooping

• 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


Controlling Loops with Variables, Controlling Loops with Variables, Counters and Sentinel ValuesCounters 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


Using a Using a whilewhile Loop with a Loop Loop with a Loop Control VariableControl 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


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



• 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



Example without a loop:



Example with a loop:



• 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


Using a Counter to Control LoopingUsing 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


Using Constant and Variable Sentinel Using Constant and Variable Sentinel ValuesValues

• 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


Using Constant and Variable Sentinel Using Constant and Variable Sentinel Values (continuedValues (continued))


Looping by DecrementingLooping by Decrementing

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


Avoiding Common Loop MistakesAvoiding 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


Neglecting to Initialize the Loop Neglecting to Initialize the Loop Control VariableControl Variable

• Uninitialized variables may contain unknown values in some languages


Neglecting to Alter the Loop Control Neglecting to Alter the Loop Control VariableVariable

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


Using the Wrong Comparison with the Using the Wrong Comparison with the Loop Control VariableLoop Control Variable

• How many times will each of these loops execute?

counter = 0

while counter < 10

perform someMethod()

counter = counter + 1

endwhile

counter = 0

while counter <= 10

perform someMethod()

counter = counter + 1

endwhile


Including Statements Inside the Loop Including Statements Inside the Loop that Belong Outside the Loopthat Belong Outside the Loop

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


Using a Using a forfor Loop 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


Using a Using a forfor Loop (continued) 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


Using a Using a do untildo until Loop 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


Recognizing the Characteristics Recognizing the Characteristics Shared by All LoopsShared 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


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


Nesting LoopsNesting 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


Nesting Loops (continued)Nesting Loops (continued)


Nesting Loops (continued)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


Using a Loop to Accumulate TotalsUsing 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


Using a Loop to Accumulate Totals Using a Loop to Accumulate Totals (continued)(continued)


SummarySummary

• 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


Summary (continued)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



• 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



• Nested loops are loops contained within other loops

• Summary reports contain only totals, no detail records

• Accumulator variables are used to accumulate totals

An Object-Oriented Approach to Programming Logic and Design

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