Programming Logic and
Design
Fourth Edition, Introductory
Chapter 2
Understanding Structure
Objectives
• Describe the features of unstructured spaghetti code
• Describe the three basic structures – sequence,
selection, and loop
• Use a priming read
• Appreciate the need for structure
• Recognize structure
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Objectives (continued)
• Describe three special structures – case, do-while,
and do-until
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Understanding Unstructured
Spaghetti Code
• Spaghetti code – logically snarled program
statements
• Can be the result of poor program design
Example: college admissions criteria
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Understanding Unstructured
Spaghetti Code (continued)
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Understanding Unstructured
Spaghetti Code (continued)
• Spaghetti code programs often work, but are difficult
to read and maintain
• Convoluted logic usually requires more code
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Understanding the Three Basic
Structures
• Structure: a basic unit of programming logic
• Any program can be constructed from only three
basic types of structures
– Sequence
– Selection
– Loop
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Understanding the Three Basic
Structures (continued)
• Sequence structure
– A set of instructions, performed sequentially with no
branching
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Understanding the Three Basic
Structures (continued)
• Selection structure
– Asks a question, then takes one of two possible
courses of action based on the answer
– Also called a decision structure or an if-then-else
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Understanding the Three Basic
Structures (continued)
• Dual-alternative if: contains two alternatives
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Understanding the Three Basic
Structures (continued)
•
Single-alternative if: contains one alternative
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Understanding the Three Basic
Structures (continued)
•
Single-alternative if
•
•
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
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Understanding the Three Basic
Structures (continued)
• Loop structure
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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
– endif: ends an if-then-else structure
– endwhile: ends a loop structure
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Understanding the Three Basic
Structures (continued)
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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)
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Understanding the Three Basic
Structures (continued)
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Understanding the Three Basic
Structures (continued)
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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
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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:
1. First ask a question
2. Take action based on the answer
3. Return to ask the question again
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Using the Priming Read (continued)
• Unstructured loop:
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Using the Priming Read (continued)
• Structured but nonfunctional loop
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Using the Priming Read (continued)
• Functional but non-structured loop
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Using the Priming Read (continued)
• Functional and structured loop
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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
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Using the Priming Read (continued)
• What is wrong with this design?
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Understanding the Reasons for
Structure
• Advantages of structure:
–
–
–
–
–
Provides clarity
Professionalism
Efficiency
Ease of maintenance
Supports modularity
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Understanding the Reasons for
Structure (continued)
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Understanding the Reasons for
Structure (continued)
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Recognizing Structure
• Any set of instructions can be expressed in structured
format
• Is this flowchart structured?
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Recognizing Structure (continued)
• Is this flowchart structured?
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Recognizing Structure (continued)
• To make it structured, pull each symbol out and rework
• B begins a selection structure
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Recognizing Structure (continued)
• Pull up on the flowline from the left side of B
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Recognizing Structure (continued)
• Next, pull up the flowline on the right side of B
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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
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Recognizing Structure (continued)
• Now pull up the flowline on the right side of D
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Recognizing Structure (continued)
• Bring together the loose ends of D and of B
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Three Special Structures – Case, Do
While, and Do Until
• Many languages allow three additional structures:
– case structure
– do-while structure
– 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
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Three Special Structures – Case, Do
While, and Do Until (continued)
• Using a case structure for multiple alternatives
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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
– Ensures that the loop statements are always used at
least once
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Three Special Structures – Case, Do
While, and Do Until (continued)
• do-while loop executes as long as the question’s
answer is Yes or True
• do-until loop executes as long as the question’s
answer is No or False (until it becomes Yes or
True)
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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
• A posttest loop can be replaced with a sequence
followed by a pretest while loop
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Three Special Structures – Case, Do
While, and Do Until (continued)
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Three Special Structures – Case, Do
While, and Do Until (continued)
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Three Special Structures – Case, Do
While, and Do Until (continued)
• How can this design be made structured?
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Three Special Structures – Case, Do
While, and Do Until (continued)
• Repeat the needed step to enforce structure
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Summary
• Spaghetti code: snarled program logic
• Three basic structures: sequence, selection, loop
• These three can be combined by stacking and
nesting
• Priming read: statement that reads the first input
data record
• Structured techniques promote clarity,
professionalism, efficiency, and modularity
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Summary (continued)
• Flowchart can be made structured by untangling
• case structure: handles questions with multiple
alternatives
• while loop: a pretest loop that asks the question
first
• while loop statements may never be executed if
the answer is No
• do-while and do-until loops: posttest loops
that ask the question last
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Summary (continued)
• 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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