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Programming Logic and
Design
Fifth Edition, Comprehensive
Chapter 2
Understanding Structure
Objectives
• Learn about the features of unstructured spaghetti
code
• Understand the three basic structures: sequence,
selection, and loop
• Use a priming read
• Appreciate the need for structure
• Recognize structure
• Learn about three special structures: case, dowhile, & do-until
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Understanding Unstructured
Spaghetti Code
• Spaghetti code: logically snarled program
statements
– Can be the result of poor program design
• Spaghetti code programs often work, but are difficult
to read and maintain
• Convoluted logic usually requires more code
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Understanding Unstructured
Spaghetti Code (continued)
• Example: College Admissions
– Admit students who score >= 90 on admissions test if
upper 75 percent of high-school graduating class
– Admit students who score >= 80 on test if upper 50
percent of high-school graduating class
– Admit students who score >= 70 on admission test if
upper 25 percent of high-school graduating class
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Understanding Unstructured
Spaghetti Code (continued)
Figure 2-2 Spaghetti code example
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Understanding the Three Basic
Structures
• Structure: basic unit of programming logic
• Any program can be constructed from only three
basic types of structures
– Sequence
• Perform actions in order
• No branching or skipping any task
– Selection (decision)
• Ask a question, take one of two actions
• Dual-alternative or single-alternative
– Loop
• Repeat actions based on answer to a question
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Understanding the Three Basic
Structures (continued)
• Sequence structure
Figure 2-3 Sequence structure
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Understanding the Three Basic
Structures (continued)
• Selection structure
Figure 2-4 Selection structure
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Understanding the Three Basic
Structures (continued)
• Dual-alternative if: contains two alternatives
if someCondition is true then
do oneProcess
else
do theOtherProcess
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Understanding the Three Basic
Structures (continued)
• Single-alternative if: contains one alternative
Figure 2-5 Single-alternative selection structure
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Understanding the Three Basic
Structures (continued)
• Single-alternative if
if employee belongs to dentalPlan then
deduct $40 from employeeGrossPay
• Else clause is not required
• Null case: situation where nothing is done
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Understanding the Three Basic
Structures (continued)
• Loop structure
– Repeats a set of actions based on the answer to a
question
– Also called repetition or iteration
– Question is asked first in the most common form of
loop
Figure 2-6 Loop structure
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Understanding the Three Basic
Structures (continued)
• Loop structure
while testCondition continues to be true
do someProcess
while quantityInInventory remains low
continue to orderItems
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Understanding the Three Basic
Structures (continued)
• All logic problems can be solved using only these
three structures
• Structures can be combined in an infinite number of
ways
• Stacking: attaching structures end-to-end
• End-structure statements: indicate the end of a
structure
– The endif statement ends an if-then-else structure
– The endwhile ends a loop structure
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Understanding the Three Basic
Structures (continued)
Figure 2-7 Structured flowchart and pseudocode
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Understanding the Three Basic
Structures (continued)
• Any individual task or step in a structure can be
replaced by a structure
• Nesting: placing one structure within another
• Indent the nested structure’s statements
• Block: group of statements that execute as a
single unit
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Understanding the Three Basic
Structures (continued)
Figure 2-8 Flowchart and pseudocode showing a sequence
nested within a selection
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Understanding the Three Basic
Structures (continued)
Figure 2-9 Selection in a sequence within a selection
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Understanding the Three Basic
Structures (continued)
Figure 2-10 Flowchart and pseudocode for loop within selection within
sequence within selection
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Understanding the Three Basic
Structures (continued)
• Each structure has one entry and one exit point
• Structures attach to others only at entry or exit points
Figure 2-11 The three structures
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Using the Priming Read
• Priming read (or priming input):
– Reads the first input data record
– Outside the loop that reads the rest of the records
– Helps keep the program structured
• Analyze a flowchart for structure one step at a time
• Watch for unstructured loops that do not follow this
order:
– First ask a question
– Take action based on the answer
– Return to ask the question again
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Using the Priming Read (continued)
• Unstructured loop
Figure 2-12 Unstructured flowchart of a number-doubling program
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Using the Priming Read (continued)
• Structured but nonfunctional loop
Figure 2-15 Structured, but nonfunctional, flowchart of
number-doubling problem
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Using the Priming Read (continued)
• Functional but nonstructured loop
Figure 2-16 Functional, but nonstructured, flowchart
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Using the Priming Read (continued)
• Functional and structured loop
Figure 2-17 Functional, structured flowchart and pseudocode for the
number-doubling problem
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Using the Priming Read (continued)
• Priming read sets up the process so the loop can be
structured
• To analyze a flowchart’s structure, try writing
pseudocode for it
start
get inputNumber
while not eof
calculatedAnswer = inputNumber * 2
print calculatedAnswer
get inputNumber
endwhile
stop
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Using the Priming Read (continued)
• What is wrong with this design?
Figure 2-18 Structured but incorrect solution to the number-doubling problem
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Understanding the Reasons for
Structure
•
•
•
•
•
Provides clarity
Professionalism
Efficiency
Ease of maintenance
Supports modularity
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Recognizing Structure
• Any set of instructions can be expressed in
structured format
• Any task to which you can apply rules can be
expressed logically using sequence, selection, loop
• It can be difficult to detect whether a flowchart is
structured
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Recognizing Structure (continued)
• Is this flowchart structured?
Figure 2-22 Example 2
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Recognizing Structure (continued)
• Is this flowchart structured?
Figure 2-23 Example 3
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Recognizing Structure (continued)
• Single process like A is part of an unacceptable
structure
– At least the beginning of a sequence structure
Figure 2-24 Untangling Example 3, first step
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Recognizing Structure (continued)
• B begins a selection structure
– Sequences never have decisions in them
– Logic never returns to B
Figure 2-25 Untangling Example 3, second step
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Recognizing Structure (continued)
• Pull up on the flowline from the left side of B
Figure 2-26 Untangling Example 3, third step
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Recognizing Structure (continued)
• Next, pull up the flowline on the right side of B
Figure 2-27 Untangling Example 3, fourth step
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Recognizing Structure (continued)
• Pull up the flowline on the left side of D and untangle
it from the B selection by repeating C
Figure 2-28 Untangling Example 3, fifth step
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Recognizing Structure (continued)
• Now pull up the flowline on the right side of D
Figure 2-29 Untangling Example 3, sixth step
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Recognizing Structure (continued)
• Bring together the loose ends of D and of B
Figure 2-30 Finished flowchart and pseudocode for untangling Example 3
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Three Special Structures – CASE, DOWHILE, and DO-UNTIL
• Many languages allow three additional structures:
– The case structure
– The do-while structure
– The do-until structure
• CASE Structure:
– Decisions with more than two alternatives
– Tests a variable against a series of values and takes
action based on a match
– Nested if-then-else statements will do what a
case structure does
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Three Special Structures – Case, Do
While, and Do Until (continued)
• Using nested if-then-else for multiple
alternatives
Figure 2-31 Flowchart and pseudocode of tuition decisions
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Three Special Structures – Case, Do
While, and Do Until (continued)
• Using a case structure for multiple alternatives
Figure 2-32 Flowchart and pseudocode of case structure
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Three Special Structures – Case, Do
While, and Do Until (continued)
• do-while and do-until loops
– Question is asked at the end of the loop structure
– Loop statements always used at least once
Figure 2-33 The while loop, which
is a pretest loop
Figure 2-34 Structure of a do-while or
do-until loop, which are posttest loops
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Three Special Structures – Case, Do
While, and Do Until (continued)
• do-while: executes as long as the question’s
answer is Yes or True
• do-until: executes as long as the question’s
answer is No or False
do
wash a dish
until all dishes are washed
do
wash a dish
while more dishes remain to be washed
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Three Special Structures – Case, Do
While, and Do Until (continued)
• while loop with question at beginning is called a
pretest loop
• do-while and do-until with question at end are
called posttest loops
– Posttest loop can be replaced with a sequence
followed by a pretest while loop
pay a bill
while there are more bills to pay
pay a bill
endwhile
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Three Special Structures – Case, Do
While, and Do Until (continued)
Figure 2-35 Flowchart and pseudocode for do-while loop
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Three Special Structures – Case, Do
While, and Do Until (continued)
Figure 2-36 Flowchart and pseudocode for sequence followed by while loop
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Three Special Structures – Case, Do
While, and Do Until (continued)
• How can this design be made structured?
Figure 2-37 Unstructured loop
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Three Special Structures – Case, Do
While, and Do Until (continued)
• Repeat the needed step to enforce structure
Figure 2-38 Sequence and structured loop that accomplish the
same tasks as Figure 2-37
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Summary
• Spaghetti code: snarled program logic
• Three basic structures: sequence, selection, loop
– Combined by stacking and nesting
• Priming read: statement that reads the first input
data record
• Structured techniques promote clarity,
professionalism, efficiency, and modularity
• Flowchart can be made structured by untangling
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Summary (continued)
• case structure: questions with multiple alternatives
• while loop: a pretest loop asks the question first
• while loop statements never execute if the
answer is No
• do-while and do-until loops: posttest loops
that ask the question last
• do-while and do-until loop statements are
always executed at least once
– Posttest loop can be replaced by a sequence
followed by a while loop
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