Programming Logic and Design Eighth Edi(on

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Programming Logic and Design

Eighth Edi(on

Chapter 3

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Objec3ves

In this chapter, you will learn about:

• The disadvantages of unstructured spagheA code

• The three basic structures—sequence, selec3on, and

loop

• Using a priming input to structure a program

• The need for structure

• Recognizing structure

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The Disadvantages of Unstructured

SpagheA Code

• Spaghe' code

–  Logically snarled program statements –  OKen a complicated mess

–  Programs oKen work but are diﬃcult to read and maintain –  Confusing and prone to error

• Unstructured programs

–  Do not follow the rules of structured logic

• Structured programs

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Understanding the Three Basic

Structures

• Structure

–  Basic unit of programming logic

–  Sequence structure

•  Perform ac3ons or tasks in order •  No branching or skipping any task

–  Selec:on structure (decision structure)

•  Ask a ques3on, take one of two ac3ons •  OKen called if-‐then-‐else

•  Dual-‐alterna:ve ifs or single-‐alterna:ve ifs

–  Loop structure

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Understanding the Three Basic

Structures

(con3nued)

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Understanding the Three Basic

Structures

(con3nued)

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Understanding the Three Basic

Structures

(con3nued)

• _{Dual-‐alterna:ve
ifs}

–  Contains two alterna3ves

–  _{The
if-then-else
structure}

if someCondition is true then

do oneProcess

else

do theOtherProcess

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Understanding the Three Basic

Structures

(con3nued)

• _{Single-‐alterna:ve
ifs}

–  _{An
else
clause
is
not
required}

• _{null
case}

–  Situa3on where nothing is done

if employee belongs to dentalPlan then

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Understanding the Three Basic

Structures

(con3nued)

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Understanding the Three Basic

Structures

(con3nued)

• Loop structure

–  Repeats a set of ac3ons while a condi3on remains true

•  Loop body

–  Also called repe::on or itera:on

–  Condi3on is tested ﬁrst in the most common form of loop –  _{The
while…do
or
while
loop}

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Understanding the Three Basic

Structures

(con3nued)

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Understanding the Three Basic

Structures

(con3nued)

• Loop structure

while testCondition continues to be true

do someProcess

endwhile

while you continue to be hungry

take another bite of food

determine if you still feel hungry

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Understanding the Three Basic

Structures

(con3nued)

• _{All
logic
problems
can
be
solved
using
only
sequence,}

selec3on, and loop

• _{Structures
can
be
combined
in
an
inﬁnite
number
of}

ways

• Stacking structures

–  A_aching structures end-‐to-‐end

• End-‐structure statement

–  Indicates the end of a structure

–  _{The
endif
statement
ends
an
if-then-else
structure} –  _{The
endwhile
statement
ends
a
loop
structure}

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Understanding the Three Basic

Structures

(con3nued)

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Understanding the Three Basic

Structures

(con3nued)

• Any individual task or step in a structure can be

replaced by a structure

• Nes:ng structures

–  Placing one structure within another

–  Indent the nested structure’s statements

• Block

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Understanding the Three Basic

Structures

(con3nued)

Figure 3-‐7 Flowchart and pseudocode showing nested structures—

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Understanding the Three Basic

Structures

(con3nued)

Figure 3-‐8 Flowchart and pseudocode showing nested structures—

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Understanding the Three Basic

Structures

(con3nued)

Figure 3-‐9 Flowchart and pseudocode for a selec3on within

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Understanding the Three Basic

Structures

(con3nued)

• Structured programs have the following

characteris3cs:

–  Include only combina3ons of the three basic structures –  Each structure has a single entry point and a single exit

point

–  Structures can be stacked or connected to one another only at their entry or exit points

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Using a Priming Input to Structure

a Program

• Priming input (or priming read)

–  Reads the ﬁrst input data record

–  Is outside the loop that reads the rest of the records –  Helps keep the program structured

• Analyze a ﬂowchart for structure one step at a 3me

• Watch for unstructured loops that do not follow this

order

–  First ask a ques3on

–  Take ac3on based on the answer –  Return to ask the ques3on again

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Using a Priming Input to Structure

a Program

(con3nued)

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Using a Priming Input to Structure

a Program

(con3nued)

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Understanding the Reasons for

Structure

• Clarity—unstructured programs are confusing

• Professionalism—other programmers expect it

• Eﬃciency—most languages support it

• Maintenance —other programmers ﬁnd it easier to

read

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Figure 3-‐18 Example 1

Structured Flowcharts?

Recognizing Structure

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Recognizing

Structure

(con3nued)

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Figure 3-‐21 Steps to structure the dog-‐

washing process

Recognizing

Structure

(con3nued)

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Figure 3-‐22 Structured dog-‐washing

ﬂowchart and pseudocode

Recognizing

Structure

(con3nued)

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Figure 3-‐23 Modularized version of the

dog-‐washing program

Recognizing

Structure

(con3nued)

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Summary

• SpagheA code

–  Statements that do not follow rules of structured logic

• Three basic structures

–  Sequence, selec3on, and loop

–  Combined by stacking and nes3ng

• Priming input

–  Statement that reads the ﬁrst input value prior to star3ng a structured loop

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Summary

(con3nued)

• Structured techniques promote:

–  Clarity

–  Professionalism –  Eﬃciency

–  Modularity

Programming Logic and Design Eighth Edi(on