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PROGRAMMING LOGIC AND DESIGN

FIFTH EDITION, COMPREHENSIVE

Chapter 5: Looping

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Objectives

Programming Logic and Design, Fifth Edition, Comprehensive

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Learn about the advantages of looping

Control loops with counters and sentinel values

Nest loops

Learn to avoid common loop mistakes

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


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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

Figure 5-1 The while loop

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Controlling Loops with Counters and Sentinel Values


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As long as a Boolean expression remains true,

whileloops 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


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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)


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Figure 5-2 A whileloop that prints Hello four times

Loop control variable is initialized prior to entering the loop

Loop control variable is testedIf result is true, loop body entered

Loop control variable must be altered in loop body

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Using a Definite while Loop with a Counter(continued)


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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


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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)


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Figure 5-4 Typical execution of the looping bank balance program

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Using an Indefinite while Loop with a Sentinel Value


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Figure 5-5

Crucial steps

that must

occur in everyloop.

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Nested Loops


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Nested loops:

loops within loops

Outer loop:

loop that contains another loop

Inner loop:

loop that is contained

Needed when values of two (or more) variablesrepeat to produce every combination of values

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Figure 5-7 Flowchart and pseudocode for AnswerSheet program

The program creates an answer sheet for an exam that contains 5 partswith three questions in each part.

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Mixing Constant and Variable Sentinel Values


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Number of times a loop executes can depend on aconstant or a value that varies

May not want to repeat every pass through a loopthe same number of times

Examples:

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 employeeOuter loop controlled by value of employee name

Inner loop controlled by production level

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Figure 5-8


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Mixing Constant

and Variable

Sentinel Values

Program that

produces 100 labels

for every employee

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Mixing Constant and Variable Sentinel Values(continued)


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Figure 5-8 Program that produces 100 labels for every employee (continued)

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Figure 5-9


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Mixing Constant

and Variable

Sentinel Values

Program that

produces a variable

number of labels for

every employee

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Avoiding Common Loop Mistakes


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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)


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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


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Avoiding

Common Loop

Mistakes (continued)

Incorrect logic when

loop control variable

initializations are

removed from label-

making program

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Figure 5-10 Incorrect logic when loop control variable altering statements are removed fromlabel-making program (continued)

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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 incrementslabelCounter

Inner loop executes infinitely

Always incorrect to create a loop that cannot

terminate

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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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Example:

Correct: loop performed 10 times

counter = 0

while counter < 10

print Hello

counter = counter + 1

end while

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Example (continued):

Error: loop performed 11 times

counter = 0

while counter

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Mistake: including statements inside the loop that

belong outside the loop

Example: calculating a users 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

projectionprogram

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Using a for Loop


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for statement or for loop is a definite loop

Provides three actions in one structure

Initializes

Evaluates Increments

Takes the form:

forvariable = initialValuetofinalValue

do something

endfor

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Using a for Loop (continued)


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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 Posttest Loops


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Loop body may never execute in while loop and

for loop

Use posttest loop when loop body must execute at

least once do-untilloop

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)33


Figure 5-15 Inner loop from label

production program in Figure 5-9

Figure 5-16 Label production

program using a do-until loop

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Recognizing the Characteristics Shared by All Loops


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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-untilloop Loop-controlling question placed at end of steps that

repeat

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Recognizing the Characteristics Shared by All Loops (cont.)

Characteristics of all

structured loops:

Loop-controlling

question must provide

entry or exit from

repeating structure

Loop-controlling

question provides the

onlyentry to or exitfrom the repeating

structure

Exactly one loop-

controlling value

Provides only entry to or

exit from the loop

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Common Loop Applications


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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)


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Accumulate total real estate prices

Declare numeric variable at beginning

Initialize the accumulator to 0

Read each transactions 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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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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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 acceptableranges

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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Figure 5-20

Reprompting

a user once

after an

invalid

month is

entered

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Figure 5-21

Reprompting

a user

continuously

after an

invalid

month is

entered

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Summary


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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 controlvariables

Alter each at the appropriate time

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Summary (continued)


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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)


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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)

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Characteristics of all structured loops:

Loop-controlling question provides entry or exit from

repeating structure

Loop-controlling question provides the only entry or exitfrom repeating structure

Accumulator: variable that gathers values

Loops used to ensure user data is valid by

reprompting the user

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