Programming Logic and Design Fifth Edition, Comprehensive

1
Programming Logic and Design Fifth Edition,
Comprehensive

• Chapter 2
• Understanding Structure

2
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,
  do-while, and do-until

3
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

4
Understanding Unstructured Spaghetti Code
(continued)

• Example College Admissions
• Admit students who score gt 90 on admissions test
  if upper 75 percent of high-school graduating
  class
• Admit students who score gt 80 on test if upper
  50 percent of high-school graduating class
• Admit students who score gt 70 on admission test
  if upper 25 percent of high-school graduating
  class

5
Understanding Unstructured Spaghetti Code
(continued)
Figure 2-2 Spaghetti code example
6
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

7
Understanding the Three Basic Structures
(continued)

• Sequence structure

Figure 2-3 Sequence structure
8
Understanding the Three Basic Structures
(continued)

• Selection structure

Figure 2-4 Selection structure
9
Understanding the Three Basic Structures
(continued)

• Dual-alternative if contains two alternatives

if someCondition is true then do
oneProcess else do theOtherProcess
10
Understanding the Three Basic Structures
(continued)

• Single-alternative if contains one alternative

Figure 2-5 Single-alternative selection structure
11
Understanding the Three Basic Structures
(continued)

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

if employee belongs to dentalPlan then deduct
40 from employeeGrossPay
12
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
13
Understanding the Three Basic Structures
(continued)

• Loop structure

while testCondition continues to be true do
someProcess while quantityInInventory remains
low continue to orderItems
14
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

15
Understanding the Three Basic Structures
(continued)
Figure 2-7 Structured flowchart and pseudocode
16
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

17
Understanding the Three Basic Structures
(continued)
Figure 2-8 Flowchart and pseudocode showing a
sequence nested within a
selection
18
Understanding the Three Basic Structures
(continued)
Figure 2-9 Selection in a sequence within a
selection
19
Understanding the Three Basic Structures
(continued)
Figure 2-10 Flowchart and pseudocode for loop
within selection within sequence within selection
20
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
21
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

22
Using the Priming Read (continued)

• Unstructured loop

Figure 2-12 Unstructured flowchart of a
number-doubling program
23
Using the Priming Read (continued)

• Structured but nonfunctional loop

Figure 2-15 Structured, but nonfunctional,
flowchart of
number-doubling problem
24
Using the Priming Read (continued)

• Functional but nonstructured loop

Figure 2-16 Functional, but nonstructured,
flowchart
25
Using the Priming Read (continued)

• Functional and structured loop

Figure 2-17 Functional, structured flowchart and
pseudocode for the number-doubling problem
26
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

start get inputNumber while not
eof calculatedAnswer inputNumber 2 print
calculatedAnswer get inputNumber endwhile stop
27
Using the Priming Read (continued)

• What is wrong with this design?

Figure 2-18 Structured but incorrect solution to
the number-doubling problem
28
Understanding the Reasons for Structure

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

29
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

30
Recognizing Structure (continued)

• Is this flowchart structured?

Figure 2-22 Example 2
31
Recognizing Structure (continued)

• Is this flowchart structured?

Figure 2-23 Example 3
32
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
33
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
34
Recognizing Structure (continued)

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

Figure 2-26 Untangling Example 3, third step
35
Recognizing Structure (continued)

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

Figure 2-27 Untangling Example 3, fourth step
36
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
37
Recognizing Structure (continued)

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

Figure 2-29 Untangling Example 3, sixth step
38
Recognizing Structure (continued)

• Bring together the loose ends of D and of B

Figure 2-30 Finished flowchart and pseudocode
for untangling Example 3
39
Three Special Structures CASE, DO-WHILE, 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

40
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
41
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
42
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
43
Three Special Structures Case, Do While, and Do
Until (continued)

• do-while executes as long as the questions
  answer is Yes or True
• do-until executes as long as the questions
  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
44
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
45
Three Special Structures Case, Do While, and Do
Until (continued)
Figure 2-35 Flowchart and pseudocode for
do-while loop
46
Three Special Structures Case, Do While, and Do
Until (continued)
Figure 2-36 Flowchart and pseudocode for
sequence followed by while loop
47
Three Special Structures Case, Do While, and Do
Until (continued)

• How can this design be made structured?

Figure 2-37 Unstructured loop
48
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
49
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

50
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