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
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

4. 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
Programming Logic and Design, Fifth Edition, Comprehensive

5. Understanding Unstructured Spaghetti Code (continued)
Figure 2-2 Spaghetti code example
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

7. Understanding the Three Basic Structures (continued)
Sequence structure
Figure 2-3 Sequence structure
Programming Logic and Design, Fifth Edition, Comprehensive

8. Understanding the Three Basic Structures (continued)
Selection structure
Figure 2-4 Selection structure
Programming Logic and Design, Fifth Edition, Comprehensive

9. Understanding the Three Basic Structures (continued)
Dual-alternative if: contains two alternatives
if someCondition is true then
do oneProcess
else
do theOtherProcess
Programming Logic and Design, Fifth Edition, Comprehensive

10. Understanding the Three Basic Structures (continued)
Single-alternative if: contains one alternative
Figure 2-5 Single-alternative selection structure
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

13. Understanding the Three Basic Structures (continued)
Loop structure
while testCondition continues to be true
do someProcess
while quantityInInventory remains low
continue to orderItems
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

15. Understanding the Three Basic Structures (continued)
Figure 2-7 Structured flowchart and pseudocode
Programming Logic and Design, Fifth Edition, Comprehensive

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 structure’s statements
Block: group of statements that execute as a single unit
Programming Logic and Design, Fifth Edition, Comprehensive

17. Understanding the Three Basic Structures (continued)
Figure 2-8 Flowchart and pseudocode showing a sequence nested within a selection
Programming Logic and Design, Fifth Edition, Comprehensive

18. Understanding the Three Basic Structures (continued)
Figure 2-9 Selection in a sequence within a selection
Programming Logic and Design, Fifth Edition, Comprehensive

19. Understanding the Three Basic Structures (continued)
Figure Flowchart and pseudocode for loop within selection within sequence within selection
Programming Logic and Design, Fifth Edition, Comprehensive

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 The three structures
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

22. Using the Priming Read (continued)
Unstructured loop
Figure Unstructured flowchart of a number-doubling program
Programming Logic and Design, Fifth Edition, Comprehensive

23. Using the Priming Read (continued)
Structured but nonfunctional loop
Figure Structured, but nonfunctional, flowchart of number-doubling problem
Programming Logic and Design, Fifth Edition, Comprehensive

24. Using the Priming Read (continued)
Functional but nonstructured loop
Figure Functional, but nonstructured, flowchart
Programming Logic and Design, Fifth Edition, Comprehensive

25. Using the Priming Read (continued)
Functional and structured loop
Figure Functional, structured flowchart and pseudocode for the number-doubling problem
Programming Logic and Design, Fifth Edition, Comprehensive

26. 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
endwhile
stop
Programming Logic and Design, Fifth Edition, Comprehensive

27. Using the Priming Read (continued)
What is wrong with this design?
Figure Structured but incorrect solution to the number-doubling problem
Programming Logic and Design, Fifth Edition, Comprehensive

28. Understanding the Reasons for Structure
Provides clarity
Professionalism
Efficiency
Ease of maintenance
Supports modularity
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

30. Recognizing Structure (continued)
Is this flowchart structured?
Figure Example 2
Programming Logic and Design, Fifth Edition, Comprehensive

31. Recognizing Structure (continued)
Is this flowchart structured?
Figure Example 3
Programming Logic and Design, Fifth Edition, Comprehensive

32. Recognizing Structure (continued)
Single process like A is part of an unacceptable structure
At least the beginning of a sequence structure
Figure Untangling Example 3, first step
Programming Logic and Design, Fifth Edition, Comprehensive

33. Recognizing Structure (continued)
B begins a selection structure
Sequences never have decisions in them
Logic never returns to B
Figure Untangling Example 3, second step
Programming Logic and Design, Fifth Edition, Comprehensive

34. Recognizing Structure (continued)
Pull up on the flowline from the left side of B
Figure Untangling Example 3, third step
Programming Logic and Design, Fifth Edition, Comprehensive

35. Recognizing Structure (continued)
Next, pull up the flowline on the right side of B
Figure Untangling Example 3, fourth step
Programming Logic and Design, Fifth Edition, Comprehensive

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 Untangling Example 3, fifth step
Programming Logic and Design, Fifth Edition, Comprehensive

37. Recognizing Structure (continued)
Now pull up the flowline on the right side of D
Figure Untangling Example 3, sixth step
Programming Logic and Design, Fifth Edition, Comprehensive

38. Recognizing Structure (continued)
Bring together the loose ends of D and of B
Figure Finished flowchart and pseudocode for untangling Example 3
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

40. Three Special Structures – Case, Do While, and Do Until (continued)
Using nested if-then-else for multiple alternatives
Figure Flowchart and pseudocode of tuition decisions
Programming Logic and Design, Fifth Edition, Comprehensive

41. Three Special Structures – Case, Do While, and Do Until (continued)
Using a case structure for multiple alternatives
Figure Flowchart and pseudocode of case structure
Programming Logic and Design, Fifth Edition, Comprehensive

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 The while loop, which is a pretest loop
Figure Structure of a do-while or do-until loop, which are posttest loops
Programming Logic and Design, Fifth Edition, Comprehensive

43. 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
Programming Logic and Design, Fifth Edition, Comprehensive

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
endwhile
Programming Logic and Design, Fifth Edition, Comprehensive

45. Three Special Structures – Case, Do While, and Do Until (continued)
Figure Flowchart and pseudocode for do-while loop
Programming Logic and Design, Fifth Edition, Comprehensive

46. Three Special Structures – Case, Do While, and Do Until (continued)
Figure Flowchart and pseudocode for sequence followed by while loop
Programming Logic and Design, Fifth Edition, Comprehensive

47. Three Special Structures – Case, Do While, and Do Until (continued)
How can this design be made structured?
Figure Unstructured loop
Programming Logic and Design, Fifth Edition, Comprehensive

48. Three Special Structures – Case, Do While, and Do Until (continued)
Repeat the needed step to enforce structure
Figure Sequence and structured loop that accomplish the same tasks as Figure 2-37
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive

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
Programming Logic and Design, Fifth Edition, Comprehensive