Programming Logic and Design Fourth Edition, Introductory

1
Programming Logic and Design Fourth Edition,
Introductory

• Chapter 2
• Understanding Structure

2
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

3
Objectives (continued)

• Describe three special structures case,
  do-while, and do-until

4
Understanding Unstructured Spaghetti Code

• Spaghetti code logically snarled program
  statements
• Can be the result of poor program design
• Example college admissions criteria

5
Understanding Unstructured Spaghetti Code
(continued)
6
Understanding Unstructured Spaghetti Code
(continued)

• Spaghetti code programs often work, but are
  difficult to read and maintain
• Convoluted logic usually requires more code

7
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

8
Understanding the Three Basic Structures
(continued)

• Sequence structure
• A set of instructions, performed sequentially
  with no branching

9
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

11
Understanding the Three Basic Structures
(continued)

• Single-alternative if contains one alternative

12
Understanding the Three Basic Structures
(continued)

• Single-alternative if
• Else clause is not required
• Null case situation where nothing is done

13
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

15
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

16
Understanding the Three Basic Structures
(continued)
17
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 structures statements
• Block group of statements that execute as a
  single unit

18
Understanding the Three Basic Structures
(continued)
19
Understanding the Three Basic Structures
(continued)
20
Understanding the Three Basic Structures
(continued)
21
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

22
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

23
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

26
Using the Priming Read (continued)

• Functional and structured loop

27
Using the Priming Read (continued)

• Priming read sets up the process so the loop can
  be structured
• To analyze a flowcharts structure, try writing
  pseudocode for it

28
Using the Priming Read (continued)

• What is wrong with this design?

29
Understanding the Reasons for Structure

• Advantages of structure
• Provides clarity
• Professionalism
• Efficiency
• Ease of maintenance
• Supports modularity

30
Understanding the Reasons for Structure
(continued)
31
Understanding the Reasons for Structure
(continued)
32
Recognizing Structure

• Any set of instructions can be expressed in
  structured format
• Is this flowchart structured?

33
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

35
Recognizing Structure (continued)

• Pull up on the flowline from the left side of B

36
Recognizing Structure (continued)

• Next, pull up the flowline on the right side of B

37
Recognizing Structure (continued)

• Pull up the flowline on the left side of D and
  untangle it from the B selection by repeating C

38
Recognizing Structure (continued)

• Now pull up the flowline on the right side of D

39
Recognizing Structure (continued)

• Bring together the loose ends of D and of B

40
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

41
Three Special Structures Case, Do While, and Do
Until (continued)

• Using nested if-then-else for multiple
  alternatives

42
Three Special Structures Case, Do While, and Do
Until (continued)

• Using a case structure for multiple alternatives

43
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

44
Three Special Structures Case, Do While, and Do
Until (continued)

• do-while loop executes as long as the questions
  answer is Yes or True
• do-until loop executes as long as the questions
  answer is No or False (until it becomes Yes or
  True)

45
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

46
Three Special Structures Case, Do While, and Do
Until (continued)
47
Three Special Structures Case, Do While, and Do
Until (continued)
48
Three Special Structures Case, Do While, and Do
Until (continued)

• How can this design be made structured?

49
Three Special Structures Case, Do While, and Do
Until (continued)

• Repeat the needed step to enforce structure

50
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

51
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

52
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