Programming Logic and Design Fifth Edition, Comprehensive

1. Programming Logic and Design Fifth Edition, Comprehensive Chapter 2 Understanding Structure

2. Programming Logic and Design, Fifth Edition, Comprehensive 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. 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

4. 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

5. Programming Logic and Design, Fifth Edition, Comprehensive 5 Understanding Unstructured Spaghetti Code (continued)

6. 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

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

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

9. Programming Logic and Design, Fifth Edition, Comprehensive 9 Understanding the Three Basic Structures (continued) Dual-alternative if: contains two alternatives

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

11. 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

12. 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

13. Programming Logic and Design, Fifth Edition, Comprehensive 13 Understanding the Three Basic Structures (continued) Loop structure

14. 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

15. Programming Logic and Design, Fifth Edition, Comprehensive 15 Understanding the Three Basic Structures (continued)

16. 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

17. Programming Logic and Design, Fifth Edition, Comprehensive 17 Understanding the Three Basic Structures (continued)

18. Programming Logic and Design, Fifth Edition, Comprehensive 18 Understanding the Three Basic Structures (continued)

19. Programming Logic and Design, Fifth Edition, Comprehensive 19 Understanding the Three Basic Structures (continued)

20. 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

21. 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

22. Programming Logic and Design, Fifth Edition, Comprehensive 22 Using the Priming Read (continued) Unstructured loop

23. Programming Logic and Design, Fifth Edition, Comprehensive 23 Using the Priming Read (continued) Structured but nonfunctional loop

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

25. Programming Logic and Design, Fifth Edition, Comprehensive 25 Using the Priming Read (continued) Functional and structured loop

26. 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

27. Programming Logic and Design, Fifth Edition, Comprehensive 27 Using the Priming Read (continued) What is wrong with this design?

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

29. 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

30. Programming Logic and Design, Fifth Edition, Comprehensive 30

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

32. 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

33. 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

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

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

36. 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

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

38. Programming Logic and Design, Fifth Edition, Comprehensive 38 Recognizing Structure (continued) Bring together the loose ends of D and of B

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

45. Programming Logic and Design, Fifth Edition, Comprehensive 45 Three Special Structures – Case, Do While, and Do Until (continued)

46. Programming Logic and Design, Fifth Edition, Comprehensive 46 Three Special Structures – Case, Do While, and Do Until (continued)

47. 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?

48. 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

49. 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

50. 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