An ObjectOriented Approach to Programming Logic and Design

1
An Object-Oriented Approach to Programming Logic
and Design

• Chapter 4
• Understanding Structure

2
Objectives

• Describe the role of structure in object-oriented
  methods
• Draw flowcharts
• Understand unstructured spaghetti code
• Understand the three basic structures of
  sequence, selection, and loop
• Build structured methods

3
Objectives (continued)

• Use a priming read
• Understand the need for structure
• Recognize structure
• Describe two special structures
• case
• do until

4
The Role of Structure in Object-Oriented Methods

• Recall that classes can contain non-static
  instance methods such as
• public get to retrieve the objects data
• public set to set values for the objects data
• Other public methods to accomplish tasks using
  the objects data

5
The Role of Structure in Object-Oriented Methods
(continued)

• An application can also contain static methods
• No object exists for these methods, but they may
  manipulate objects instantiated from prewritten
  classes
• All methods should be structured methods

6
The Role of Structure in Object-Oriented Methods
(continued)

• A structured method
• Follows a specific set of rules for logical
  design
• Has a single entry and exit point

7
Drawing Flowcharts

• Symbols on a flowchart
• Lozenge racetrack-shaped marks the beginning or
  end of a method
• Annotation symbol three-sided box attached to
  flowchart by a dashed line provides details for
  a step
• Processing statement rectangle contains an
  action taken
• Input/output parallelogram denotes data coming
  in from or going out on a device

8
Drawing Flowcharts (continued)
9
Understanding Unstructured Spaghetti Code

• Spaghetti code unstructured code that is
  confusing and complex due to poor structure
• Flowchart of spaghetti code shows crossed flow
  paths with no clear path from beginning to end
• Unstructured code is difficult to read,
  understand, and modify

10
Understanding Unstructured Spaghetti Code
(continued)
11
Three Basic Structures of Sequence, Selection,
and Loop

• Structure a basic unit of programming logic
• All programs are composed of 3 structures
• Sequence
• Selection
• Loop

12
Three Basic Structures of Sequence, Selection,
and Loop (continued)

• Sequence structure performs one or more actions
  in sequence, with no branching, skipping, or
  looping continues step by step

13
Three Basic Structures of Sequence, Selection,
and Loop (continued)

• Selection structure allows one of two or more
  alternative paths to be taken
• if-then-else when only 2 alternatives

if hours-worked is more than 40 then calculate
overtimePay else calculate regularPay
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Three Basic Structures of Sequence, Selection,
and Loop (continued)

• Loop structure allows repetition or iteration
• Asks a question and performs an action if a
  certain answer is given, then asks the question
  again.
• Also called a while loop or a while-do loop

while testCondition continues to be true, do
someProcess
15
Building Structured Methods

• All logic problems can be solved with only the
  sequence, selection, and looping structures
• Structures can be stacked, one after another
• Structures can be nested, one within another

16
Building Structured Methods
17
Building Structured Methods (continued)
18
Building Structured Methods (continued)

• Block group of statements that execute as a
  single unit
• All pseudocode statements inside a block should
  be indented
• Indentation in pseudocode reflects the logic
  shown graphically in a flowchart

19
Building Structured Methods (continued)
20
Building Structured Methods (continued)

• endif always matches the most recent if that is
  not already matched
• endwhile always matches with the most recent
  while that is not already matched

21
Building Structured Methods (continued)
22
Building Structured Methods (continued)

• Each structure has 1 entry and 1 exit point
• Other structures may attach only at an entry or
  exit point

23
Using a Priming Read

• Priming read (or priming input) the first read
  or data input statement that occurs before and
  outside the loop that handles the rest of the
  input
• Priming read is required to keep a method
  structured

24
Using a Priming Read (continued)
25
Using a Priming Read (continued)

• Rules for a structured loop
• You ask a question
• If the answer indicates a task is to be
  performed, do so
• After you perform the task, go back to ask the
  question again

26
Using a Priming Read (continued)
Example Its structured, but doesnt work!
27
Using a Priming Read (continued)

• Example It works, but isnt structured!

28
Using a Priming Read (continued)
Example It works, and its structured!
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Using a Priming Read (continued)

• public void numberDoubling()
• numeric userNumber
• numeric answer
• while userNumber not 0
• answer userNumber 2
• print answer
• input userNumber
• endwhile
• return

30
Understanding the Reasons for Structure

• Why structured methods are better
• Clarity structured methods are less confusing
• Professionalism it is the expected standard in
  industry today
• Efficiency todays programming languages are
  built expecting structure
• Maintenance easier to modify programs later
• Modularity facilitates reuse of modules

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Understanding the Reasons for Structure
(continued)
32
Understanding the Reasons for Structure
(continued)
33
Recognizing Structure

• Example Is this flowchart segment structured?

• Yes, it has a loop and a selection within the
  loop

34
Recognizing Structure (continued)

• Example Is this flowchart segment structured?

• No, it is not constructed from the 3 basic
  structures

35
Recognizing Structure (continued)

• To untangle spaghetti code, pull one strand at
  a time and follow it
• Lets untangle this
• flowchart ?

36
Recognizing Structure (continued)

• Untangling Example Steps 1 2

37
Recognizing Structure (continued)

• Untangling Example Step 3 the No side of
  Question B

38
Recognizing Structure (continued)

• Untangling Example Step 4 the Yes side of
  Question B

39
Recognizing Structure (continued)

• Untangling Example Step 5 left side of
  Question D repeat step C to untangle it

40
Recognizing Structure (continued)

• Untangling Example Step 6 right side of
  Question D leads to the end

41
Recognizing Structure (continued)
42
Describing Two Special Structures case and do
until

• case structure allows more than 2 alternative
  paths
• do until loop an alternative to the while loop
• case and do until are not needed, but are
  convenient, acceptable structures

43
The case Structure

• Use a case when there are several distinct values
  for a variable, each of which requires a
  different action
• case eliminates the need for a set of nested if
  statements

44
The case Structure (continued)

• Example using nested if statements

45
The case Structure (continued)

• Example using a case statement instead

46
The do until Loop

• do until loop is similar to the while loop, but
  tests the condition at the end of the loop
• while loop tests the condition at the beginning
  of the loop If the answer is no, the action is
  not performed
• do until loop performs the action, then tests the
  condition the action is always performed at
  least once

47
The do until Loop (continued)
48
The do until Loop (continued)

• while loop pseudocode
• pay bills
• while there are more bills to pay
• pay bills
• endwhile
• do until loop pseudocode
• do
• pay bills
• until all bills are paid

49
The do until Loop (continued)

• Comparison using a do until loop

50
The do until Loop (continued)

• Comparison using a while loop

51
Summary

• Flowchart symbols
• Lozenge method header and return
• 3-sided box Annotation symbol
• Rectangle statement
• Parallelogram input/output action
• Spaghetti code is unstructured, hard to read and
  modify
• Structured code uses 3 basic structures
  sequence, selection, and loop

52
Summary (continued)

• Structures can be stacked and nested
• Priming read the first read or data input prior
  to beginning a loop
• Structured techniques result in clarity,
  professionalism, efficiency, and modularity
• if-then-else handles 2 path choices
• case structure handles 2 or more path choices

53
Summary (continued)

• while loop tests the condition first, then does
  the action the action may be performed 0 or more
  times
• do until loop does the action, then tests the
  condition it always performs the action at least
  once